FidoNet is a worldwide computer network that is used for communication between bulletin board systems (BBSes). It uses a store-and-forward system to exchange private (email) and public (forum) messages between the BBSes in the network, as well as other files and protocols in some cases.
The FidoNet system was based on a number of small interacting programs. Only one of these interacted with the BBS system directly and was the only portion that had to be ported to support other BBS software. This greatly eased porting, and FidoNet was one of the few networks that was widely supported by almost all BBS software, as well as a number of non-BBS online services. This modular construction also allowed FidoNet to easily upgrade to new data compression systems, which was important in an era using modem-based communications over telephone links with high long-distance calling charges.
The rapid improvement in modem speeds during the early 1990s, combined with the rapid decrease in price of computer systems and storage, made BBSes increasingly popular. By the mid-1990s there were almost 40,000 FidoNet systems in operation, and it was possible to communicate with millions of users around the world. Only UUCPNET came close in terms of breadth or numbers; FidoNet's user base far surpassed other networks like BITNET.
The broad availability of low-cost Internet connections starting in the mid-1990s lessened the need for FidoNet's store-and-forward system, as any system in the world could be reached for equal cost. Direct dialing into local BBS systems rapidly declined. Although FidoNet had shrunk considerably since the late 1990s, it has remained in use even today despite internet connectivity becoming universally available.
History
Origins

There are two major accounts of the development of the FidoNet, differing only in small details.
Tom Jennings' account
Around Christmas 1983, Tom Jennings started work on a new MS-DOS–hosted bulletin board system that would emerge as Fido BBS. Jennings set up the system in San Francisco some time in early 1984. Another early user was John Madil, who was trying to set up a similar system in Baltimore on his Rainbow 100. Fido started spreading to new systems, and Jennings eventually started keeping an informal list of their phone numbers, with Jennings becoming #1 and Madil #2.[1]
Jennings released the first version of the FidoNet software in June 1984. In early 1985 he wrote a document explaining the operations of the FidoNet, along with a short portion on the history of the system. In this version, FidoNet was developed as a way to exchange mail between the first two Fido BBS systems, Jennings' and Madil's, to "see if it could be done, merely for the fun of it". This was first supported in Fido V7, "sometime in June 84 or so".[2][3][4]
Ben Baker's account
In early 1984, Ben Baker was planning on starting a BBS for the newly forming computer club at the McDonnell Douglas automotive division in St. Louis. Baker was part of the CP/M special interest group within the club.[5] He intended to use the seminal, CP/M-hosted, CBBS system, and went looking for a machine to run it on. The club's president told Baker that DEC would be giving them a Rainbow 100 computer on indefinite loan, so he made plans to move the CBBS onto this machine. The Rainbow contained two processors, an Intel 8088 and a Zilog Z80, allowing it to run both MS-DOS and CP/M, with the BBS running on the latter. When the machine arrived, they learned that the Z80 side had no access to the I/O ports, so CBBS could not communicate with a modem. While searching for software that would run on the MS-DOS side of the system, Baker learned of Fido through Madil.[1]
The Fido software required changes to the serial drivers to work properly on the Rainbow. A porting effort started, involving Jennings, Madil and Baker. This caused all involved to rack up considerable long distance charges as they all called each other during development, or called into each other's BBSes to leave email. During one such call "in May or early June", Baker and Jennings discussed how great it would be if the BBS systems could call each other automatically, exchanging mail and files between them.[1] This would allow them to compose mail on their local machines, and then deliver it quickly, as opposed to calling in and typing the message in while on a long-distance telephone connection. Jennings responded by calling into Baker's system that night and uploading a new version of the software consisting of three files: FIDO_DECV6 (the new version of the BBS program itself), FIDONET, and NODELIST.BBS. The new version of FIDO BBS had a timer that caused it to exit at a specified time, normally at night, and as it exited it would run the separate FIDONET program. NODELIST was the list of Fido BBS systems, which Jennings had already been compiling.[1]
The FIDONET program was what later became known as a mailer. FIDO was modified to use a previously unused numeric field in the message headers to store a node number for the machine the message should be delivered to. When FIDONET ran, it would search through the email database for any messages with a number in this field. FIDONET collected all of the messages for a particular node number into a file known as a message packet. After all the packets were generated, one for each node, the FIDONET program would look up the destination node's phone number in NODELIST.BBS, and call the remote system. Provided that FIDONET was running on that system, the two systems would handshake and, if this succeeded, the calling system would upload its packet, download a return packet if there was one, and disconnect. FIDONET would then unpack the return packet, place the received messages into the local system's storage, and move onto the next packet. When there were no remaining packet, it would exit, and run the FIDO BBS program.[6]
In order to lower long distance charges, the mail exchanges were timed to run late at night, normally 4 AM.[2] This would later be known as national mail hour,[7] and, later still, as Zone Mail Hour.
Up and running
By June 1984 Version 7 of the system was being run in production, and nodes were rapidly being added to the network. By August there were almost 30 systems in the nodelist, 50 by September, and over 160 by January 1985. As the network grew, the maintenance of the nodelist became prohibitive, and errors were common. In these cases people would start receiving phone calls at 4 AM, from a caller that would say nothing and then hang up. In other cases the system would be listed before it was up and running, resulting in long distance calls that accomplished nothing.[2]
In August 1984 Jennings handed off control of the nodelist to the group in St. Louis, mostly Ken Kaplan and Ben Baker. Kaplan had come across Fido as part of finding a BBS solution for his company, which worked with DEC computers and had been given a Rainbow computer and a USRobotics 1200bit/s modem.[8] From then on, joining FidoNet required one to set up their system and use it to deliver a netmail message to a special system, Node 51. The message contained various required contact information. If this message was transmitted successfully, it ensured that at least some of the system was working properly. The nodelist team would then reply with another netmail message back to the system in question, containing the assigned node number. If delivery succeeded, the system was considered to be working properly, and it was added to the nodelist.[2] The first new nodelist was published on 21 September 1984.[1]
Nets and nodes
Growth continued to accelerate, and by the spring of 1985 the system was already reaching its limit of 250 nodes. In addition to the limits on growth of what was clearly a popular system, nodelist maintenance continued to grow more and more time consuming.[1]
It was also realized that Fido systems were generally clustered – of the 15 systems running by the start of June 1984, 5 of them were in St. Louis.[1]  A user on Jennings's system in San Francisco that addressed emails to different systems in St. Louis would cause calls to be made to each of those BBSes in turn.  Local calls were normally free or charged at a low rate.  Additionally, the initial call setup, generally the first minute of the call, was normally billed at a higher rate than continuing an existing connection. Therefore, it would make sense to deliver all the messages from all the users in San Francisco to all of the users in St. Louis in a single call. Packets were generally small enough to be delivered within a minute or two, so delivering all the messages in a single call could greatly reduce costs by avoiding multiple first-minute charges. Once delivered, the packet would be broken out into separate packets for local systems, and delivered using multiple local free calls.
The team settled on the concept of adding a new network number patterned on the idea of area codes.[N 1] A complete network address would now consist of the network and node number pair, which would be written with a slash between them. All mail travelling between networks would first be sent to their local network host, someone who volunteered to pay for any long distance charges. That single site would collect up all the netmail from all of the systems in their network, then re-package it into single packets destined to each network. They would then call any required network admin sites and deliver the packet to them. That site would then process the mail as normal, although all of the messages in the packet would be guaranteed to be local calls.[1]
The network address was placed in an unused field in the Fido message database, which formerly always held a zero. Systems running existing versions of the software already ignored the fields containing the new addressing, so they would continue to work as before; when noticing a message addressed to another node they would look it up and call that system. Newer systems would recognize the network number and instead deliver that message to the network host. To ensure backward compatibility, existing systems retained their original node numbers through this period.[1]
A huge advantage of the new scheme was that node numbers were now unique only within their network, not globally. This meant the previous 250 node limit was gone, but for a variety of reasons this was initially limited to about 1,200. This change also devolved the maintenance of the nodelists down to the network hosts, who then sent updated lists back to Node 51 to be collected into the master list. The St. Louis group now had to only maintain their own local network, and do basic work to compile the global list.[1]
At a meeting held in Kaplan's living room in St. Louis on 11 April 1985[N 2] the various parties hammered out all of the details of the new concept. As part of this meeting, they also added the concept of a region, a purely administrative level that was not part of the addressing scheme. Regional hosts would handle any stragglers in the network maps, remote systems that had no local network hosts. They then divided up the US into ten regions that they felt would have roughly equal populations.[1]
By May, Jennings had early versions of the new software running. These early versions specified the routing manually through a new ROUTE.BBS file that listed network hosts for each node. For instance, an operator might want to forward all mail to St. Louis through a single node, node 10. ROUTE.BBS would then include a list of all the known systems in that area, with instructions to forward mail to each of those nodes through node 10. This process was later semi-automated by John Warren's NODELIST program.[9] Over time, this information was folded into updated versions of the nodelist format, and the ROUTES file is no longer used.[10]
A new version of FIDO and FIDONET, 10C, was released containing all of these features. On 12 June 1985 the core group brought up 10C, and most Fido systems had upgraded within a few months.[9] The process went much smoother than anyone imagined, and very few nodes had any problems.[1]
Echomail
Some time during the evolution of Fido, file attachments were added to the system, allowing a file to be referenced from an email message. During the normal exchange between two instances of FIDONET, any files attached to the messages in the packets were delivered after the packet itself had been up or downloaded. It is not clear when this was added, but it was already a feature of the basic system when the 8 February 1985 version of the FidoNet standards document was released, so this was added very early in Fido's history.
At a sysop meeting in Dallas, the idea was raised that it would be nice if there was some way for the sysops to post messages that would be shared among the systems.[11] In February 1986 Jeff Rush, one of the group members, introduced a new mailer that extracted messages from public forums that the sysop selected, in a manner similar to the way the original mailer handled private messages. The new program was known as a tosser/scanner. The tosser produced a file that was similar (or identical) to the output from the normal netmail scan, however, these files were then compressed and attached to a normal netmail message as an attachment. This message was then sent to a special address on the remote system. After receiving netmail as normal, the scanner on the remote system looked for these messages, unpacked them, and put them into the same public forum on the original system.[7]
In this fashion, Rush's system implemented a store and forward public message system similar to Usenet, but based on, and hosted by, the FidoNet system. The first such echomail forum was one created by the Dallas area sysops to discuss business, known as SYSOP. Another called TECH soon followed. Several public echos soon followed, including GAYNET and CLANG. These spawned hundreds of new echos, and led to the creation of the Echomail Conference List (Echolist) by Thomas Kenny in January 1987.[12] Echomail produced world-spanning shared forums, and its traffic volume quickly surpassed the original netmail system. By the early 1990s, echo mail was carrying over 8 MB of compressed message traffic a day, many times that when uncompressed.[7]
Echomail did not necessarily use the same distribution pathways as normal netmail, and the distribution routing was stored in a separate setup file not unlike the original ROUTES.BBS. At the originating site a header line was added to the message indicating the origin system's name and address. After that, each system that the message traveled through added itself to a growing PATH header, as well as a SEENBY header. SEENBY prevented the message from looping around the network in the case of mis-configured routing information.[7]
Echomail was not the only system to use the file attachment feature of netmail to implement store-and-forward capabilities. Similar concepts were used by online games and other systems as well.
Zones and points
The evolution towards the net/node addressing scheme was also useful for reducing communications costs between continents, where timezone differences on either end of the connection might also come into play. For instance, the best time to forward mail in the US was at night, but that might not be the best time for European hosts to exchange. Efforts towards introducing a continental level to the addressing system started in 1986.[7]
At the same time, it was noted that some power users were interested in using FidoNet protocols as a way of delivering the large quantities of echomail to their local machines where it could be read offline. These users did not want their systems to appear in the nodelist - they did not (necessarily) run a bulletin board system and were not publicly accessible.[7] A mechanism allowing netmail delivery to these systems without the overhead of nodelist maintenance was desirable.
In October 1986 the last major change to the FidoNet network was released, adding zones and points. Zones represented major geographical areas roughly corresponding to continents. There were six zones in total, North America, South America, Europe, Oceania, Asia, and Africa. Points represented non-public nodes, which were created privately on a BBS system. Point mail was delivered to a selected host BBS as normal, but then re-packaged into a packet for the point to pick up on-demand. The complete addressing format was now zone:net/node.point, so a real example might be Bob Smith@1:250/250.10.[7] Points were widely used only for a short time, the introduction of offline reader systems filled this role with systems that were much easier to use. Points remain in use to this day, but are less popular than when they were introduced.
Other extensions
Although FidoNet supported file attachments from even the earliest standards, this feature tended to be rarely used and was often turned off. File attachments followed the normal mail routing through multiple systems, and could back up transfers all along the line as the files were copied. A solution was offered in the form of file requests, which made file transfers driven by the calling system and used one-time point-to-point connections instead of the traditional routing. Two such standards became common, "WaZOO" and "Bark", which saw varying support among different mailers. Both worked in a similar fashion, with the mailer calling the remote system and sending a new handshake packet to request the files.[13][14]
Although FidoNet was, by far, the best known BBS-based network, it was by no means the only one. From 1988 on, PCBoard systems were able to host similar functionality known as RelayNet, while other popular networks included RBBSNet from the Commodore 64 world, and AlterNet. Late in the evolution of the FidoNet system, there was a proposal to allow mail (but not forum messages) from these systems to switch into the FidoNet structure.[15] This was not adopted, and the rapid rise of the internet made this superfluous as these networks rapidly added internet exchange, which acted as a lingua franca.

Peak
FidoNet started in 1984 and listed 100 nodes by the end of that year. Steady growth continued through the 1980s, but a combination of factors led to rapid growth after 1988. These included faster and less expensive modems, and rapidly declining costs of hard drives and computer systems in general. By April 1993 the FidoNet nodelist contained over 20,000 systems. At that time it was estimated that each node had, on average, about 200 active users. Of these 4 million users in total, 2 million users commonly used echomail, the shared public forums, while about 200,000 used the private netmail system.[7] At its peak, FidoNet listed approximately 39,000 systems.[2][N 3]
Throughout its lifetime, FidoNet was beset with management problems and infighting. Much of this can be traced to the fact that the inter-net delivery cost real money, and the traffic grew more rapidly than decreases caused by improving modem speeds and downward trending long distance rates. As they increased, various methods of recouping the costs were attempted, all of which caused friction in the groups. The problems were so bad that Jennings came to refer to the system as the "fight-o-net".[16]
Decline

As modems reached speeds of 28.8kbit/s, the overhead of the TCP/IP protocols were no longer so egregious and dial-up Internet became increasingly common. By 1995 the bulletin board market was reeling as users abandoned local BBS systems in favour of larger sites and web pages, which could be accessed worldwide for the same cost as accessing a local BBS system. This also made FidoNet less expensive to implement, because inter-net transfers could be delivered over the Internet as well, at little or no marginal cost. But this seriously diluted the entire purpose of the store-and-forward model, which had been built up specifically to address a long-distance problem that no longer existed.
The FidoNet nodelist started shrinking, especially in areas with widespread availability of internet connections. This downward trend continues, but has levelled out at approximately 2,500 nodes.[N 4] FidoNet remains popular in areas where Internet access is difficult to come by, or expensive.
Resurgence
There is now (~2014) a retro movement which is resulting in a slow increase in internet connected BBS and nodes. Telnet, Rlogin, and SSH are being used between systems. This means you can telnet to many BBS worldwide as cheaply as ones next door. Also Usenet and internet mail has been added, along with long file names to many newer versions of BBS software, some being free-ware, resulting in increasing use. The deaf and blind are also able to access this better than the internet as a whole as interfaces for them deal mostly with ASCII text which exists in most BBS. They find it helps them communicate without the complications of pictures and audio in their internet mail and communication in general. Nodelists are no longer declining in all cases.
FidoNet organizational structure
FidoNet is governed in a hierarchical structure according to FidoNet policy, with designated coordinators at each level to manage the administration of FidoNet nodes and resolve disputes between members.[17] Network coordinators are responsible for managing the individual nodes within their area, usually a city or similar sized area. Regional coordinators are responsible for managing the administration of the network coordinators within their region, typically the size of a state, or small country. Zone coordinators are responsible for managing the administration of all of the regions within their zone. The world is divided into six zones, the coordinators of which elect one of themselves to be the International Coordinator of FidoNet.
Technical structure
FidoNet was historically designed to use modem-based dial-up (POTS) access between bulletin board systems, and much of its policy and structure reflected this.
The FidoNet system officially referred only to transfer of Netmail—the individual private messages between people using bulletin boards—including the protocols and standards with which to support it. A netmail message would contain the name of the person sending, the name of the intended recipient, and the respective FidoNet addresses of each. The FidoNet system was responsible for routing the message from one system to the other (details below), with the bulletin board software on each end being responsible for ensuring that only the intended recipient could read it. Due to the hobbyist nature of the network, any privacy between sender and recipient was only the result of politeness from the owners of the FidoNet systems involved in the mail's transfer. It was common, however, for system operators to reserve the right to review the content of mail that passed through their system.
Netmail allowed for the attachment of a single file to every message. This led to a series of piggyback protocols that built additional features onto FidoNet by passing information back and forth as file attachments. These included the automated distribution of files, and transmission of data for inter-BBS games.
By far the most commonly used of these piggyback protocols was Echomail, public discussions similar to Usenet newsgroups in nature. Echomail was supported by a variety of software that collected up new messages from the local BBSes' public forums (the scanner), compressed it using ARC or ZIP, attached the resulting archive to a Netmail message, and sent that message to a selected system. On receiving such a message, identified because it was addressed to a particular user, the reverse process was used to extract the messages, and a tosser put them back into the new system's forums.
Echomail was so popular that for many users, Echomail was the FidoNet. Private person-to-person Netmail was relatively rare.
Geographical structure
FidoNet is politically organized into a tree structure, with different parts of the tree electing their respective coordinators. The FidoNet hierarchy consists of zones, regions, networks, nodes and points broken down more-or-less geographically.
The highest level is the zone, which is largely continent-based:
Zone 1 is North America
Zone 2 is Europe, Former Soviet Union countries, and Israel
Zone 3 is Australasia
Zone 4 is Latin America (except Puerto Rico)
Zone 5 was Africa
Zone 6 was Asia, Israel and the Asian parts of Russia, (which are listed in Zone 2). On 26 July 2007 zone 6 was removed, and all remaining nodes were moved to zone 3.[18]
Each zone is broken down into regions, which are broken down into nets, which consist of individual nodes.  Zones 7-4095 are used for othernets; groupings of nodes which use Fido-compatible software to carry their own independent message areas without being in any way controlled by FidoNet's political structure. Using un-used zone numbers would ensure that each network would have a unique set of addresses, avoiding potential routing conflicts and ambiguities for systems that belonged to more than one network.
FidoNet addresses
FidoNet addresses explicitly consist of a zone number, a network number (or region number), and a node number. They are written in the form Zone:Network/Node.[19] The FidoNet structure also allows for semantic designation of region, host, and hub status for particular nodes, but this status is not directly indicated by the main address.
For example, consider a node located in Tulsa, Oklahoma, United States with an assigned node number is 918, located in Zone 1 (North America), Region 19, and Network 170. The full FidoNet address for this system would be 1:170/918.  The region was used for administrative purposes, and was only part of the address if the node was listed directly underneath the Regional Coordinator, rather than one of the networks that were used to divide the region further.
FidoNet policy requires that each FidoNet system maintain a nodelist of every other member system. Information on each node includes the name of the system or BBS, the name of the node operator, the geographic location, the telephone number, and software capabilities. The nodelist is updated weekly, to avoid unwanted calls to nodes that had shut down, with their phone numbers possibly having been reassigned for voice use by the respective telephone company.
To accomplish regular updates, coordinators of each network maintain the list of systems in their local areas. The lists are forwarded back to the International Coordinator via automated systems on a regular basis. The International Coordinator would then compile a new nodelist, and generate the list of changes (nodediff) to be distributed for node operators to apply to their existing nodelist.
Routing of FidoNet mail
In a theoretical situation, a node would normally forward messages to a hub. The hub, acting as a distribution point for mail, might then send the message to the Net Coordinator. From there it may be sent through a Regional Coordinator, or to some other system specifically set up for the function. Mail to other zones might be sent through a Zone Gate.
For example, a FidoNet message might follow the path:
1:170/918 (node) to 1:170/900 (hub) to 1:170/0 (net coordinator) to 1:19/0 (region coordinator) to 1:1/0 (zone coordinator). From there, it was distributed 'down stream' to the destination node(s).
Originally there was no specific relationship between network numbers and the regions they reside in. In some areas of FidoNet, most notably in Zone 2, the relationship between region number and network number are entwined. For example, 2:201/329 is in Net 201 which is in Region 20 while 2:2410/330 is in Net 2410 which is in Region 24. Zone 2 also relates the node number to the hub number if the network is large enough to contain any hubs. This effect may be seen in the nodelist by looking at the structure of Net 2410 where node 2:2410/330 is listed under Hub 300. This is not the case in other zones.
In Zone 1, things are much different. Zone 1 was the starting point and when Zones and Regions were formed, the existing nets were divided up regionally with no set formula. The only consideration taken was where they were located geographically in respect to the region's mapped outline. As net numbers got added, the following formula was used.

Region number × 20

Then when some regions started running out of network numbers, the following was also used.

Region number × 200

Region 19, for instance, contains nets 380-399 and 3800-3999 in addition to those that were in Region 19 when it was formed.
Part of the objective behind the formation of local nets was to implement cost reduction plans by which all messages would be sent to one or more hubs or hosts in compressed form (ARC was nominally standard, but PKZIP is universally supported); one toll call could then be made during off-peak hours to exchange entire message-filled archives with an out-of-town uplink for further redistribution.
In practice, the FidoNet structure allows for any node to connect directly to any other, and node operators would sometimes form their own toll-calling arrangements on an ad-hoc basis, allowing for a balance between collective cost saving and timely delivery. For instance, if one node operator in a network offered to make regular toll calls to a particular system elsewhere, other operators might arrange to forward all of their mail destined for the remote system, and those near it, to the local volunteer.  Operators within individual networks would sometimes have cost-sharing arrangements, but it was also common for people to volunteer to pay for regular toll calls either out of generosity, or to build their status in the community.
This ad-hoc system was particularly popular with networks that were built on top of FidoNet. Echomail, for instance, often involved relatively large file transfers due to its popularity. If official FidoNet distributors refused to transfer Echomail due to additional toll charges, other node operators would sometimes volunteer. In such cases, Echomail messages would be routed to the volunteers' systems instead.
The FidoNet system was best adapted to an environment in which local telephone service was inexpensive and long-distance calls (or intercity data transfer via packet-switched networks) costly. Therefore, it fared somewhat poorly in Japan, where even local lines are expensive, or in France, where tolls on local calls and competition with Minitel or other data networks limited its growth.
Points
As the number of messages in Echomail grew over time, it became very difficult for users to keep up with the volume while logged into their local BBS. Points were introduced to address this, allowing technically savvy users to receive the already compressed and batched Echomail (and Netmail) and read it locally on their own machines.[20]
To do this, the FidoNet addressing scheme was extended with the addition of a final address segment, the point number. For instance, a user on the example system above might be given point number 10, and thus could be sent mail at the address 1:170/918.10.
In real-world use, points are fairly difficult to set up. The FidoNet software typically consisted of a number of small utility programs run by manually edited scripts that required some level of technical ability. Reading and editing the mail required either a "sysop editor" program, or a BBS program to be run locally.
In North America (Zone 1), where local calls are generally free, the benefits of the system were offset by its complexity. Points were used only briefly, and even then only to a limited degree. Dedicated offline mail reader programs such as Blue Wave, Squiggy and Silver Xpress (OPX) were introduced in the mid-1990s, and quickly rendered the point system obsolete. Many of these packages supported the QWK offline mail standard.
In other parts of the world, especially Europe, this was different. In Europe, even local calls are generally metered, so there was a strong incentive to keep the duration of the calls as short as possible. Point software employs standard compression (ZIP, ARJ, etc.) and so keeps the calls down to a few minutes a day at most. In contrast to North America, pointing saw rapid and fairly widespread uptake in Europe.
Many regions distribute a pointlist in parallel with the nodelist. The pointlist segments are maintained by Net- and Region Pointlist Keepers and the Zone Point List Keeper assembles them into the Zone pointlist. At the peak of FidoNet there were over 120,000 points listed in the Zone 2 pointlist. Listing points is on a voluntary basis and not every point is listed, so how many points there really were is anybody's guess. As of June 2006, there are still some 50,000 listed points. Most of them are in Russia and Ukraine.
Technical specifications
FidoNet contained several technical specifications for compatibility between systems.  The most basic of all is FTS-0001,[21] with which all FidoNet systems are required to comply as a minimal requirement.  FTS-0001 defined:
Handshaking - the protocols used by mailer software to identify each other and exchange meta information about the session.
Transfer protocol (XMODEM) - the protocols to be used for transferring files containing FidoNet mail between systems.
Message format - the standard format for FidoNet messages during the time which they were exchanged between systems.
Other specifications that were commonly used provided for echomail, different transfer protocols and handshake methods (e.g.: Yoohoo/Yoohoo2u2, EMSI), file compression, nodelist format, transfer over reliable connections such as the Internet (Binkp), and other aspects.
Zone mail hour
Since computer bulletin boards historically used the same telephone lines for transferring mail as were used for dial-in human users of the BBS, FidoNet policy dictates that at least one designated line of each FidoNet node must be available for accepting mail from other FidoNet nodes during a particular hour of each day.[22]
Zone Mail Hour, as it was named, varies depending on the geographic location of the node, and was designated to occur during the early morning.  The exact hour varies depending on the time zone, and any node with only one telephone line is required to reject human callers. In practice, particularly in later times, most FidoNet systems tend to accept mail at any time of day when the phone line is not busy, usually during night.
FidoNet deployments

Most FidoNet deployments were designed in a modular fashion.  A typical deployment would involve several applications that would communicate through shared files and directories, and switch between each other through carefully designed scripts or batch files.  However, monolithic software that encompassed all required functions in one package is available, such as D'Bridge.  Such software eliminated the need for custom batch files and is tightly integrated in operation.  The preference of deployment was that of the operator and there were both pros and cons of running in either fashion.
Arguably the most important piece of software on a DOS-based Fido system was the  FOSSIL driver, which was a small device driver which provided a standard way for the Fido software to talk to the modem.[23]  This driver needed to be loaded before any Fido software would work.  An efficient FOSSIL driver meant faster, more reliable connections.
Mailer software was responsible for transferring files and messages between systems, as well as passing control to other applications, such as the BBS software, at appropriate times.  The mailer would initially answer the phone and, if necessary, deal with incoming mail via FidoNet transfer protocols.  If the mailer answered the phone and a human caller was detected rather than other mailer software, the mailer would exit, and pass control to the BBS software, which would then initialise for interaction with the user.  When outgoing mail was waiting on the local system, the mailer software would attempt to send it from time to time by dialing and connecting to other systems who would accept and route the mail further.  Due to the costs of toll calls which often varied between peak and off-peak times, mailer software would usually allow its operator to configure the optimal times in which to attempt to send mail to other systems.
BBS software was used to interact with human callers to the system.  BBS software would allow dial-in users to use the system's message bases and write mail to others, locally or on other BBSes.  Mail directed to other BBSes would later be routed and sent by the mailer, usually after the user had finished using the system.  Many BBSes also allowed users to exchange files, play games, and interact with other users in a variety of ways (i.e.: node to node chat).
A scanner/tosser application, such as FastEcho, FMail, TosScan and Squish, would normally be invoked when a BBS user had entered a new FidoNet message that needed to be sent, or when a mailer had received new mail to be imported into the local messages bases.  This application would be responsible for handling the packaging of incoming and outgoing mail, moving it between the local system's message bases and the mailer's inbound and outbound directories.  The scanner/tosser application would generally be responsible for basic routing information, determining which systems to forward mail to.
In later times, message readers or editors that were independent of BBS software were also developed.  Often the System Operator of a particular BBS would use a devoted message reader, rather than the BBS software itself, to read and write FidoNet and related messages. One of the most popular editors in 2008 was GoldED+.  In some cases FidoNet nodes, or more often FidoNet points, had no public bulletin board attached, and existed only for the transfer of mail for the benefit of the node's operator. Most nodes in 2009 had no BBS access, but only points, if anything.
The original Fido BBS software, and some other FidoNet-supporting software from the 1980s, is no longer functional on modern systems.  This is for several reasons, including problems related to the Y2K bug.  In some cases, the original authors have left the BBS or shareware community, and the software, much of which was closed source, has been rendered abandonware.
Several DOS based legacy FidoNet Mailers such as FrontDoor, Intermail, MainDoor and D'Bridge from the early 1990s can still be run today under Windows without a modem, by using the freeware NetFoss Telnet FOSSIL driver, and by using a Virtual Modem such as NetSerial. This allows the mailer to dial an IP address or hostname via Telnet, rather than dialing a real POTS phone number. There are similar solutions for Linux such as MODEMU (modem emulator) which has limited success when combined with DOSEMU (DOS emulator).
Mail Tossers such as FastEcho and FMail are still used today under both Windows and Linux/DOSEMU.

There are several modern Windows based FidoNet Mailers available today with source code, including Argus, Radius, and Taurus. MainDoor is another Windows based Fidonet mailer, which also can be run using either a modem or directly over TCP/IP. Two popular free and open source software FidoNet mailers for Unix-like systems are the binkd (cross-platform, IP-only, uses the binkp protocol) and qico (supports modem communication as well as the IP protocol of ifcico and binkp).
On the hardware side, Fido systems were usually well-equipped machines, for their day, with quick CPUs, high-speed modems and 16550 UARTs, which were at the time an upgrade.  As a Fidonet system was usually a BBS, it needed to quickly process any new mail events before returning to its 'waiting for call' state. In addition, the BBS itself usually necessitated lots of storage space.  Finally, a FidoNet system usually had at least one dedicated phoneline.  Consequently, operating a Fidonet system often required significant financial investment, a cost usually met by the owner of the system.
FidoNet availability
While the use of FidoNet has dropped dramatically compared with its use up to the mid-1990s, it is still used in many countries and especially Russia and former republics of the USSR.  Some BBSes, including those that are now available for users with Internet connections via telnet, also retain their FidoNet netmail and echomail feeds.
Some of FidoNet's echomail conferences are available via gateways with the Usenet news hierarchy using software like UFGate.  There are also mail gates for exchanging messages between Internet and FidoNet.  Widespread net abuse and e-mail spam on the Internet side has caused some gateways (such as the former 1:1/31 IEEE fidonet.org gateway) to become unusable or cease operation entirely.
FidoNews
FidoNews is the newsletter of the FidoNet community.  Affectionately nicknamed The Snooze, it is published weekly.  It was first published in 1984.  Throughout its history, it has been published by various people and entities, including the short-lived International FidoNet Association.
See also

PODSnet
RelayNet
UUCP

Notes

Citations

Further reading


Rearrangements, especially those that can participate in cascade reactions, such as the aza-Cope rearrangements, are of high practical as well as conceptual importance in organic chemistry, due to their ability to quickly build structural complexity out of simple starting materials. The aza-Cope rearrangements are examples of heteroatom versions of the Cope rearrangement, which is a [3,3]-sigmatropic rearrangement that shifts single and double bonds between two allylic components.  In accordance with the Woodward-Hoffman rules, thermal aza-Cope rearrangements proceed suprafacially.[1]  Aza-Cope rearrangements are generally classified by the position of the nitrogen in the molecule (see figure):

The first example of an aza-Cope rearrangement was the ubiquitous cationic 2-aza-Cope Rearrangement, which takes place at temperatures 100-200°C lower than the Cope rearrangement due to the facile nature of the rearrangement.[2]  The facile nature of this rearrangement is attributed both to the fact that the cationic 2-aza-Cope is inherently thermoneutral, meaning there's no bias for the starting material or product, as well as to the presence of the charged heteroatom in the molecule, which lowers the activation barrier.  Less common are the 1-aza-Cope rearrangement and the 3-aza-Cope rearrangement, which are the microscopic reverse of each other.  The 1- and 3-aza-Cope rearrangements have high activation barriers and limited synthetic applicability, accounting for their relative obscurity.[3][4][5]
To maximize its synthetic utility, the cationic 2-aza-Cope rearrangement is normally paired with a thermodynamic bias toward one side of the rearrangement.  The most common and synthetically useful strategy couples the cationic 2-aza-Cope rearrangement with a Mannich cyclization, and is the subject of much of this article.  This tandem aza-Cope/Mannich reaction is characterized by its mild reaction conditions, diastereoselectivity, and wide synthetic applicability.  It provides easy access to acyl-substituted pyrrolidines, a structure commonly found in natural products such as alkaloids, and has been used in the synthesis of a number of them, notably strychnine and crinine.
[6]
Larry E. Overman and coworkers have done extensive research on this reaction.[1]
The cationic 2-aza-Cope rearrangement

The cationic 2-aza-Cope rearrangement, most properly called the 2-azonia-[3,3]-sigmatropic rearrangement, has been thoroughly studied by Larry E. Overman and coworkers.  It is the most extensively studied of the aza-Cope rearrangements due to the mild conditions required to carry the arrangement out, as well as for its many synthetic applications, notably in alkaloid synthesis.  Thermodynamically, the general 2-aza-Cope rearrangement does not have a product bias, as the bonds broken and formed are equivalent in either direction of the reaction, similar to the Cope rearrangement.  The presence of the ionic nitrogen heteroatom accounts for the more facile rearrangement of the cationic 2-aza-Cope rearrangement  in comparison to the Cope rearrangement. Hence, it is often paired with a thermodynamic sink to bias a rearrangement product.[1]
In 1950, Horowitz and Geissman reported the first example of the 2-aza-Cope rearrangement, a surprising result in a failed attempt to synthesize an amino alcohol.[2]  This discovery identified the basic mechanism of the rearrangement, as the product was most likely produced through a nitrogen analog of the Cope rearrangement.  Treatment of an allylbenzylamine (A) with formic acid and formaldehyde leads to an amino alcohol (B).  The amino alcohol converts to an imine under addition of acid (C), which undergoes the cationic 2-aza-Cope rearrangement (D).  Water hydrolyses the iminium ion to an amine (E).  Treating this starting material with only formaldehyde showed that alkylation of the amine group occurred after the cationic 2-aza-Cope rearrangement, a testament to the quick facility of the rearrangement.[2]

Due to the mild heating conditions of the reaction carried out, unlike the more stringent ones for a purely hydrocarbon Cope rearrangement, this heteroatomic Cope rearrangement introduced the hypothesis that having a positive charge on a nitrogen in the cope rearrangement significantly reduces the activation barrier for the rearrangement.[2]
Reaction mechanism
Rate acceleration due to positively charged nitrogen

The aza-Cope rearrangements are predicted by the Woodward-Hoffman rules to proceed suprafacially.  However, while never explicitly studied, Overman and coworkers have hypothesized that, as with the base-catalyzed oxy-Cope rearrangement, the charged atom distorts the sigmatropic rearrangement from a purely concerted reaction mechanism (as expected in the Cope rearrangement), to one with partial diradical/dipolar character, due to delocalization of the positive charge onto the allylic fragment, which weakens the allylic bond.  This results in a lowered activation barrier for bond breaking.  Thus the cationic-aza-Cope rearrangement proceeds more quickly than more concerted processes such as the Cope rearrangement.[6][7]
Transition state and stereochemistry
The cationic 2-aza-Cope rearrangement is characterized by its high stereospecificity, which arises from its high preference for a chair transition state.   In their exploration of this rearrangement's stereospecificity, Overman and coworkers used logic similar to the classic Doering and Roth experiments,
[8] which showed that the Cope rearrangement prefers a chair conformation.[9]  By using the cationic 2-aza-Cope/Mannich reaction on pyrrolizidine precursors, they showed that pyrrolizidines with cis substituents from E-alkenes and trans substituents from Z-alkenes are heavily favored, results that are indicative of a chair transition state.  If a boat transition state was operative, the opposite results would have been obtained (detailed in image below).[9]  As is the trend with many reactions, conversion of the Z-enolate affords lower selectivity due to 1,3 diaxial steric interactions between the enolate and the ring, as well as the fact that substituents prefer quasi-equatorial positioning.  This helps explain the higher temperatures required for Z-enolate conversion.[6][9]
The boat transition state is even less favored by the cationic-2-aza-Cope rearrangement than it is for the Cope rearrangement: in analogous situations to where the Cope rearrangement takes on a boat transition state, the aza-Cope rearrangement continues in the chair geometry.[1][6][10]  These results are in accord with computational chemistry results, which further assert that the transition state is under kinetic control.[11]

Significantly, these stereochemical experiments imply that the cationic 2-aza-Cope rearrangement (as well as Mannich cyclization) occur faster than enol or iminium tautomerization.  If they were not, no meaningful stereochemistry would have been observed, highlighting the facility of this fast reaction.[1]
Additional considerations for stereochemistry
The aza-Cope/Mannich reaction, when participating in ring-expanding annulations, follows the stereochemistry dictated by the most favorable chair conformation, which generally places bulky substituents quasi-equatorially.  The vinyl and amine components can have either syn or anti relationships when installed on a ring.  This relationship is typically dictated by the amine substituent: bulky substituents lead to syn aza-Cope precursors.  While anti vinyl and amine substituents generally only have one favored transition state, leading to a cis fused ring system, the favored product of syn substituents can change, dictated by steric interactions with solvents or large N-substituents, which may take preference over bulky substituents and change the transition state.[12][13]

For simple aza-Cope/Mannich reactions that do not participate in ring-expanding annulation, namely condensations of amino alcohols and ethers, bond rotation occurs more quickly than the Mannich cyclization, and racemic products are observed.[14]  This can be avoided by using a chiral auxiliary substituent on the amine.  Reactions tethered to rings cannot undergo these bond rotations.[1]

Possible thermodynamic sinks for biasing a rearrangement product
Horowitz and Geissman's first example demonstrates a possible thermodynamic sink to couple with the cationic 2-aza-Cope rearrangement, where the product is biased by the phenyl substituent through aryl conjugation, then captured by hydrolysis of the iminium.  Other methods of biasing a product include using substituents which are more stable on substituted carbons, releasing ring strain (for instance, by pairing the rearrangement with cyclopropane opening), intramolecular trapping (pictured), and pairing the rearrangement with the Mannich cyclization.[1][15]

The aza-Cope/Mannich Reaction

The aza-Cope/Mannich reaction is a synthetically powerful reaction, as it is able to create complex cyclic molecules from simple starting materials.  This tandem reaction provides a thermodynamic bias towards one rearrangement product, as the Mannich cyclization is irreversible and its product, an acyl substituted pyrrolidine ring, more stable than that of the rearrangement.[1][16]
The first aza-Cope/Mannich reaction
Overman and coworkers recognized that the cationic 2-aza-Cope rearrangement could potentially be synthetically powerful if an appropriate thermodynamic sink could be introduced.  Their logic was to incorporate a nucleophilic substituent into the starting material, namely an alcohol group, which acts only after rearrangement, converted into an enol primed to attack the iminium ion.
This first report of the reaction was a reaction between aldehydes and 2-alkoxy-3-butenamines, which formed an amino alcohol whose aza-Cope/Mannich reaction product was an acyl-substituted pyrrolidine ring.  This simple procedure only involved mild heating for several hours.  Significantly, the aza-Cope/Mannich reaction occurs in a single step with excellent yield.  This procedure is easily applied to condensation of amino ethers (shown below), where the alcohol has been methylated first.[16]  After the aza-Cope/Mannich reaction is carried out, the ketone is formed by addition of NaOH.[16] The amine, in this simple case, cannot form the iminium ion from basic ketones; subsequent methods found ways of incorporating ketones into the reaction.[16][17] The utility of this reaction is evident in the fact that even when a less stable isomer is formed, the reaction proceeds, demonstrating its high thermodynamic favorability.[12][17]

Reaction mechanism

The general product of the reaction can potentially occur via two separate pathways:  the aza-Cope/Mannich reaction, or an aza-Prins cyclization/pinacol rearrangement.
These mechanisms have different stereochemical properties, which elucidate the dominance of the aza-Cope/Mannich reaction.  The aza-Cope/Mannich reaction forces each atom in the [1,5] diene analog to undergo sp2 hybridization, erasing the starting material's stereochemistry at the labelled R' position, while the aza-Prins/pinacol rearrangement retains stereochemistry at the labelled R' position, pointing to a simple test that reveals the active mechanism.  An enantiomerically pure starting material at the "R'" position should lead to a racemic product if the dominant mechanism is the aza-Cope/Mannich reaction, while the stereochemistry should be retained if the dominant mechanism is an aza-Prins cyclization/pinacol rearrangement pathway.  A simple experiment verified that the product was racemic, providing clear evidence of the aza-Cope Mannich reaction as the operative mechanism.  Further experiments verified this, using the knowledge that the carbenium ion formed in an aza-Prins/pinacol pathway would be effected by its substituent's ability to stabilize its positive charge, thus changing the reactivity of the pathway.  However, a variety of substituents were shown to have little effect on the outcome of the reaction, again pointing to the aza-Cope Mannich reaction as the operative mechanism.[14]
Recent literature from the Shanahan lab supports the rare aza-Prins/pinacol pathway only associated with significantly increased alkene nucleophilicity and iminium electrophilicity.[1][6]
[18][19]
The aza-Cope/Mannich reaction shows high diastereoselectivity, generally in accordance the results of the stereochemical experiments elucidating the transition state of the cationic 2-aza-Cope rearrangement, which follows as this tandem reaction pathway was an integral part of these experiments.  The stereochemistry of the rearrangement is slightly more complicated when the allyl and amine substituents are installed on a ring, and thus cis or trans to one another.
The aza-Cope/Mannich reaction starting material, the amino alcohol, can also be thought of as related to the oxy-Cope rearrangement(below), both for its rate acceleration due to ionic involvement, as well as the analogous enol collapsing function of the Mannich cyclization and the keto-enol tautomerization in the oxy-Cope rearrangement.[7]

Synthetic applications of the 2-aza-Cope/Mannich reaction
The aza-Cope/Mannich reaction is often the most efficient way to synthesize pyrrolidine rings, and thus has a number of applications in natural product total syntheses.  Because of its diastereoselectivity this reaction has added to the catalog of asymmetric synthesis tools, as seen in the many examples of asymmetric alkaloids synthesized using the reaction.  As we have seen in the first aza-Cope/Mannich reaction and in the elucidation of the reaction's stereochemistry, the aza-Cope/Mannich reaction can be used to form pyrrolidine rings and pyrrolizidine rings.  It can be used to create many additional ring structures useful in synthesis, such as indolizidine cycles and indole rings.[1][7]
(−)-Strychnine total synthesis

The classic example demonstrating the utility of this reaction is the Overman synthesis of strychnine.  Strychnine is a naturally occurring highly poisonous alkaloid, found in the Strychnos genus of trees and climbing shrubs.  Strychnine is commonly used as a small vertebrate pesticide.
The first strychnine total synthesis, by R. B. Woodward,[20] represented a major step in natural product synthesis: no molecule approaching its complexity had been synthesized before.  The next total syntheses were not reported until the late 80s, using similar methods, namely by using an intermediate available by degradated strychnine.  All of these syntheses used harsh conditions.  The Overman synthesis bysteps these problems, and is the first asymmetric total synthesis of strychnine, taking advantage of the diastereoselectivity and mild reaction conditions of the aza-Cope/Mannich reaction.  The aza-Cope/Mannich reaction step proceeded in near quantitative yield.  The Overman synthesis is accordingly several orders of magnitude more efficient than its predecessors.[20]

Overman's synthesis of strychnine represents a useful example of the preparation of precursors necessary for the aza-Cope/Mannich rearrangement, representing effective usage of an epoxide ring opening.  The key steps of the synthesis of the rearrangement substrate leading to the starting materials necessary for the aza-Cope/Mannich reaction included a Stille reaction to piece together two precursors, an epoxidation of a double bond using tert-Butyl hydroperoxide, a Wittig reaction to convert the ketone to an alkene, and a cyclization step.  Amine alkylation (not shown), transforms the molecule to the rearrangement substrate.  Significantly, this molecule shows the enantiomeric precision of the aza-Cope/Mannich reaction, as a simple enantiomeric starting material dictates the final enantiomer: the enantiomer of strychnine was produced by using the enantiomer of the starting material.[20][21]

The Overman synthesis, with in-depth details of the synthesis of the rearrangement substrate, as well as the final steps of the reaction, is detailed here: Overman synthesis of (−)-Strychnine.
Synthesis of (−)-crinine
Crinine is an alkaloid of the Amaryllidaceae family, and its asymmetric total synthesis was one of the first using the aza-Cope/Mannich reaction.  This synthesis represents a significant step in the development of the aza-Cope/Mannich reaction, as it takes advantage of several of the most useful synthetic strategies characteristic of the reaction.  This reaction takes advantage of the cationic-2-aza-Cope rearrangement's high diastereoselectivity, as well as usage of the cyanomethyl group to protect the amine during vinyllithium addition and as a leaving group to promote iminium formation, assisted by addition of silver nitrate.[22]
This synthesis is one example of many of the cyanomethyl group providing a synthetically useful route towards pyrrolidine and indolizidine formation.

Synthesis of bridged tricyclic alkaloids
Overman and coworkers developed methods to synthesize complicated bridged tricyclic structures using the aza-Cope/Mannich reaction.  These aza-tricyclic structures are found in the complex Stemona alkaloid family, as well as in potential drugs such as some immunosuppressants.  The example shown is a facile reaction combining a 1-aza-bicyclo[2.2.1]heptane salt starting material with paraformaldehyde at 80°C to form the pivotal aza-tricyclic structure of the Stemona alkaloid molecules.  Saliently, despite unfavorable orbital overlap due to the sterics of this ring system, the reaction proceeds with 94% yield, highlighting the power of this reaction even under unfavorable conditions.[23]

General ring opening and expansion

The aza-Cope/Mannich reaction, when coupled with existing ring cycles, is often used to create indolizidine cycles (a pyrrolidine connected to a cyclohexane ring).  This typical ring annulation, where the cyclopentane moiety is opened with the rearrangement and closed with the Mannich cyclization to form a six membered ring attached to a pyrrolidine ring, while the most popular aza-Cope/Mannich annulation, is not the only one.  Seven-membered ring cycles are also possible to synthesize, as the enol and iminium ions stay in close enough proximity to undergo Mannich cyclization.[22]  Macrocycle synthesis has not been reported using this reaction, due to the lack of proximity between the enol and iminium.[6]
Vinyl oxazolidines can also be used as rearrangement substrates.  This rearrangement first creates the vinyl oxazolidine from attack on the cyclohexanone by the aminobutenol, which then undergoes the aza-Cope/Mannich reaction using heat and acid (Lewis or protic).  This example breaks and then forms a five-membered ring.  More complex examples attach the oxazolidine to another ring, presenting additional methods for the formation of indolizidine cycles.[24]
Scope of the aza-Cope/Mannich reaction
The aza-Cope/Mannich reaction has numerous advantages in comparison to other methods.  The gentle conditions of the reaction aren't matched: light heating, normally no higher than 80°C, a wide range of solvents, and addition of 1 stoichiometric equivalent of acid, commonly Camphorsulfonic acid (CSA) or a Lewis acid.  Other routes toward pyrrolidine synthesis cannot compete with the stereospecificity, widescale applications in structures containing pyrrolidine derivatives, and large scope of possible starting materials. The reaction exhibits high diastereoselectivity, and is robust, proceeding even when faced with poor orbital overlap in the transition state.[1]

The advantages of the aza-Cope/Mannich reaction have motivated research on the synthesis of the starting materials for the reaction, which split into two main categories: amine addition and iminium formation (red) and installation of the vinyl substituent (blue).  A wide variety of N-substituents (R), alkyl and aryl, can be used in the reaction, some of which affect the stereochemical outcome of the reaction.  Vinyl groups are generally limited to those which are either 1,1 or 1,2-Disubstituted (vinyl with substituents at R1, and R1,R2 respectively), with a wide range of electronic and steric variety tolerated.[1]
Amine addition and iminium formation
Epoxide ring opening
The ring strain of epoxides provide useful methodology for installation of an amine group two atoms away from an alcohol group.  The epoxide may be first broken by bromide nucleophilic attack.  Primary amines, aromatic amines, or lithium anilides can also be used as nucleophiles.  Protective O-methylation often follows this step and proceeds easily.

When sterics allow for attack on only the appropriate carbon (the terminal carbon as opposed to the second carbon), direct attack by an intramolecular nitrogen is effective, as is the case with strychnine synthesis.[16][25]
Iminium ion formation
The most common way to generate the iminium ion from the installed amine is by adding formaldehyde or paraformaldehyde, which undergoes acid-catalyzed condensation to form the iminium. Overman's strychnine synthesis typifies this method.[6][25]  Occasionally, intramolecular carbonyls are used.[9]  Other methods for iminium ion formation include using cyanomethyl groups or using oxazolidines as carbonyl precursors.

Amine alkylation
Amine alkylation represents a common method to get to imine precursors.  Amine alkylation by direct SN2 reaction is only occasionally useful in producing starting materials due to the high propensity of amines to overalkylate.[25]
Reductive amination is a more common and effective alkylating procedure, typified in the first aza-Cope rearrangement.[16][26][27]
The most useful and standard method of amine alkylation is to have the amine form an amide bond, and subsequently reduce it, often with LiAlH4.[9]
Oxazolidine use
Ketones and sterically hindered aldehydes are not suitable for the basic aza-Cope/Mannich reaction, as the amine cannot form an iminium ion with them. Dehydrative oxazoline formation followed by heating in the presence of a full equivalent of acid present a way to get around this issue.
Overman has reported the use of oxizolidines to generate the iminium ion requisite for the reaction.  Upon formation, Overman showed that cyclohexanones can be used for the carbonyl component in pyrrolidine synthesis.[17]  This reaction proceeded with various forms of cyclohexanones.  When an acyclic ketone was substituted, the reaction proceeded with low yield, highlighting the thermodynamic favorability of releasing cyclohexanone from the double bonded carbonyl, as it creates unfavorable bond strain in the chair conformation.  This represents one of the most convenient constructions of the 1-azaspiro[4,5]decane ring system, a useful natural product.[17]

Installation of the vinyl substituent
Vinylation of ketones
Vinylation can offer additional synthetic advantages, allowing for expanded functionality of the reaction.[23] Organolithium reagents are typically used.  Often, a substituent or protecting group will be added to the nitrogen, although this isn't always necessary.  The addition of lithium to the reaction has a major effect on starting material stereochemistry, as the nitrogen coordinates to it.  Starting materials affected by this coordination generally result in anti aza-Cope precursors, while those that aren't, such as those containing highly substituted, sterically hindered amines, result in syn precursors.  Thus the nature of the nitrogen substituent is of high importance.[6][25]

Cyanomethyl group use
Cyanomethyl groups represent an easy way to protect an iminium ion during allylic vinylation of the ketone.  Cyanamide groups and analogs have been often used in the generation iminium ions.  They are typically installed by nucleophilic addition onto an iminium ion, generally produced by amine alkylation with formaldehyde.  The iminium ion is thus masked.[28]  It follows that usage of a cyanomethyl group provides an efficient way to control the aza-Cope/Mannich reaction.  The cyanomethyl group protects the nitrogen at the 2-position during formation of the other allylic analog by logic similar to cyanide-type umpolung.  It then later provides a good leaving group for formation of the iminium ion, in accordance with its usage in iminium ion generation.[29]  Iminium ion generation from cyanomethyl groups is normally promoted by addition of silver nitrate, although other silver and copper compounds have been used.  This added step allows for more precise control of iminium ion generation.[6][29]  Importantly, these preparatory reactions must be carried out at -78°C to prevent cyanomethyl/vinyllithium interaction.  This method also allows for many different possible N-substituents, and can be used to simplify the synthesis of octahydroindoles and pyrroles.[1][29]

The 1- and 3-aza-Cope rearrangements

The 1- and 3-aza-Cope rearrangements are obscure in comparison to the cationic 2-aza-Cope rearrangement due to their activation energies, which are comparatively much higher than that of the cationic 2-aza-Cope rearrangement.
The 1- and 3-aza-Cope have a bias towards imine formation as opposed to enamine formation, as carbon-nitrogen π-bonding is stronger than carbon-nitrogen σ-bonding, meaning the 3-aza-Cope rearrangement is thermodynamically favored, while the 1-aza-Cope rearrangement is not: the imine is nearly 10kcal/mol less in energy.  Thus the 3-aza Cope's large activation barriers are kinetically based.[30]  Research on both the 1 and 3-aza-Cope rearrangements has focused on finding good driving forces to lowering the activation barriers.   Several versions of these rearrangements have been optimized for synthetic utility.  The 1-aza-Cope rearrangement is normally paired with thermodynamic driving forces.  The 3-aza-Cope rearrangements are generally performed cationically to lower the kinetic barrier to its thermodynamically favorable product.[30][31]
These rearrangements follow much of the mechanistic logic of the cationic 2-Aza-Cope rearrangement.  The 1- and 3-aza-Cope rearrangements both occur preferentially via chair transition states (and retain stereochemistry, similarly to the cationic 2-aza-Cope rearrangement), and are sped up with the introduction of a positive charge, as this gives the transition state more diradical/dipolar character.[31]  The 3-aza-Cope rearrangement (and thus also the 1-aza-Cope rearrangement, which goes through the same transition state)  is expected to show even less aromatic character in its transition state in comparison to the Cope rearrangement and cationic-2-aza-Cope rearrangement, contributing to the higher temperatures required (close to the temperatures required for the Cope Rearrangement, at times even higher, from 170 to 300 degrees) to overcome the kinetic activation barriers for these arrangements.[3][31][32]
The 3-aza-Cope rearrangement

The 3-aza-Cope reaction was discovered soon after the 2-aza-Cope rearrangement was identified, due to its analogous relationship to the Claisen rearrangement.  Indeed, in early papers, this version of the aza-Cope rearrangement is often referred to as the amino-Claisen rearrangement, a misrepresentation of the rearrangement, as this would imply that both a nitrogen and oxygen are in the molecule.[3]  This rearrangement can be used to form heterocyclic rings involving carbon, most commonly piperidine.
One of the first examples of this arrangement was identified by Burpitt, who recognized the rearrangement occurring in ammonium salts, which, due to their charged nature, proceeded exothermically without addition of heat—importantly, without a tetrasubstituted nitrogen, the rearrangement did not proceed.[33]  Following this logic, much of the research on the 3-aza-Cope rearrangement has focused on charged zwitterionic versions of this reaction, as the charge distribution helps lower the activation barrier: in certain cases, the rearrangement can occur at temperatures as low as -20°C.[30][34]

HIll and Gilman first reported a general uncharged 3-aza-Cope rearrangement in 1967.  Upon creation of appropriately substituted enamines, intense heating afforded an almost complete rearrangement to the imine product.  However, this rearrangement pathway has limited utility.[35]
The 1-aza-Cope rearrangement

The first discovered 1-aza-Cope reaction was a simple analog to the generic Cope reaction and required intense heat to overcome its large thermodynamic activation barrier; most subsequent work on the 1-aza-Cope rearrangement has thus focused on pairing the arrangement with a driving thermodynamic force to avoid these harsh reaction conditions.  It has been hypothesized that the 1-aza-Cope rearrangement rate-determining transition state has partial diradical and dipolar transition state character due to the presence of the heteroatom.[4]
Fowler and coworkers have come up with a scheme that mobilizes the 1-aza-Cope rearrangement as a synthetically useful route.[3]  Fowler and coworkers recognized that because the barrier for the reaction lies in the nitrogen’s thermodynamic preference to stay as an imine, stabilizing the nitrogen could have a thermodynamically beneficial effect.  To that end, Fowler and coworkers installed a carbonyl group on the nitrogen, hypothesizing that the lone pair of the nitrogen would be stabilized by participation in an amide bond, and that the electronegativity of this amide group would lower the LUMO of the imine group, making the transition state more favorable.[3]  Using this strategy, Fowler and coworkers were able to use the 1-aza-Cope rearrangement to create piperidine and pyridine derivatives.  This strategy was shown to be relatively robust, allowing for the formation of products even when forced through a boat transition state, when perturbed with substituent effects, or put in competition with alternative rearrangements.[3]  Also significant is the relative ease of production of the reactants, which uses a Diels-Alder reaction paired with relatively simple workup steps, allowing for syntheses using complex cycling.[3]

Other methods of overcoming this thermodynamic barrier include pairing it with cyclopropane ring strain release, which allows the reaction to proceed at much lower temperatures.[30][36]

See also
Reviews by Overman[1][37] and Siegfried Blechert.[38]

When were aza-Cope rearrangements first identified?


Calypso Deep, located in the Ionian Sea south-west of Pylos, Greece, is the deepest part of the Mediterranean Sea, with a maximum depth of 5,267m (17,280ft), at Coordinates: .[1][2]
See also
Extremes on Earth
NESTOR Project

What is the deepest point in the Mediterranean Sea?