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Paper text:
macrolide antibiotics have been the focus of widespread research due to increasing bacterial resistance [ 1 , 2 ] .
there have been significant synthetic and theoretical efforts to generate new core structures to address this challenge .  
structure elucidation of a large number of obtained molecules , shows the existence of two parts .
the first one is a macrocyclic system from 12 to 40 links with several asymmetric centers and lactone function ; the second is a sugar part .
the two main classes of these macrolides are presented by two molecules ; the first is erythromycin a which is an active antibiotic against a large number of bacteria , and the second is amphotericin b which presents a strong anti - fungal .  

still and galynker   have shown that conformational properties of middle and large size ( 8 to 14 atoms ) might induce a diastereoselection phenomenon for the reactions carried out on these compounds .
more precisely , macrocycles which have a double bond ( c = c , c = o ) and correctly situated substitutes adopt privileged conformations
.   peripheral attack of the reactive by the less hindered face of -system conduct to a higher stereoselective formation of a new asymmetric center .  
gre et al .   have shown also in some cases the possibility of a stereochemical control induced by tricarbonyliron .
so , our objective is to verify if this notion can be extended for cycles with large size .   in this paper
, we propose to study the 18-membered ,-unsaturated macrocycle in order to determine the most favored conformations and the influence of fe(co)3 on conformational flexibility of these macrocycles .
in our study , the main method of calculation , which we have used , is molecular mechanics .
this method for structure determination includes a quantum mechanical ( vescf ) -system calculation in the iterative sequence .
we also used the molecular dynamics ( hyperchem ) for the conformational research , with following options : 1000k , in vacuo , step size : 0.001  ps , and relaxation time : 0.1  ps .  
these calculations were carried out with two software packages : hyperchem ( 8.01 ) , for geometry optimization , and conformational search and chem3d ( 8.0 ) , for structural representation .  
then , our objective is to search the favored conformations , on the basis of energy and geometric considerations with statistical calculations using boltzmann distribution . in this part of our work , we have undertaken a conformational study of macrocycle 18 ( figure 1 ) , symmetrical which we will design 18s ( n1 = n2 = 5 ) , dissymmetrical which we will design 18d ( n1 = 4 , n2 = 6 ) , which represent the core group for many antibiotics .   
we will also try to evaluate the stereoselectivity of addition reactions carried out on functional groups appended to the tricarbonyliron moiety .
the most stable structures can be characterized by three structural characters : the diene group , the ,-unsaturated ester group , and the two saturated chains .
thus , we have obtained eight types of conformations which are present in the majority of cases in a 6  kcal / mol energy range above the global minimum .
the conformation types are classed from 1 to 8 [ 1315 ] .   for types ( 2 , 4 , 6 , 8) ,
the two planes of two conformational sites diene and ,-unsaturated ester group were pseudoparallels ; but for types ( 1 , 3 , 5 , 7 ) , the two planes of the two sites are pseudoantiparallelsure ( figure 2 ) .
we remark also that for two conformations which we distinguish by the arrangement between the two sites , the dipole moment values are higher for pseudoparallel arrangement and lower in the opposite case ( for the macrocycle 18s (t2 ) = 2.17  d and (t1 ) = 1.97  d ) . in 1 
kcal / mol difference , the macrocycle 18d is characterized by the first conformer type 6 , which is the most favored with 20.1% rate followed by a type 4 with 18.3% .
then , the macrocycle 18s is presented preferably in the type t5 ( 17.2% ) and type t3 ( 15.0% ) .  
the conformer populations of macrocycle 18d are lightly greater than these of macrocycle 18s . for the most favored conformer geometry , the ,-unsaturated ester group has s - cis conformation with an angle 1 : o19-c2-c3-c4 = 14.5 for macrocycle 18d and 1 : o19-c2-c3-c4 = 25.0 for cycle 18s .  
the diene group has s - trans conformation with a torsion angle 2 : c11-c12-c13-c14 = 169.4 for 18d and 2 : c10-c11-c12-c13 = 179.5 for 18s .
these macrocycles have a very high conformational flexibility .   however , mobility of dissymmetric macrocycles is lightly less important than that of symmetric macrocycles .  
they present many privileged conformations that do not a priori foresee a diastereoselection for envisaged reactions .
this is in agreement with still 's works , on macrocycle 17 , which yields many different conformations .
we have studied also the exerted effect by tricarbonyliron on conformational flexibility of these macrocycles .
we note that organometallic complex can intervene by a very high steric hindrance and also introducing an important rigidification of skeleton .
the results of conformational analysis of two complexed macrocycles 18 show that tricarbonyliron has a considerable influence on cycles , because the number of possible conformations was reduced to four types [ 1719 ] .   in 1  kcal / mol energetic difference ,
the complexed macrocycle 18s shows three favored conformations and only one favored conformation for 18d .  
the peopling rate of the most stable conformers was increased for complexed macrocycles compared with these without tricarbonyliron ( table 2 ) .   for macrocycle 18d , which was presented by a preferred conformer with 20.1% without complex ,
we remark also that macrocycles 18s and 18d were presented , respectively , in t1 type with 28.5% and t8 type with 50.1% for the most favored conformers .  
the dihedral angle value of dienic system was comprised between 1.0 and 5.4 for 18s cycle and between 4.1 and 13.9 for 18d cycle .
the lower deviations of registered dihedral angles compared with normal values were imposed essentially by a cyclic chain .  
the presence of tricarbonyliron motif imposes a minimum steric modifications and introduces an asymmetric element .
so , this creates a favorable environment to discrimination between two faces of macrocycles increasing thus a peripheral attack proportion ( figure 4 ) .  
this reasoning was found again in methyl acetates with fluorine containing auxiliaries where intramolecular interaction li - f creates a steric hindrance around one of two faces causing a diastereofacial selectivity of 94.6  :  5.4 . the lactone function and
this is in agreement with still 's works that affirm that ch3i addition will be executed on the clear face by peripheral attack .
the study carried out by cox and ley , on fe(co)3 complexes has shown that the presence of complex , which has an important steric effect , induced a diastereoselectivity in addition reactions ( figure 3 ) .
we conclude that our study shows the existence of a very high conformational flexibility increasing in a majority of noncomplexed macrolactones .  
finally , the obtained diastereoselectivity for complexed macrolactones is the result of the stereochemical control effect of the tricarbonyliron .
it has introduced an asymmetric element , an important steric effect increasing a peripheral attack proportion , and has contributed to the rigidity of the structure .
this last factor constitutes a tool of the stereochemical remote control , which permits us to foresee a priori the phenomenon of the stereoselectivity for alkylation reactions .
conformational analysis of 18-ring membered macrolactones has been carried out using molecular mechanics calculations and molecular dynamics . 
 a high conformational flexibility of macrolactones was obtained , and an important stereoselectivity was observed for the complexed macrolides . for 18d macrolactone , which was presented by a most favored conformer with 20.1% without complex , 
 it was populated with 50.1% in presence of fe(co)3 .