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Title:
Chlorine-Atom-Controlled Terminal-Epoxide-Initiated Bicyclization Cascade Enables a Synthesis of the Potent Cytotoxins Haterumaimides J and K

Paper sections:

<p>Our laboratory has been investigating the chemistry and biology of the potently cytotoxic labdane diterpenoids in the lissoclimide family.1–3 Nearly two dozen such natural products were isolated from sea squirts as described by the groups of Malochet-Grivois/Roussakis,4 Ueda/Uemura,5 and Schmitz.6 Among these compounds, many of which were named haterumaimides, several showed potent cytotoxicity against the P388 murine leukemia cell line (Figure 1); dichlorolissoclimide (1) and chlorolissoclimide (2) were also very active against KB cells and non-small-cell lung cancer,4b,c and were later shown by Pelletier and co-workers to be inhibitors of eukaryotic translation.7 With the Alexanian group, we completed semisyntheses of haterumaimide Q (3) and chlorolissoclimide from sclareolide, featuring in the latter case a selective radical C–H chlorination reaction to install the salient C2-halogen atom.1,8 In a separate report focused more on the biological properties of these compounds, we disclosed a more general, π-cyclization-based approach, SAR data among several natural and unnatural congeners, and an X-ray cocrystal structure of chlorolissoclimide bound to the eukaryotic 80S ribosome.2 A serious limitation of our previous efforts became clear: we were never able to access haterumaimides J and K (hatJ and hatK; 4 and 5, respectively), the two compounds reported to be the most cytotoxic in the family. Notably, these targets bear oxygenation at C18, but C3 is unfunctionalized. This arrangement of structural features motivated the distinct synthesis design described herein, which is based on rarely studied terminal-epoxide-initiated polycyclizations.</p><p>In spite of the significant number of trans-decalin-type diterpenoid alcohols and acids that are C3-unfunctionalized but are oxygenated at C18 or C19 (terpenoid numbering, see Figure 2a), the previous use of simple terminal epoxides as activators to induce polycyclizations is essentially limited to the single report of Goldsmith and Phillips from a half century ago,9 with important later work on more functionalized systems by the groups of van Tamelen10 and Corey.11 In the seminal study,8 an epoxide of type 6 (Figure 2b), wherein the terminator was a m-methoxyphenyl ring, was shown to generate as the major product a compound of type 8a (equatorial, C18 oxygenation) in low yield; other mono-cyclized and oxabicyclized products of unconfirmed relative C4–C5 configuration were also observed (the trans-C5–C10 ring junction of course remains constant). As a result, we were uncertain about the relative preference for the stereochemical outcome of bicyclizations of type 6; however, with the required C2-chlorine substituent in place (see 7), we postulated that a preference for its equatorial disposition in transition structures would lead selectively to the arrangement 9a needed for a synthesis of hatJ. We hypothesized that the relatively small A-value of a chlorine atom (ca. 0.5 kcal/mol) would still be enough for effective diastereocontrol because of the exacerbation of nonbonded interactions resulting from the two other putative axial groups at C4 and C10 in the transition structure leading to the undesired product 9b. Moreover, the diastereomer of 7 (chlorine and epoxide arranged syn) should permit the selective formation of compounds with C19 oxygenation (not shown, see below).</p><p>We therefore embarked on a synthesis of cyclization precursors related to 7. The control of relative configuration between the epoxide and the chlorine-bearing stereogenic center was paramount to the effectiveness of this approach. We focused on furan as our choice of nucleophilic terminating group, because oxidative ring opening would afford the requisite functionality to complete the remainder of the synthesis. The utility of this particular approach had previously been established in seminal studies by Tanis and co-workers.12</p><p>Enantiopure alcohol 10
(Scheme 1) was made from epichlorohydrin, isopropenylmagnesium bromide, and lithiated trimethylsilylacetylene, as previously reported for its enantiomer by Danishefsky and co-workers.13 Zirconocene-dichloride-catalyzed methylalumination of the alkyne,14 followed by iododealumination, generated nearly symmetrical diene 11. Deoxychlorination proved challenging in the face of facile competing elimination processes that generated conjugated dienes. Kartika's conditions were uniquely effective, and provided chloride 12 without minimal loss of enantiopurity.15</p><p>1,1-Disubstituted alkenes, especially those bearing two unbranched alkyl groups, are notoriously poor substrates for asymmetric oxidation, owing to the very similar enantiotopic (or diastereotopic in the case of 12) π-faces presented to the catalysts in question.16 For the case at hand, we also needed to address the issue of chemoselectivity with respect to the alkenyl iodide,17 which competed with the nonhalogenated alkene in some preliminary epoxidation experiments. Fortunately, dihydroxylation under Sharpless AD conditions proved to be chemoselective. Figure 3 shows the outcome of representative experiments aimed at the selective generation of anti product 14 using enantiomerically enriched 12. A ca. 1:2 anti/syn mixture was obtained in the absence of ligand, and the typical AD-mix ligands showed little selectivity. However, application of the less frequently adopted pyrimidine-based ligands (DHQ)2PYR and (DHQD)2PYR16,18 led to enhanced selectivity, such that anti-product 14 could be obtained as the major product of a ca. 6:1 mixture.</p><p>The formation of the epoxide 15 from 14 was uncomplicated by the chloride substituent. This building block was joined to furan-containing alkyl iodide 16 in an efficient net reductive B-alkyl Suzuki coupling proceeding through the presumed intermediacy of the methoxy-9-BBN ate complex.19 After some optimization of Lewis acid and solvent combinations, the key epoxide-initiated, furan-terminated bicyclization of 17 was realized with ethylaluminum dichloride, affording 18 in 45–65% yield and with virtually complete diastereoselectivity (>20:1), which is consistent with the chlorine atom's ability to direct the stereochemical outcome of the reaction (see below).20,21 The inclusion of 2,6-di-t-butylpyridine (DTBP) was critical for reaction reproducibility. Silylation of the neopentylic alcohol and oxidative ring opening of the furan with in situ-carboxylate methylation22 yielded ketoenoate 20.</p><p>Diastereoselective hydrogenation of the alkene, a salt-free Wittig methylenation, and careful partial reduction of the ester provided aldehyde 21. This electrophile was subjected to our previously developed aldol-based introduction of the hydroxysuccinimide motif,2,23 which was complicated by the competitive formation of variable and often substantial amounts of the "non-Evans syn" diastereomer in some cases. A simple change of conditions for boron enolate formation (replacement of n-Bu2BOTf with Cy2BOTf) alleviated the uncertainty of this previously capricious reaction, leading to a highly diastereoselective imide introduction. Desilylation of the neopentylic silyl ether afforded hatJ (4). Acetylation provided hatK (5).</p><p>Notably, the same epoxide-initiated, furan-terminated bicyclization reaction of the syn-diastereomer of the chloroepoxide (24,24
Figure 4) led to the selective formation of diastereomer 25, with C19 oxygenation (axial hydroxymethyl group). Clearly, the chlorine atom can play a powerful role as a single atom auxiliary21 for cationic polycyclizations, in this case overturning the intrinsic selectivity for C18 oxygenation observed in the deschloro analogue (not shown).20 With the easy incorporation and easy reductive removal of the chlorine atom auxiliary, this reaction type could be widely applied to terpenoids bearing C19 oxygenation including, among others, members of the ent-kaurene family.25</p><p>Because our assays of synthetic chlorolissoclimide against the P388 cell line had demonstrated a 15-fold lower cytotoxicity than originally reported by the Malochet-Grivois group (see Figure 1), we wished to see if the reported high potency of hats J and K against P388 could be recapitulated with our synthetic samples and to evaluate their potency against more important human cancer cell lines. These compounds were tested against P388, HUT78 (cutaneous T-cell lymphoma), A2058 (aggressive melanoma), and DU145 (aggressive prostate cancer) cell lines (Table 1), with our previously reported data for chlorolissoclimide shown for comparison. While we again observed much lower activity against P388 than previously reported, hats J and K are the most potent compounds that we have yet made—natural or unnatural—in the lissoclimide series at ca. 30 nM each. For this reason, we tested them against the human hematological cancer HUT78, and again found nanomolar activities. Consistent with our previous work, 4 and 5 were less active against the solid tumor cell lines A2058 and DU145.</p><p>Our synthesis of haterumaimides J and K showcases the power of underutilized terminal-epoxide-initiated polycyclizations for terpenoid synthesis. Moreover, this work reveals the utility of chlorine (and potentially other halogen) atoms as single-atom auxiliaries to control the stereochemical outcome of such polycyclizations.26 The chlorine atom's ability to override intrinsic selectivities for C18 oxygenation might be widely applied toward polycyclic terpenoids of the ent-kaurane class. Our synthesis of haterumaimide J was complete in 14 steps from known secondary alcohol 10, and 18 steps from commercial precursors. Further applications of both terminal epoxides and of halogen atom auxiliaries to complex terpenoid synthesis are ongoing in our laboratory.</p>
Haterumaimide J (hatJ) is reportedly the most cytotoxic member of the lissoclimide family of labdane diterpenoids. The unusual functional group arrangement of hatJ\xe2\x80\x94C18 oxygenation and C2 chlorination\xe2\x80\x94resisted our efforts at synthesis until we adopted an approach based on rarely studied terminal epoxide-based cation-\xcf\x80 bicyclizations that is described herein. Using the C2-chlorine atom as a key stereocontrol element and a furan as a nucleophilic terminator, the key structural features of hatJ were rapidly constructed. The 18-step stereoselective synthesis features applications of chiral pool starting materials, and catalyst-, substrate-, and auxiliary-based stereocontrol. Access to hatJ and its acetylated congener hatK permitted their biological evaluation against aggressive human cancer cell lines.