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Keck, G.E. et al.

Gary E. Keck, Robert L. Giles, Victor J. Cee, Carrie A. Wager, Tao Yu, Matthew B. Kraft,

Total synthesis of epothilones B and D: stannane equivalents for beta-keto ester dianions.

Studies leading to a total synthesis of epothilones B and D are described. The overall synthetic plan was based on late-stage fragment assembly of two segments representing C(1)-C(9) and C(10)-C(21) of the structure. The C(1)-C(9) fragment was prepared by elaboration of commercially available (2R)-3-hydroxy-2-methylpropanoate at both ends of the three-carbon unit. Introduction of carbons 1-4 containing the gem-dimethyl unit was achieved in a convergent manner using a diastereoselective addition of a stannane equivalent of a beta-keto ester dianion. An enantioselective addition of such a stannane equivalent for a beta-keto ester dianion was also used to fashion one version of the C(10)-C(21) subunit; however, the fragment assembly (using bimolecular esterification followed by ring-closing metathesis) with this subunit failed. Therefore, fragment assembly was achieved using a Wittig reaction; this was followed by macrolactonization to close the macrocycle. The C(10)-C(21) subunit needed for this approach was prepared in an efficient manner using the Corey-Kim reaction as a key element. Other key reactions in the synthesis include a stereoselective SmI(2) reduction of a beta-hydroxy ketone and a critical opening of a valerolactone with aniline which required extensive investigation.[1]

References

Total synthesis of epothilones B and D: stannane equivalents for beta-keto ester dianions. Keck, G.E., Giles, R.L., Cee, V.J., Wager, C.A., Yu, T., Kraft, M.B. J. Org. Chem. (2008) [Pubmed]

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