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Ester dienolate [2,3]-Wittig rearrangement in natural product synthesis: diastereoselective total synthesis of the triester of viridiofungin A, A2, and A4
Authors:Pollex Annett  Millet Agnès  Müller Jana  Hiersemann Martin  Abraham Lars
Institution:Institut für Organische Chemie, Technische Universit?t Dresden, 01062 Dresden, Germany.
Abstract:structure: see text] An ester dienolate 2,3]-Wittig rearrangement was utilized to access the alkylated citric acid skeleton 6 that is characteristic for the viridiofungins and other members of the alkyl citrate family of secondary natural products. The 2,3]-sigmatropic rearrangement of (Z,Z)-15 provided the rearrangement product (+/-)-syn-16 in moderate yield and with very good diastereoselectivity. A Julia-Kocienski olefination efficiently served to connect the polar head (+/-)-syn-26 with the lipophilic tail (32a-c) of the viridiofungins. Amide formation between the racemic viridiofungin precursors 35a-c and the enantiomerically pure amino acid L-tyrosine methyl ester followed by preparative reversed-phase HPLC provided the isopropyl dimethyl ester of viridiofungin A ((+)-39a), A2 ((+)-39b), and A4 ((+)-39c) as well as the nonnatural diastereomers (-)-38a-c.
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