Expeditious asymmetric synthesis of a stereoheptad corresponding to the C(19)-C(27)-ansa chain of rifamycins: formal total synthesis of Rifamycin S |
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Authors: | Turks Māris Huang Xiaogen Vogel Pierre |
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Institution: | Laboratory of Glycochemistry and Asymmetric Synthesis, Swiss Federal Institute of Technology, BCH 1015, Lausanne-Dorigny, Switzerland. |
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Abstract: | In the presence of sulfur dioxide and an acid promoter, (-)-(1E,3Z)-2-methyl-1-((1S)-1-phenylethoxy)penta-1,3-dien-3-yl isobutyrate reacts with (Z)-3-(trimethylsilyloxy)pent-2-ene giving a silyl sulfinate intermediate that undergoes, in the presence of palladium catalyst, a desilylation and retro-ene elimination of SO(2) with formation of (-)-(1Z,2S,3R,4S)-1-ethylidene-2,4-dimethyl-5-oxo-3-((1S)-1-phenylethoxy)-heptyl isobutyrate as major product. This ethyl ketone undergoes cross-aldol reaction with (2S)-2-methyl-3-(tert-butyldimethylsilyl)oxy]propanal giving an aldol that is reduced into a stereoheptad corresponding to the C(19)-C(27)-segment of Rifamycins with high diastereoselectivity and enantiomeric excess. |
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Keywords: | aldol reaction asymmetric synthesis Diels–Alder reaction dienes polypropionates sulfur dioxide |
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