An Unusual Reversal of Stereoselectivity in a Boron Mediated Aldol Reaction: Enantioselective Synthesis of the C1–C6 Segment of the Epothilones

Enantioselective syntheses of differentially protected C1–C6 fragments, (3S)-3-hydroxy-4,4-dimethyl-5-oxoheptanoic acid 4, (5S)-7-[1,1-bis(methylethyl)-2-methyl-1-silapropoxy]-5-hydroxy-4,4-dimethylheptan-3-one 5 and (4S)-2-(2,2-dimethyl-1,3-dioxan-4-yl)-2-methylpentan-3-one 23, common to both epothilones A and B, are reported.