New route to (+)-(20R)-De-AB-cholesta-8(14),22-dien-9-one and (+)-(20S)-De-AB-isocholesta-8(14),22-dien-9-one from (S)- and (R)-2,3-O-isopropylideneglyceraldehyde

Abstract

The optically active (+)-(20R)-de-AB-cholesta-8(14),22-dien-9-one (1) and (+)-(20S)-de-AB-isocholesta-8(14),22-dien-9-one (2) have been synthesised via stereo- and regio-selective Michael addition to the (3S)-methylcyclopentanone (4) derived from (S)-2,3-O-isopropylideneglyceraldehyde (5) and its (R)-isomer; the methyl group at C₂₀ in (1) and (2) was introduced by Claisen rearrangement of the allyl vinyl ether (12) followed by decarbonylation of the resulting homoaldehydes (13) and (14), respectively.