Methyl 3β-pyranyloxy-(21S)-methoxy-14β,21-epoxy-5β-(20S)-norcholanate (6a) and the axial acetal 6b each gave isodigitoxigenin (3a) on treatment with hydrochloric acid in aqueous acetic acid. Extension of this reaction to cholanate ester 7b gave 30-acetoxy-14β,21-epoxy-5β-bufanolide (4). The epoxide 9 prepared from 3β-acetoxy-14β,21-epoxy-5p-chol-20 (21)-enic acid (8b) spontaneously cyclized to give 7-lactones of type 11 rather than 8-lactone derivatives. In another potential approach to bufadienolides, 3β-pyranyloxy-23-mesyloxy-14β,21-epoxy-20-methoxy-5β-(20)-norcholane (7d) was converted into 3β-acetoxy-14β,21-epoxy-20-formylpregn-20 (21)-ene (15a). Condensation of aldehyde 15a with malonic acid took an unexpected course which culminated in formation of olefin 18.
ASJC Scopus subject areas
- Organic Chemistry