Tetracyclic triterpenes. Part 16. Synthesis of 31-norcucurbitane and fusidane derivatives in the skeletal rearrangements of 9,11-epoxy-4β-demethyl-5α-lanostanes

Abstract

The BF₃·Et₂O-catalysed rearrangement of 9β,11β-epoxy-4β-demethyl-5α-lanostan-7-one derivatives 11 and 12 in acetic anhydride resulted in formation of 19(10→9β)abeo compounds (31-norcucurbitanes) 20–22 or 23 and 24, respectively. The extent of a rearrangement was dependent on the substituent at position 3 of the steroid. Reaction of the 9α,11α-epoxide 17 under similar conditions gave fusidenone 25 (31-norprotostane) in high yield which had the carbon skeleton characteristic of the steroidal antibiotic, fusidic acid.