Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

Participation of the 19-methoxy and 19-acetoxy group in 5α,6α-epoxides IIIa and IVa on treatment with aqueous perchloric acid is investigated and compared with acid cleavage of previously studied 3β-acetoxy epoxides IIIb and IVb. The methoxy group in IIIa participates by a 5(O)ⁿ process. The participation is predominant on treatment with perchloric acid but is completely suppressed by the attack of an external nucleophile on treatment with hydrobromic acid. The acetoxy group in IVa participates by a 6(O)^π,n process. Participation is predominant on treatment with perchloric acid and is only partially suppressed by a competitive external nucleophile attack on the action of hydrobromic acid. This behavior constitutes a difference to the 3β-acetoxy derivative IVb where hydrobromic acid completely suppresses the participation in favor of an external attack.