Triterpenoids. Part VIII. Allylic oxidation by N-bromosuccinimide

Abstract

Treatment of β-amyrin acetate with N-bromosuccinimide in aqueous dioxan gives variable amounts of 3β-acetoxyolean-12-ene-11-one as the major product, together with 3β-acetoxyolean-12-en-11α-ol, 3β-acetoxy-12-bromoolean-12-en-11α-ol, and 12α-bromo-16-oxotaraxeryl acetate. 3β-Acetoxyolean-12-en-11-one is the sole product, obtained in essentially quantitative yield, when the reaction mixture is irradiated with visible light. Similar photo-oxidations afford high yields of the corresponding αβ-unsaturated ketones from α-amyrin acetate, taraxeryl acetate, α- and β-amyrin and their benzoates, olean-12-ene, urs-12-ene, olean-12-en-3-one, urs-12-en-3-one, and cholesteryl acetate.