Reaction of 18β,19β-Epoxylupan-21-one Derivatives with Acids: A Way to 21,22-Disubstituted Lup-18-ene Triterpenoids

Abstract

Depending on the conditions and the acid employed, 18β,19β-epoxy-28-hydroxy- 21-oxolupan-3β-yl acetate (2a) and 18β,19β-epoxy-21-oxolupane-3β,28-diyl diacetate (2b) on treatment with acid gave three types of products: (i) 28-nor derivatives: 21-oxo-28-norlupa-16,18-dien-3β-yl acetate (6), 19β-hydroxy-21-oxo-28-norlup-17-en-3β-yl acetate (7) and 17ξ-hydroxy-21-oxo-28-norlup-18-en-3β-yl acetate (8), (ii) lupa-12,18-dien-21-ones 4a and 4b, and (iii) 22β-substituted lup-18-en-21-one derivatives of the type 5. The formation of 22β-substituted derivatives of the type 5 probably proceeds in the enol form of epoxy ketone 2. Opening of the epoxide ring with shift of the double bond to position 19(21) and attack by nucleophilic species at C-22 followed by elimination of water and re-formation of the 22-oxo group leads to derivatives of the type 5.