Preparation and Conformational Analysis of 1,2-Seco Derivatives of 19β,28-Epoxy-18α-oleanane

Abstract

Oxidation of 19β,28-epoxy-18α-oleanan-3-one (1) with chromium(VI) oxide in acetic acid leads to the formation of the 1β,3β;19β,28-diepoxy-3-hydroxy-1,2-seco-18α-oleanano- 2,1α-lactone (2). Its structure follows from spectral data, molecular modelling. Lactone 2 was converted to its acetate 3, methyl 19β,28-epoxy-1,3-dioxo-1,2-seco-18α-oleanan-2-oate (4) and to the stereoisomers at C(3) of methyl 1,3;19β,28-diepoxy-1-oxo-1,2-seco-18α-oleanan-2-oate (6 and 7) and dimethyl 19β,28-epoxy-3-hydroxy-1,2-seco-18α-oleanan-1,2-dioate (8 and 9). Lactone 2 reacts slowly with diazomethane which is indicative for its equilibrium with a small amount of free acid. Alkaline hydrolysis of compound 2 leads to compounds 8 and 9; the reaction involves hydride transfer of a Cannizzaro reaction type. A high rotational barriers were found in compounds 8 and 9. A combination of NMR methods and molecular modelling revealed that most sterically hindered bond in both compounds is the C(1)-C(10) single bond.