Preparation and conformational analysis of 1,2-seco derivatives of 19β,28-epoxy-18α-oleanane

Oxidation of 19 beta ,28-epoxy-18 alpha -oleanan-3-one (1) with chromium(VI) oxide in acetic acid leads to the formation of the 1 beta ,3 beta ;19 beta, 28-diepoxy-3-hydroxy-1,2-seco-18 alpha -oleanano-2,1 alpha -lactone (2). Its structure follows from spectral data, molecular modelling. Lactone 2 was converted to its acetate 3, methyl 19 beta ,28-epoxy-1,3-dioxo-1,2-seco-18 alpha -oleanan-2-oate (4) and to the stereoisomers at C(3) of methyl 1,3;19 beta ,28-diepoxy-1-oxo-1,2-seco-18 alpha -oate (6 and 7) and dimethyl 19 beta ,28-epoxy-3-hydroxy-1,2-seco-18 alpha -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.