Reinvestigation of the action of hydrogen peroxide on ursolic acid acetate

The action of hydrogen peroxide on ursolic acid acetate in boiling acetic acid has been reinvestigated, and the three oxidation products, designated as U_I, U_II and U_III, have been reisolated. The structure of U_I has been revised to 8a and that of U_II has been established as 11a on the basis of spectral and chemical evidence. Additional spectral data are provided in support of the structure of U_III (5a). The stereoelectronic factors responsible for the unusual stability of the epoxide function in U_I towards acid-catalysed rearrangement to the 12-keto-derivative (9a) and the facile conversion of the latter to the enol-acetate (10a), compared to the normal behaviour of the structurally similar compounds, 8b and 9d of the oleanane series, have been rationalised.