OXIDATIVE TRANSFORMATIONS OF TRITERPENOIDS OF THE URSANE AND OLEANANE SKELETA WITH HYDROGEN PEROXIDE. INTRODUCTION OF 11,12-DOUBLE BOND AND 13(28)OXIDE MOIETY IN THE URSANE SYSTEM

Article abstract of DOI:10.1016/S0040-4020(01)91841-1

Treatment of oleanolic acid acetate (1b) with H2O2 in boiling HOAc gave the epoxy-γ-lactone 2b and the 12-hydroxy-γ-lactone 3b.The total absence of the 12-ketodihydro acid 4b among the oxidation products of 1b has been rationalised.The results of this reaction with several isomeric pairs of triterpenoids of the ursane and oleanane skeleta bearing functional groups at C-17 other than a carboxyl group show that for any appreciable oxidation involving the intermediacy of a 12β,13β-epoxide 6a or 6b, the 17-carboxyl group is an essential requirement.An intramolecular epoxidation of 12,13-double bond of 1a and 1b by 17-percarboxy acid formed in situ has been envisaged.The reactions aiming at introducing 11,12-double bond and 13(28)-oxide moiety in the ursane system are described.The desired compound 2c was obtained by treatment of the triol 1p with TsOH.With H2O2/TsOH, 1p, however, gave besides 2c, the rearranged product 8a.The difference in the chemical behaviour of 1p and 1q has been explained.Treatment of 2e with H2O2 in boiling HOAc gave the epoxide 2f.The mechanism of formation of 2a and 2b is discussed in the light of this observation.