FORMATION OF OLEANAN-12-ONE TRITERPENOID FROM TARAXEROL VIA WAGNER-MEERWEIN/PINACOL REARRANGEMENT

摘要

Oleanane-type triterpenoids, both natural and semisynthetic, are reported to exhibit interesting biological or phannacological properties such as anti-HIV (human immunodeficiency virus) and anticancer activities [1]. The major strategy in the semisynthesis of oleanane derivatives is transformations of functional groups on the pentacyclic oleanane skeleton [2]. The occurrence of C-12-or C-15-oxygenated oleananes has been little reported in the literature [3]. Taraxerane-type triterpenoids such as taraxerone can be converted to I5-hydroxyolean-12-enes through acid-catalyzed rearrangement of their 14,15-epoxy derivatives [4]. We report herein the rearrangement of taraxerol (1) in the presence of excessive MCPBA (m-chloroperoxybenzoic acid) to 3P,15-dihydroxyolean-12-ene [5] and novel 3p,15-dihydroxyoleanan-12-one. Since taraxerane triterpenoids are readily available from plants of the family Euphorbiaceae, this Wagner-Meerwein/pinacol-type approach is applicable for the preparation of new semisynthetic oleanane derivatives.