Oxidative Functionalization of Trinor-18α-olean-17(22)-ene Derivatives. Annulation of the E-Ring by an Intramolecular Aldol Reaction

Abstract

cis -Dihydroxylation of trinor-18α-olean-17(22)-ene 2 with osmium tetroxide led to diol 9 . Its cleavage with lead tetraacetate gave tetracyclic ketoaldehyde 10 . By comparison, the ozonation of trinor-18α-olean-17(22)-ene 2 in the presence of p -toluenesulfonic acid gave the corresponding ketoacetal 12 . Both products were subjected to an intramolecular aldol reaction under the acidic conditions and yielded unusual triterpenes bearing a bicyclo[4.3.1]decane fragment ( 22 ). Further manipulation of the protective groups afforded compounds useful in triterpene synthesis, especially in the preparation of potentially biologically active saponins based on a tetracyclic terpene core.