Approaches to the total synthesis of triterpenes. VIII. The ABC + D/E approach. Synthesis and X-ray structure determination of 1β,4a β-dimethyl-7-methoxy-1α- (7′,7′-ethyleneketal-6′,6′-dimethyl-3′-ketooctyl)-3,4,4a,9,10,10aα-hexahydro- 2(l-H)phenanthrone

The title compound was made following a projected synthetic route to pentacyclic triterpenes. The key step in the route is the alkylative trapping of the enolate derived from the enol trimethylsilyl ether 8. The stereochemical consequence of this reaction is confirmed by a single crystal X-ray structure determination on 4, which although of no further utility in the projected synthesis, nevertheless served as a useful template for this determination. In this way, ongoing work in a parallel series of compounds rests on a firm stereochemical footing.