SPONGIAN PENTACYCLIC DITERPENES - STEREOSELECTIVE SYNTHESIS OF (-)-DENDRILLOL-1

A formal total synthesis of the spongian diterpene (-)-dendrillol-1 (3), through a consice approach that could be used for the synthesis of other pentacyclic spongian diterpenes, is reported. The synthesis is based on the intramolecular acetalization of an acid-dialdehyde 4, which is prepared from (+)-podocarp-8(14)-en-13-one (5) via a sequence of transformations involving (a) introduction of a latent dialdehyde unit on 5 by photochemical reaction with acetylene, (b) reductive carboxylation at C-13 of photoadduct 6 to obtain acid 18, and (c) elaboration of the dialdehyde moiety at C-8 and C-14 of 18 by ozonolysis. Several procedures that have been examined for the reductive carboxylation at C-13 of 6 are described. A simple three-step procedure to effect the conversion of a podocarp-8-en-13-one system into a C-17-functionalized beyerane compound is also reported.