Triterpenoid total synthesis. Part 5. Synthetic disproof of the triterpene structure proposed for naurol A, a cytotoxic metabolite of a Pacific sponge

Abstract

Naurol A is a cytotoxic metabolite isolated from a Pacific sponge, and 1 has been proposed as its structure. A mixture of (±)-1 and meso-1′ was synthesized from 4-tert-butyldimethylsilyloxy-3-methylcyclohex-2-en-1-one (6) employing the Stille coupling (10 + 11→1 + 1′) as the key step. Although the synthetic sample (1 + 1′) was a diastereomeric mixture at C-11, its spectral data (IR, UV, 1H and 13C NMR and MS) were significantly different from those reported for naurol A. It was therefore concluded that the structure 1 proposed for naurol A was in error.