Stereoselective total synthesis of (±)-isocostic and (±)-3-oxoisocostic acids

Abstract

Stereoselective total synthesis of the title compounds, two new sesquiterpenes isolated from a Mexican plant, are described. The key step in the synthesis is based on classical santon in syntheses by the stereoselective Michael addition of the potassium salt of ethyl α-cyanoacetate to the dienone(4), to give the cis adduct (5), in an 8 : 2 ratio with the trans isomer. Further appropriate transformation of this cis adduct (5) leads to formation of either isocostic acid (1) or 3-oxoisocostic acid (2).