Intramolecular photocycloadditions-cyclobutane fragmentation: total synthesis of (.+-.)-pentalenene, (.+-.)-pentalenic acid, and (.+-.)-deoxypentalenic acid

Abstract

Pentalenene, pentalenic acid, and deoxypentalenic acid, important metabolites in the biosynthesis of the pentalenolactones, have been synthesized from methyl isobutyrate through a common intermediate. The initial key step involves a novel conjugate addition-cycloacylation sequence on an acetylenic diester. The 1,6-diene which results is converted in two steps to a 1,6-diene diester which undergoes a highly stereoselective photocycloaddition to set three of the necessary stereocenters. Reductive cleavage of one cyclobutane bond produces a functionalized spirobicyclo(4.4)nonanone which is converted in three steps to dione 10, the pivotal intermediate. Differential functionalization of this system provides efficient, stereocontrolled routes to pentalenene, pentalenic acid, and deoxypentalenic acid, all in racemic form.