Synthetic studies on terpenoids. Part XVIII. Stereocontrolled synthesis of (±)-1,2,3,4,4a,9,10,10aα-octahydro-1α-methylphenanthrene-1β,4aβ-dicarboxylic acid and the 7-methoxy-analogue: a potential intermediate for diterpenoid syntheses

Abstract

A stereocontrolled synthetic route to (±)-1,2,3,4,4a,9,10,10aα-octahydro-1α-methylphenanthrene-1β,4aβ-di-carboxylic acid (Ia) and the 7-methoxy-analogue (Ib) is described. The present approach differs basically from earlier ones and proceeds through polyhydrobenzindenone intermediates. 2,3,4,5-Tetrahydro-3-methylbenz[e] inden-1-one (Va) and the corresponding 7-methoxy-compound (Vb) were transformed through several steps into cis-9b-acetonyl-3a,4,5,9b-tetrahydro-3-methylbenz[e]inden-1-one (XIIIa) and its methoxy-derivative (XIIIb). These unsaturated diketones underwent intramolecular Michael condensation, affording tetracyclic bridged ketones (XIVa and b), which were converted into the acids (Ia and b). In the methoxy-series, disproportionation of a polyhydrobenzindenone derivative was noted.