Preparation and evaluation of a perhydroazulene intermediate for lactarane sesquiterpene synthesis

Abstract

A facile preparation of 6,9,9-trimethylbicyclo[5.3.0]deca-1,6-dien-3-one (6) from 2,5,5-trimethylcyclohexane-1,3-dione is described. Enolate reactions of 6 do not yield norcaradiene products, but proceed in the conventional manner (e.g., α-methylation and O-silylation). Nucleophilic addition reactions at the carbonyl function, and epoxidation of the γ,δ double bond are also observed. Under a variety of conditions, singlet oxygen reacts with the diene moiety in 6 to give both endoperoxide (13) and ene reaction (14) products in roughly equal amounts. The endoperoxide rearranges to cyclic hemiketal (15) on treatment with base.