Lactones 17. Synthesis of Bicyclic Lactones with the Methyl- or gem-Dimethylcyclopropane System

A new method of synthesis of bicyclic γ-lactones with the methyl- or gem-dimethylcyclopropane system from γ,δ-unsaturated esters is described. The reaction of γ-halo-δ-lactones with DBU is the key step of this synthesis. The mechanism of the dehydrohalogenation-cyclization process of γ-halo-δ-lactones is proposed. The structures of products were confirmed by both spectroscopic and crystallographic methods.