A novel synthesis of tricyclo[5.1.0.03,5]octane-2,6-dione derivatives via double Michael addition-induced cyclopropanation reactions

Tricyclo[5.1.0.03,5]octane-2,6-diones 1a–q were prepared in moderate yields by the reaction of α,β-unsaturated esters 2 with dihalomethane in the presence of butyllithium, and by the reaction of dichloromethyl α,β-unsaturated ketones 3 with sodium ethoxide, respectively. This reaction was newly explained by a mechanism involving intermolecular double Michael additions of enolate anion 4 of ketones 3 and the following cyclopropanation.