Olefination of cis-1,5-dimethylbicyclo[3. 3. 0]octane-3,7-dione with (carbethoxyethylidene)triphenylphospharane under controlled microwave heating has been investigated. It reacts with carbon disulfide, strong base and an alkylating agent in dipolar aprotic solvents to yield racemic mixtures of 2-dialkylthio-methylene compounds.