The development of a new, synthetically useful methylenecyclohexane annulation method is described. Transmetallation (MeLi, tetrahydrofuran,-78° C) of 5-chloro-2-trimethylstannyl-1- pentene (8) affords the novel bifunctional reagent 5-chloro-2-lithio-1-pentene (9), which, at temperatures higher than approximately-50° C, self-destructs to give methylenecyclobutane (14). Reagent 9 can be converted into the Grignard reagent 10 and the organocopper (I) ...