Cyclization of 1,1-disubstituted alkenes to cyclopentenes

A general method for the cyclization of an unactivated 1,1-disubstituted al kene such as 1 to the corresponding cyclopentene 5 is described. Brominatio n followed by the addition of strong base in the same pot gave the vinyl br omide 3, which reacted further in situ to give the alkylidene carbene 4. Th is then inserted 1,5 into a C-H bond to give the cyclopentene 5.