IODINE MONOBROMIDE (IBR) AT LOW-TEMPERATURE - ENHANCED DIASTEREOSELECTIVITY IN ELECTROPHILIC CYCLIZATIONS OF HOMOALLYLIC CARBONATES

Iodine monobromide affords superior diastereoselectivity in low-temperature electrophilic cyclizations of homoallylic carbonates. Solvent and temperature effects and the scope and limitations of the method are discussed; optimal selectivity is obtained in toluene at -80 to -85-degrees-C. The latter protocol generally furnishes significantly enhanced selectivity, vis-a-vis the original procedure employing 12 in acetonitrile at -20-degrees-C; for example, the IBr-induced cyclization of 14 affords a 25.8:1 mixture of 15 and 16, whereas I2 gives an 8.4:1 ratio. An equilibration experiment established that the diastereoselectivity derives primarily or exclusively from kinetic control of the cyclization process.