Iodine monobromide (IBr) at low temperature: enhanced diastereoselectivity in electrophilic cyclizations of homoallylic carbonates

1993 
Iodine monobromide affords superior distereoselectivity 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 o C. The latter protocol generally furnishes significantly enhanced selectivity, vis-a-vis the original procedure employing I 2 in acetonitrile at -20 o C; for example, the IBr-induced cyclization of 14 affords a 25.8:1 mixture of 15 and 16, whereas I 2 gives an 8.4:1 ratio. An equilibration experiment established that the diastereoselectivity derives primarily or exclusively from kinetic control of the cyclization process
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