RELATIVE STEREOCONTROL IN AN INTRAMOLECULAR 4+3 CYCLOADDITION REACTION

The first complete study of relative stereocontrol in an intramolecular 4+3 cycloaddition has been performed. Thus, CH2Cl2 solutions of alkoxyallylic sulfones 5, 7, and 9 were treated with TiCl4 at -78-degrees-C to give diastereomeric cycloadducts in fair yield. For 5 and 7 diastereoselectivity was low, but suggested that allylic cation geometry plays a small but definitive role in determining the diastereomer distribution in the cycloadducts. For the cycloaddition reaction of 9, cation geometry is irrelevant and the cycloadduct is obtained in very high diastereomeric purity.