ON THE CONFIGURATION AND CONFORMATION OF OXYALLYLS IN MEDIUM AND LARGE RINGS

Calculations demonstrate that the preferred shape of oxyallyls in ring systems depends on the ring size, consistent with experimentally observed stereoselectivity in cycloadditions to furan. For nine-membered rings, the U-configuration is preferred. For ten-membered rings, the sickle shape is the major configuration. As the ring becomes larger (n greater than or equal to 12), the W-configuration dominates.