[2+2] cycloaddition of chlorosulfonyl isocyanate to allenyl-sugar ethers

The direction and magnitude of asymmetric induction in the [2+2] cycloaddit ion of chlorosulfonyl isocyanate to 3-O-allenyl-alpha-D-xylofuranoses was i nvestigated. It is shown that gem terminal dimethylallenes react more readi ly than methyl free congeners. The configuration of adducts was established by X-ray and CD-spectroscopy of alkylidene cephams. A stereochemical model of the transition state for the [2+2] cycloaddition of chlorosulfonyl isoc yanate and allenyl ethers is proposed, based on the lowest energy conformat ion of the cumulene. (C) 2000 Elsevier Science Ltd. All rights reserved.