Stereochemistry of the [2+2] cycloaddition of chlorosulfonyl isocyanate to chiral alkoxyallenes derived from 1,3-alkylidene-L-erythritol and -D-threitol

The [2+2] cycloaddition of chlorosulfonyl isocyanate to alkoxyallenes derived from ethylidene and benzylidene erythritols and threitols proceeds with a moderate asymmetric induction in the case of the erythritols and with a very low induction in the case of threitols. This indicates that the erythritol derivatives may exist in solution in one predominant conformation while the threitol derivatives behave as a conformational ensemble. The conformations of alkoxyallenes were studied with variety of NMR techniques as well as using ab initio calculations. The results thus obtained were in a full agreement with our predictions based on the stereo selectivity of cycloaddition. The azetidinones obtained by the cycloaddition were subjected to intramolecular alkylation at the nitrogen atom to provide the corresponding tricyclic cephams. The absolute configurations of the resultant azetidinones and cephams were assigned using NMR and CD spectroscopy. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).