An Approach to Clavams and 1-Oxacephams from Hydroxy Acids

[2 + 2] Cycloaddition of chlorosulfonyl isocyanate to chiral alkyl vinyl ethers bearing a sterogenic center in the alkyl part of the ether afford the corresponding azetidin-2-ones with relatively good asymmetric induction in certain cases. Reactions were shown to depend on steric requirements of the ligands at the stereogenic center. The model that rationalizes the stereochemical outcome is based on the s-cis conformation of the vinyl ether in which the bulkiest of the ligands is situated in the plane of the double bond, and the next most demanding substituent is placed gauche to the double bond.