Synthesis of 7-substituted 5-dethia-5-oxacephams via [2+2]cycloaddition ofchlorosulfonyl isocyanate to sugar vinyl ethers

[2+2]Cycloaddition of chlorosulfonyl isocyanate (CSI) to (Z) prop-1-enyl, ( Z) and (E) 4'-trimethylsilyl-but-1'-enyl ethers of 1,2-O-isopropylidene-5-O -trityl-a-D-xylo nose proceeds with high stereoselectivity to afford respec tive azetidin-2-ones. Subsequent transformations of adducts consisting in d etritylation, tosylation of a terminal hydroxy group and intramolecular alk ylation of the nitrogen atom give corresponding tetracyclic 5-oxacephams.