Stereocontrolled formation of oxacephams from carbohydrates. 
The [2 + 2] cycloaddition of chlorosulfonyl isocyanate to (Z)-4-O-propenyl ethers 16, 17, 29 and 30 proceeds with an excellent stereoselectivity in the case of ether 16 and with moderate stereoselectivity in remaining cases.  Adducts were transformed into corresponding oxacephams: 37 in the first case, a mixture of 37/40 in the second and third case, and a mixture of 50/51 in the last case.  In all instances addition to the si-re side of the olefin dominates.  Oxacephams 41 and 52 with opposite R-configuration at the bridgehead carbon C-5a can be obtained by the alternate methodology based on the alkylation of nitrogen in 4-vinyloxyazetidin-2-one by protected 6-O-triflate 24 or 25, followed by cyclization via intramolecular displacement of the vinyloxy group.  Compounds 37, 40, 41, 50, 51 and 52 constitute a convenient entry leading to polyfunctionalized oxacephams.