Transformation of the 7-isopropylidene group in 5-dethia-5-oxacephams

[2 + 2] Cycloaddition of CSI to 5-O-protected-3-0-allenyl-1,2-O-isopropylid ene-alpha -D-xylofuranoses 8 and 9 gave respective p-lactams 10-13 having a n exo-propylidene group, in moderate stereoselectivity. Compounds 10, 11 an d tetracyclic cephams 14, 15 obtained from 10 and 11 or 12 and 13, were use d as substrates for a variety of transformations leading to the introductio n of isopropyl, hydroxyisopropyl, oxygen and nitrogen functions, or to the p-lactam carbonyl group. These reactions proceeded in high stereoselectivit y with control of the absolute configuration of the cephams formed. (C) 200 1 Elsevier Science Ltd. All rights reserved.