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

[2+2]Cycloaddition of CSI to 5-O-protected-3-O-allenyl-1,2-O-isopropylidene-α-d-xylofuranoses 8 and 9 gave respective β-lactams 10–13 having an exo-propylidene group, in moderate stereoselectivity. Compounds 10, 11 and tetracyclic cephams 14, 15 obtained from 10 and 11 or 12 and 13, were used as substrates for a variety of transformations leading to the introduction of isopropyl, hydroxyisopropyl, oxygen and nitrogen functions, α to the β-lactam carbonyl group. These reactions proceeded in high stereoselectivity with control of the absolute configuration of the cephams formed.