Unusual steric effects in sulfonyl transfer reactions

The hydrolysis of N-acyl--sultams generally occurs with ring opening and S–N fission in contrast to the C–N fission observed in analogous acyclic N-acyl sulfonamides. Similar to other -sultams, the N-acyl derivatives are at least 106 more reactive than N-acylsulfonamides. However, the -substituted 4-isopropylidene -sultam is relatively unreactive and undergoes alkaline hydrolysis with C–N fission leaving the strained 4-membered -sultam ring intact. This reduction in reactivity is shown to be due to steric strain introduced in the transition state for attack at the sulfonyl centre. (Z)-4-Ethylidene--sultam shows similar behaviour with preferential C–N fission whereas the (E)-4-ethylidene isomer and 4-isopropyl--sultam revert to hydrolytic ring opening with S–N fission