Unusual steric effects in sulfonyl transfer reactions

The hydrolysis of N-acyl-β-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 β-sultams, the N-acyl derivatives are at least 10⁶ more reactive than N-acylsulfonamides. However, the α-substituted 4-isopropylidene β-sultam is relatively unreactive and undergoes alkaline hydrolysis with C-N fission leaving the strained 4-membered β-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-β-sultam shows similar behaviour with preferential C-N fission whereas the (E)-4-ethylidene isomer and 4-isopropyl-β-sultam revert to hydrolytic ring opening with S-N fission.