Unusual steric effects in sulfonyl transfer reactions

The hydrolysis-of N-acyl-beta -sultams generally occurs with ring opening a nd S-N fission in contrast to the C-N fission observed in analogous acyclic N-acyl sulfonamides. Similar to other beta -sultams, the N-acyl derivative s are at least 10(6) more reactive than N-acylsulfonamides. However, the al pha -substituted 4-isopropylidene beta -sultam is relatively unreactive and undergoes alkaline hydrolysis with C-N fission leaving the strained 4-memb ered beta -sultam ring intact: This reduction in reactivity is shown to be due to steric strain introduced in the transition state for attack at the s ulfonyl centre. (Z)-4-Ethylidene-beta -sultam shows similar behaviour with: preferential C-N fission whereas the (E)-4-ethylidene isomer and 4-isoprop yl-beta -sultam revert to hydrolytic ring opening with S-N fission.