J. Matthew Wood

Structure–reactivity relationships in the inactivation of elastase by β-sultams

N-Acyl-β-sultams are time dependent irreversible active site directed inhibitors of elastase. The rate of inactivation is first order with respect to β-sultam concentration and the second order rate constants show a similar dependence on pH to that for the hydrolysis of a peptide substrate. Inactivation is due to the formation of a stable l ∶ l enzyme inhibitor complex as a result of the active site serine being sulfonylated by the β-sultam. Ring opening of the β-sultam occurs by S–N fission in contrast to the C–N fission observed in the acylation of elastase by N-acylsulfonamides. Structure–activity effects are compared between sulfonylation of the enzyme and alkaline hydrolysis. Variation in 4-alkyl and N-substituted β-sultams causes differences in the rates of inactivation by 4 orders of magnitude.

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.

β-sultams - a novel class of serine protease inhibitors

N-Benzoyl β-sultam is an irreversible inactivator of elastase by sulfonation of the active site serine.

Hydrolysis of a sulfonamide by a novel elimination mechanism generated by carbanion formation in the leaving group

The alkaline hydrolysis of N-alpha-methoxycarbonyl benzyl-beta-sultam occurs 10(3) times faster than the corresponding carboxylate and with rapid D-exchange at the alpha-carbon: the pH rate profile indicates pre-equilibirum CH ionisation and together with formation of benzoyl formate as a product this suggests a novel mechanism for hydrolysis.

Reactivity and the mechanisms of reactions of β-sultams with nucleophiles

Ethane-1,2-sultam has a pKa of 12.12 ± 0.06 at 30 C and its rate of alkaline hydrolysis shows a pH-dependence reflecting this so that the observed pseudo first-order rate constant at pHs above the pKa are pH independent. There is no evidence of neighbouring group participation in the hydrolysis of either N-α-carboxybenzylethane-1,2-sultam or N-(hydroxyaminocarbonylmethyl)-2-benzylethane-1,2-sultam. Oxyanions, but not amines or thiols, react with N-benzoylethane-1,2-sultam in water by a nucleophilic ring opening reaction confirmed by product analysis and kinetic solvent isotope effects. A Bronsted plot for this reaction has two distinct correlations with βnuc = 0.52 and 0.65 for weak and strong bases, respectively, although a statistically corrected plot may indicate a single correlation.