A predictive model for thiol reactivity of N-heteroaryl α-methylene–γ-lactams—a medicinally relevant covalent reac-tive group

Herein, we present a systematic study on the effects of electronically diverse heteroarenes on the rate of glutathione (GSH) addition to novel N-heteroaryl α–methylene–γ-lactam covalent reactive groups (CRGs). Despite their unique electronic and drug-like properties, heteroarenes have not been extensively studied as handles for systematically tuning the reactivity of CRGs. Informed by mechanistic insights, we evaluated 16 substrate parameters, including a new heteroaryl Hammett-type substituent constant (σHet), for their correlation with experimental reactivity (DG‡exp) as determined by 1H NMR kinetics studies. Of these parameters, electron affinity represents a robust single-parameter predictive model of CRG reactivity with thiols, as demonstrated by test sets of additional N-heteroaryl lactams (MUE = 0.4 kcal/mol) and other α,β-unsaturated amide CRGs (MUE = 0.3 kcal/mol). These N-heteroaryl lactams were subse-quently shown to inhibit cysteine protease activity (i.e., papain enzyme) to varying degrees that correlate with both the experimentally observed and predicted reactivity with GSH.