These results suggested that the structure of the pyrazole could influence the rate of aza-Michael cyclization and the propensity for the reverse reaction. To explore this possibility, an additional series of GSH capture experiments was performed. Incubation of b-amidomethyl-vinyl sulfone 7 with a 100-fold excess of GSH in phosphate buffer at 30 °C led to formation of its corresponding GSH adduct 10, although conversion was slower than was seen with 1 and required 24 h to complete. More importantly, incubation of the corresponding dihydropyrazolo pyrazin-4(5H)-one 8 with GSH resulted in the formation of 3% of the GSH adduct 10 after 8 h and 6% after 24 h (Figure ). These results demonstrate that the cyclic dihydropyrazolo buffer and adds credence to the potential use of dihydropyrazolo pyrazin-4(5H)-ones as prodrugs for their corresponding cysteine reactive vinyl sulfones. ) is a covalent inhibitor of nsP2 cysteine proteases with potent antiviral activity against New and Old World alphaviruses. Although 1 was prone to intramolecular cyclization to a cyclic dihydropyrazolo pyrazin-4(5H)-one 2 under basic conditions and in phosphate buffer, two modified procedures were developed for the synthesis of the pure acyclic b-amidomethyl-vinyl sulfones as their TFA or HCl salts that can be employed in analog development for structure-activity studies of nsP2 protease inhibitors. The primary liability of 1 as an anti-alphavirus drug lead was its poor pharmacokinetics due to very high clearance in mice, which prompted us to explore the use of dihydropyrazolo pyrazin-4(5H)-one 2 as a potential prodrug. Although cyclic 2 showed no evidence for retro aza-Michael reaction to give 1, the phenylsulfonamide-5-substituted pyrazole 8 formed detectable levels of its acyclic b-amidomethyl vinyl sulfone 7 as evidenced by capture with GSH. These results demonstrate that the dihydropyrazolo pyrazin-4(5H)-one chemotype can function as a masked form of the cysteine-reactive b-amidomethyl vinyl sulfone. Synthesis of dihydropyrazolo pyrazin-4(5H)-one analogs with substituents that further favor equilibrium with their acyclic vinyl sulfones may provide a new prodrug strategy for covalent inhibition of viral nsP2 cysteine proteases.