During the resynthesis of 1 we observed the formation of a cyclic byproduct 2 that effectively masked the vinyl sulfone warhead. In this report we document methods to synthesize pyrazolesubstituted b-amidomethyl-vinyl sulfones, such as 1, devoid of contamination from cyclic dihydropyrazolo pyrazin-4(5H)-ones. We also explored the reversibility of the cyclization reaction as a potential prodrug strategy for the cysteine-reactive acyclic b-amidomethyl-vinyl sulfone. LCMS analysis using an Ultrahigh Performance Liquid Chromatography (UPLC) reverse phase column with an extended run time achieved baseline separation of 1 and 2 (Figure ). The mass spectra of the respective peaks showed predicted molecular weights of 349 Da for both compounds , indicating that they were likely to be constitutional isomers.