There are numerous reports of reversible cysteine protease inhibitors, which include aldehydes,10−13 thio- or oxymethylketones,14 cyclic ketones,15 amidomethylketones,16 nitriles,17,18 or various 1,2-dicarbonyl motifs.19,20 The electrophilic carbon of these chemotypes reacts reversibly with the sulfur atom of an active-site cysteine forming a covalently bound tetrahedral complex.