JAK3 and Crohn disease: The API, Ritlecitinib,92 functions as a selective and irreversible JAK3 inhibitor for irritable bowel disease with additional studies in progress for further uses as a treatment for alopecia areata93 and Crohn's disease.94 The irreversible binding is covalent in nature to a specific cysteine residue (Cys-909) within the JAK3 protein.95 The original synthesis for the molecule, Ritlectinib, was described by Thorarensen et al.96 In this work, the API is considered in its tosylate salt form, 1 (see Scheme 1).97