Benzimidazole and other fused ring derivatives as selective janus kinase inhibitors

Disclosed is a compound of formula I, or a pharmaceutically acceptable prodrug, salt, hydrate, solvate, crystal form, or diastereoisomer thereof, wherein up to one of X1 to X4 may be nitrogen, and the others are substituted carbon, A is a substituted pyridine, pyrazine, pyrimidine, or 1,3,5-triazine, Q is a bond, C1-4alkyl, O, S, SO, SO2, CO, or CS, and wherein the other substituents are as defined in the specification. Also disclosed is the use of the compound for inhibiting JAK-3, and for treating diseases such as atopy, allergic asthma, atopic dermatitis, allergic rhinitis, cell mediated hypersensitivity, rheumatic diseases, transplant rejection, graft versus host disease, viral diseases, proliferative diseases and cancer.