FIELD : medicine; pharmaceuticals.SUBSTANCE : invention relates to a new compound of formula (I) or a pharmaceutically acceptable salt thereof. Compound has the properties of a selective JAK3 inhibitor, exhibits high activity in suppressing the growth of human peripheral blood monocytes and oral absorption, and also exhibits activity when inhibiting IFN-γ in vivo. In general formula (I)X is -CH = CH-, sulphur atom or oxygen atom; and n denotes integer from 0 to 2.EFFECT : compound can be used for treating diseases involving JAK3, where the disease involving JAK3 is selected from a group consisting of psoriasis, graft versus host, multiple sclerosis, inflammatory bowel disease, systemic lupus erythematosus and rheumatoid arthritis.16 cl, 2 dwg, 5 tbl, 31 ex