FIELD : medicine; pharmaceuticals.SUBSTANCE : invention relates to a novel compound of formula I or a pharmaceutically acceptable salt thereof. In formula I , where X denotes -(C=O)NH- or -NH(C=O)-; R1 denotes phenyl, where said phenyl group is optionally substituted with one to three substitutes independently selected from a group consisting of halogen, R4, (C=O)OR4, NH(C=O)OR4 and 5-member heteroaryl containing 4 nitrogen atoms; R2 denotes hydrogen, hydroxy, halogen or C1-6 alkyl, where said alkyl is optionally substituted with one substitute selected from C3-6 cycloalkyl; R3 denotes phenyl or C3-10 cycloalkyl, where said phenyl and cycloalkyl group are optionally substituted with one substitute selected from (C=O)OR4; R4 is hydrogen or C1-6 alkyl; Rx is hydrogen, hydroxy or halogen; Rz denotes hydrogen.EFFECT : compound has the properties of a coagulation factor inhibitor hFXIa and can be used for treating venous thromboembolia and pulmonary embolism, as well as in producing a drug for inhibiting thrombin, inhibiting thrombosis, treating thrombosis or preventing thrombosis.16 cl, 4 ex