In our investigation of factor Xa inhibitors, a series of 1-(6-chloronaphthalen-2-yl) sulfonyl-4- (4, 5, 6, 7-tetrahydrothiazolo [5, 4-c] pyridine-2-carbonyl) piperazines 3a–i were synthesized. In vitro inhibitory activities of the compounds against factor Xa and coagulation are summarized. Among the compounds, 3c and 3d, possessing a carbamoyl or N- methylcarbamoyl moiety, showed potent inhibitory activities when administered orally to ...