Abstract: The present invention relates to compounds to inhibit blood coagulation, and more particularly to novel diketopiperazine derivatives, pharmaceutically acceptable salts and compositions thereof, to specifically inhibit thrombin. The compound has the following general structure wherein R1, R2 and R4 consist of a hydrogen, alkyl or aryl moiety, R3 consist of an alkyl or aryl moiety, wherein R5 consists of a hydrogen, alkyl, aryl, hydroaryl, heteroaryl, hydroheteroaryl, sulfonylalkyl, sulfonylaryl, sulfonylhydroaryl, sulfonylheteroaryl or sulfonylhydroheteroaryl moiety, and wherein R6 consists of a hydrogen, alkyl, aryl, hydroaryl, heteroaryl or hydroheteroaryl moiety. Also disclosed are methods of using the compound for treating coagulation disorders such as thrombosis and heparin associated thrombocytopenia.