Libraries of tricyclic 2-substituted 4-alkylamino-5,6-dihydro[1]benzoxepino[5,4-d]pyrimidines and tetracyclic 12-substituted 1,2,4,5-tetrahydro[1]benzoxepino[4,5-e]imidazo[1,2-c]pyrimidines were synthesized as part of our research to develop new effective antiplatelet drugs. Several alkyl and aryl groups were used as substituents at the 2-position. Evaluation of the effects of the newly synthesized compounds on collagen-induced platelet aggregation revealed several promising antiplatelet candidates with potencies superior to aspirin.