3,4-Dihydro-2H,6H-pyrimido[1,2-c][1,3]benzothiazin-6-imine (PD 404182) is a virucidal heterocyclic compound active against various viruses, including HCV, HIV, and simian immunodeficiency virus. Using facile synthetic approaches that we developed for the synthesis of pyrimido[1,2-c][1,3]benzothiazin-6-imines and related tricyclic derivatives, the parallel structural optimizations of the central 1,3-thiazin-2-imine core, the benzene part, and the cyclic amidine part of PD 404182 were investigated. Replacement of the 6-6-6 pyrimido[1,2-c][1,3]benzothiazin-6-imine framework with 5-6-6 or 6-6-5 derivatives led to a significant loss of anti-HIV activity, and introduction of a hydrophobic group at the 9- or 10-positions improved the potency. In addition, we demonstrated that the PD 404182 derivative exerts anti-HIV effects at an early stage of viral infection.