AbstractTwenty‐one 1,3‐diaminobenzo[f]quinazolines (2, R = H, alkyl, halogen, methoxy) were synthesized as planar tricyclic analogs of the antimalarial agent pyrimethamine (1). The synthetic methods included i) condensation of 1‐cyano‐2‐naphthylamines with cyanamide in the presence of pyridine hydrochloride, ii) cyclization of N1,N5‐bis(2‐naphthyl)biguanide hydrochlorides in diphenyl ether at 250°, iii) reaction of 2‐naphthylamine hydrochlorides with excess sodium dicyanamide in boiling 1‐octanol, and iv) selenium dioxide dehydrogenation of 1,3‐diamino‐5,6‐dihydrobenzo[f]quinazolines. A number of new 2‐naphthols and 2‐naphthylamines were synthesized as intermediates. Substituent effects on the ultraviolet absorption spectra of 1,3‐diamino‐benzo[f]quinazolines were investigated and found to be additive. These compounds are of interest as inhibitors of dihydrofolate reductase and as potential antimalarial and antitumor agents.