3,4-Dihydro-2-alkoxy-6-benzyl-4-oxopyrimidines (DABOs) have emerged as non-nucleoside inhibitors of human immunodeficiency virus type 1 [Artico et al. (1993), Antiviral Chem Chemother 4: 361-368]. With a view to increasing their potency, a new series of DABO derivatives, differently substituted at positions C-2 and/or C-5 of the pyrimidine ring and 3′ or 3′,5′ of the benzyl moiety, have been synthesized. DABOs were prepared by reacting O-methylisourea with the appropriate methyl 2-alkyl-4-phenylacetylacetate, with formation of 3,4-dihydro-2-methoxy-6-arylmethyl-4-oxopyrimidines. Subsequent displacement of the methoxy group linked at the 2-position of the pyrimidine ring by treatment with alkoxy and cycloalkoxy potassium salts led to the required derivatives. In vitro, the most potent compounds were 12e and 12p, which had an EC₅₀ of 0.8 μM and a selective index of 400.