A variety of N1-substituted pyrimido[5,4-f]benzo[1,4]thiazepines, 5, designed as conformationally constrained analogs of 1-[(2-hydroxyethoxy)methyl]-6-(phenylthio)thymidine HEPT (1), were synthesized and evaluated for their inhibition of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT). The preparation of these compounds was carried out based on a Mannich-type cyclization of 6-[(2-aminophenyl)thio]uracils followed by alkylation at N1 by a one-pot Vorbruggen reaction. The pyrimidobenzothiazepines were developed to give molecules with IC50 values in the micromolar range, as exemplified by [[(2-ethoxyethyl)oxy]methyl]-pyrimido[5,4- f]benzo[1,4]thiazepine, 25, (IC50 = 0.64 microM), the most active compound of this series. The structural and electronic features of this novel class of HIV-1 RT inhibitors are presented and compared with those of HEPT (1), TIBO (2), and nevirapine (3).