The reaction of 2,2,4‐trimethyl‐1H‐2,3‐dihydro‐1,5‐benzodiazepine (1) with cinnamoyl chloride leading to the formation of 1‐cinnamoyl derivative 2 is described. Two novel benzodiazepines, 2,2,4‐trimethyl‐1H‐2,3,4,5‐tetrahydro‐1,5‐benzodiazepine (3) and 1‐cinnamoyl‐2,2,4‐trimethyl‐1H‐2,3,4,5‐tetrahydro‐1,5‐benzodiazepine (4), were synthesized by the reduction of 1 and 2 using NaBH4 in i‐PrOH and two other derivatives 5 and 6 were obtained by reaction of 4 with equimolar and dimolar quantity of cinnamoyl chloride, respectively. The structures of 1—6 were confirmed by analytical and spectral data (IR, 1H NMR, and MS). 7‐Carboxy‐2,2,4‐trimethyl‐1H‐2,3‐dihydro‐1,5‐benzodiazepine (7) was synthesized and its crystals were subjected to X‐ray analysis. Benzodiazepines 1—6 were evaluated for antiproliferative activity in vitro. Among the compounds tested, 4—6 exhibited cytotoxic activity against human cancer cell lines, namely SW707 (colon cancer), MCF‐7 (breast cancer), A549 (lung cancer), and HCV29T (bladder cancer).