In a 500 mL flask was charged (R)-2,3,4,5-tetrahydro-3-(phenylmethyl)-4-(2-thienylsulfonyl)-1H-1,4-benzodiazepine-7-carbonitrile (20.0 g, 1 eq.), imidazole-4-carboxaldehyde (5.2 g, 1.1 eq.), and CH2Cl2 (80 mL) at room temperature. Cool to 0-5° C. Add trichloroacetic acid (32 g, 4 eq.) and trichloroacetic acid anhydride (12.2 mL, 1.36 eq.) while keeping the temperature below 20° C. Stir the mixture for 30 min at 0-5° C. Add Triethylsilane (11.6 mL, 1.5 eq.). Agitate the mixture at 20-25° C. for 26 h until the reaction is complete. Add CH2Cl2 (80 mL), then cool to 0° C. Adjust the pH to 8-9 with 10% NaOH. Separate the two phases. Wash the organic phase with H2O (50 mL). Remove most of the solvents, and add EtOH (150 mL); and heat up to reflux and stir at reflux for 10 min. Cool to room temperature in a period of 60 min. Crystals appeared at 65° C. Stir at room temperature for 2 h. Filter and wash the crystals with EtOH (150 mL). Dry at 40° C. in vacuum oven over night to give the product (20.4 g, 85% yield) as a white solid.
