23.6 g (0.1 mol) of (S)-6-chloro-1,2,4,9,10,10a-hexahydro-azeto[2,1-c][1,4]benzodiazepine-4,10-dione in 300 ml of THF/DMPU 5:1 were added dropwise under argon at about -75° to a LDA solution (prepared in the usual manner from 16 ml (0.11 mol) of diisopropylamine in 200 ml of THF and 69 ml of 1.6M n-butyllithium solution in hexane). The mixture was stirred for a further 40 min. 23 ml (0.11 mol) of diphenyl chlorophosphate were added dropwise thereto at about -75° and the mixture was stirred at -75° for 35 min. In the meanwhile and separately, a further LDA solution was prepared as described above (100 ml of THF). Thereto there was now added dropwise at about -75° a solution of 12.0 ml (0.11 mol) of ethyl isocyano- acetate in 20 ml of THF/DMPU 1:1. The deprotonized ethyl isocyanoacetate was added dropwise at about -70° via a dropping funnel cooled with dry ice within 30 min. to the initially described reaction mixture. The mixture was stirred in an acetone/dry-ice bath for 2 hrs, 100 ml of 20% ammonium chloride solution were added at <-60° and the mixture was poured into 800 ml of ice-water. The mixture was extracted three times with 200 ml of methylene chloride, dried with sodium sulfate and evaporated. The brown oil was dissolved in 300 ml of ethyl acetate, whereupon 300 ml of n-hexane were added slowly. The crystals were filtered off. Further product was obtained by chromatography of the mother liquor on 300 g of silica gel (methylene chloride/acetone 9:1). After drying in a vacuum there were obtained 17.8 g (54%) of ethyl (S)-7-chloro-12,12a-dihydro-9-oxo-9H,11H-azeto[2,1-c]imidazo[1,5-a][1,4]benzodiazepine-1-carboxylate of m.p. 190°-194° (ethyl acetate).
