A solution of 2g of phosgene in 20 ml of ice-cold 1,2-dichloroethane is placed in a sulphonation flask. There is added dropwise thereto a solution of 4 g (0.012 M) of (S)-7-amino-6-chloro-5-(o-fluorophenyl)-1,3-dihydro-1,3-dimethyl-2H-1,4-benzodiazepin-2-one in 40 ml of 1,2-dichloroethane while cooling with ice and stirring in such a manner that the temperature does not exceed 10° C. The mixture is then stirred at room temperature for 45 minutes. The resulting solution of (S)-[6-chloro-5-(o-fluorophenyl)-2,3-dihydro-1,3-dimethyl-2-oxo-1H-1,4-benzodizepin-7-yl]isocyanate is cooled with ice and slowly made alkaline with 6 ml of triethylamine. A solution of 2 ml of 2-aminoethanol in 20 ml of 1,2-dichloroethane is treated with the foregoing pre-prepared solution, the mixture is stirred at room temperature for 30 minutes and then concentrated. The residue is purified on a 300 g silica gel column using methylene chloride and then methylene chloride/ethanol (20:1) as the eluting agent and crystallised from a small amount of ethyl acetate/n-hexane. There is obtained (S)-1-[6-chloro-5-(o-fluorophenyl)-2,3-dihydro-1,3-dimethyl-2-oxo-1H-1,4-benzodiazepin-7-yl]-3-(2-hydroxyethyl)urea which melts above 147° C. with decomposition; [α]25D =+192.2° (in dioxan; 1%).
