In succession, 2.2 g (20 mmole) of triethylamine and a solution of 1.4 ml (10 mmole) of isobutyl chloroformate in 20 ml of dry methylene chloride are added dropwise to a solution, cooled to -10°, of 2.82 g (10 mmole) of 4-p-nitrobenzyloxycarbonylaminobutyric acid in 50 ml of dry methylene chloride. The reaction mixture is stirred for one hour and then a strong current of hydrogen sulphide is conveyed through for 2 hours. After acidifying with 2N sulphuric acid, the organic phase is separated off, dried and concentrated by evaporation in vacuo. The resulting 4-p-nitrobenzyloxycarbonylaminothiobutyric acid can be further processed without further purification.
