To a solution prepared by dissolving 1,17 g (5,0 mM) 4-phtalimido-butyric acid and 0,56 g (5,5 mM) triethylamin in 20 ml chloroform 0,61 g (5,0 mm) pivaloylchlorid were dropped at −15° C. under stirring. The reaction mixture was stirred for 1 hour at the above temperature and then a solution prepared by dissolving 1,03 g (5,0 mM) L-prolyl-pyrrolidin-hydrochloric acid salt in a mixture of 5 ml chloroform and 1,5 ml (1,1 g, 11,0 mM) triethylamine were dropwise added to it. Reaction mixture was stirred at room temperature for 4 hours, then it was washed successively with water, 30% cc. citric acid solution, saturated aqueous sodium bicarbonate solution, water and with saturated sodium chloride solution. The organic phase was dried on calcinated magnesium sulfate and it was evaporated. Crystallisation of the residue from a mixture of 5 ml chloroform and 10 ml petrolether yielded 1,1 g (53%) N-(4-phtalimido-butanoyl)-L-prolyl-pyrrolidin which melted at 148-149° C. The compounds of the general formula I were synthetised by the method as explained above starting from the corresponding compounds having general Formulas (II) and (III).
