the following examples illustrate the method of the invention in more detail : 50 mmoles ( 9 . 3 g ) ala - pro were suspended in 100 ml ethanol , mixed with 10 moles ( 1 g ) triethylamine , 75 mmoles ( 15 . 5 g ) 2 - oxo - 4 - phenyl - butyric acid ethyl ester and 18 g molecular sieve 3 a and agitated for 2 hours under anhydrous conditions . one teaspoon of dehydrated raney nickel and 15 g molecular sieve 3 a were now added , briefly reacted and subsequently hydrogenated for 22 hours under an atmosphere of 4 bars of h 2 . the reaction mixture was filtered and evaporated to concentrate it to a small volume . conversion ( according to hplc ) relative to ala - pro used : 95 . 6 %, product yield ( according to hplc ): 95 %. the residue was taken up in 250 ml ethanol and adjusted to ph 4 . 25 with conc . hcl ( an aliquot was removed for this purpose , diluted with water and the ph adjusted , then the entire amount of hcl required was calculated ). the solution was evaporated to low bulk under reduced pressure at 40 ° c . to 50 g , filtered with celite and diluted to 100 ml total volume with ethyl acetate . 5 . 2 g maleic acid dissolved in ethyl acetate + ethanol were added and the mixture was seeded with enalapril maleate . the crystallizate formed was separated off and dried . yield : 20 . 9 g = 85 % of theory & gt ; 99 % sss diastereomer , melting point : 145 - 146 ° c ., [ α ] d 25 : 42 ° ( c = 1 in meoh ). 50 moles ( 9 . 3 g ) ala - pro were suspended in 100 ml ethanol , combined with 75 moles ( 15 . 5 g ) 2 - oxo - 4 - phenyl - butyric acid ethyl ester and 18 g molecular sieve 3 a and agitated for 2 hours under anhydrous conditions . one teaspoon of dehydrated raney nickel and 15 g molecular sieve 3 a were now added , briefly reacted and subsequently hydrogenated for 22 hours under an atmosphere of 4 bars of h 2 . the reaction mixture was filtered and evaporated to concentrate it to a small volume . conversion ( according to hplc ) relative to ala - pro used : 76 . 7 %, product yield ( according to hplc ): 35 %. the residue was taken up is 250 ml ethanol and adjusted to ph 4 . 25 with conc . hcl ( an aliquot was removed for this purpose , diluted with water and the ph was adjusted , then the entire amount was calculated ). the solution was evaporated to low bulk under reduced pressure at 40 ° c . to 50 g , filtered by celite and diluted to 100 ml total volume with ethyl acetate . 5 . 2 g maleic acid dissolved in ethyl acetate + ethanol were added and seeded with enalapril maleate . the crystallizate formed was separated off and dried . yield : 7 . 4 g = 30 % of theory ,& gt ; 99 % sss diastereomer , melting point : 145 - 146c , [ α ] d 25 : 42 ° ( c = 1 in meoh ). the same procedure was used as in example 1 ; however , instead of triethylamine , 0 . 8 molar equivalent of ethanolamine were added as the base . conversion ( according to hplc ) relative to ala - pro : 85 %, product yield ( according to hplc ): 85 %, diastereoisomer ratio before crystallization : sss : rss isomer = 88 . 12 150 mmoles h - n . sup . ε - tfa - lys - pro - oh ( 50 . 9 g ) were taken up in 120 ml n - butanol and approximately 50 ml n - butanol distilled off under vacuum . the solution was diluted with 500 ml ethanol . 22 . 5 g molecular sieve 3 a , 34 g 2 - oxo - 4 - phenyl butyric acid ethyl ester and 30 mmoles triethylamine ( 4 . 2 ml ) are added and the mixture agitated 2 hours under anhydrous conditions . three teaspoons dehydrated raney nickel were now added and the mixture was agitated for 4 hours at under an atmosphere of 4 bars of h 2 . after this time , the pressure was released , another 12 . 4 g 2 - oxo - 4 - phenyl butyric acid ethyl ester was added and the mixture was reagitated for 2 hours . then a pressure of 4 bars of hydrogen was applied and the mixture was hydrogenated a further 4 hours at this pressure . the catalyst was separated off ( it can be directly reused ) and the solution was evaporated to a small volume . conversion ( according to hplc ) relative to n . sup . ε - tfa - lys - pro : approximately 95 %, product yield ( according to hplc ): 90 %, diastereomer ratio : sss : rss - isomer = 95 . 5 . the residue was suspended in 450 ml water and 450 ml 1 , 1 , 1 - trichloroethane at 5 ° c ., the ph was adjusted to 9 . 6 in the aqueous phase with 50 % naoh and the phases were thoroughly mixed . the organic phase was separated off and the aqueous product phase was post - extracted with 100 ml 1 , 1 , 1 - trichloroethane . the ph of 4 . 6 was now adjusted to 5 in the aqueous phase and this phase was extracted twice with 400 ml 1 , 1 , 1 - trichloroethane . the organic phase was evaporated to low volume and taken up in 250 ml methyl - tert . butyl ether . the solution was cooled down to 0 ° c . and seeded . yield : 48 g ( 60 % of theory ), sss diastereomer content : & gt ; 99 %, [ α ] d 20 :- 25 . 5 ° [( c = 1 in meoh / · 1n hcl ( 1 : 1 )]. the same procedure was used as in example 4 ; however , instead of triethylamine , 0 . 2 molecular equivalent of tributylamine were added as the base . conversion ( according to hplc ) relative to n . sup . ε - tfa - lys - pro : 85 %, product yield ( according to hplc ): 75 %, diastereomer ratio before crystallization : sss : rss diastereomer = 96 . 4 : 3 . 6 . the same procedure was used as in example 4 ; however , instead of triethylamine , 0 . 8 molar equivalent ethanol amine were added as base . conversion ( according to hplc ) relative to n . sup . ε - tfa - lys - pro : 75 %, product yield ( according to hplc ): 65 %, diastereomer ratio before crystallization : sss : rss diastereomer = 93 : 7 . the same procedure was used as in example 4 ; however , instead of triethylamine , 0 . 2 molar equivalent l - prolinol were added as base . conversion ( hplc ) relative to n . sup . ε - tfa - lys - pro : 85 %, product yield ( hplc ): 65 %, diastereomer ratio before crystallization : sss : rss diastereomer = 95 : 5 . the same procedure was used as in example 4 ; however , instead of triethylamine , 0 . 2 molar equivalent ( 1r , 2s )-(-)- ephedrine were added as base . conversion ( according to hplc ) relative to . sup . ε - tfa - lys - pro : 84 %, product yield ( according to hplc ): 70 %, diastereomer ratio before crystallization : sss : rss diastereomer = 96 . 7 : 3 . 3 . the same procedure was used as in example 4 ; however , 0 . 1 molar equivalent triethylamine were added as base . conversion ( according to hplc ) relative to n . sup . ε - tfa - lys - pro : 75 %, product yield ( hplc ): 60 %, diastereomer ratio before crystallization : sss : rss diastereomer = 97 . 7 : 2 . 3 . the same procedure was used as in example 4 ; however , instead of molecular sieve and base , 25 g basic activated aluminum oxide were added . conversion ( according to hplc ) relative to n . sup . ε - tfa - lys - pro : 95 %, product yield ( hplc ): 80 %, diastereomer ratio before crystallization : sss : rss diastereomer = 95 . 5 : 4 . 5 . the same procedure was used as in example 4 ; however , instead of raney nickel , palladium 1 % on carbon was used as catalyst . conversion ( according to hplc ) relative to n . sup . ε - tfa - lys - pro : 90 %, product yield ( according to hplc ): 80 %, diastereomer ratio before crystallization : sss : rss diastereomer = 62 : 38 . 250 mmoles l - alanine benzyl ester p - toluene sulfonic acid salt ( 88 g ) and 312 . 5 mmoles 2 - oxo - 4 - phenyl butyric acid ethyl ester ( 65 g ) were suspended in 500 ml cyclohexane . after the addition of 250 mmoles n - methylmorpholine ( 25 . 3 g ), water was removed by azeotropic distillation until the end of the reaction . after the mixture had cooled , it was filtered to remove salts , evaporated to low volume and taken up in 400 ml anhydrous methanol . after the addition of 25 mmoles n - methylmorpholine ( 1 g ) and 3 teaspoons anhydrous raney nickel , the mixture was hydrogenated at 4 bars excess hydrogen pressure and room temperature until the adsorption of hydrogen stopped . the reaction mixture was filtered from the catalyst , post - washed with warm methanol and evaporated to low volume . the residue was partitioned at 5 ° c . and ph 9 . 6 between water and 1 , 1 , 1 - trichloroethane . the aqueous phase was subsequently extracted at ph 3 and 5 ° c . with 1 , 1 , 1 - trichloroethane and the organic phase evaporated to low volume . the residue was crystallized from ethanol or acetone while cold .
n - substituted α - amino acids and dipeptides are valuable intermediate products for the production of inhibitors of the angiotensin converting enzyme ; representatives are e . g . enalapril and ramipril . these compounds are prepared by means of hydrogenolytic conversion of a primary amine with a ketone with the addition of an organic or inorganic base ; among the inorganic bases , basic aluminum oxide is preferred . the use of chiral organic bases favors the production of a diastereomer .