Process for preparing 2-piperazinecarboxylic acid derivatives

Process for preparing 2-piperazinecarboxamides in the form of the enantiomers or their enantiomer mixtures of formula:

• ##STR13## where R.sub.1 is a) alkyl or alkyl substituted with phenyl or halogen, or b) --OR.sub.4 where R.sub.4 is alkyl, alkenyl, phenyl, naphthyl, alkyl substituted with phenyl or halogen, alkenyl substituted with phenyl or halogen, phenyl substituted with halogen or alkyl, or naphthyl substituted with halogen or alkyl, or c) --NR.sub.5 R.sub.6, where R.sub.5 is hydrogen, alkyl or alkyl substituted with phenyl or halogen, and R.sub.2 and R.sub.3 are identical or different and are hydrogen, alkyl, alkenyl, phenyl, naphthyl, alkyl substituted with phenyl or halogen, alkenyl substituted with phenyl or halogen, phenyl substituted with halogen or alkyl, naphthyl substituted with halogen or alkyl, or R.sub.3 is hydrogen and R.sub.2 is a residue of one of the essential L-.alpha.-amino acids selected from the group consisting of alanine, valine, leucine, isoleucine, proline, tryptophan, phenylalanine, methionine, glycine, serine, tyrosine, threonine, cysteine, asparagine, glutamine, aspartic acid, glutamic acid, lysine, arginine, histidine, and the (C.sub.1 -C.sub.4)-alkyl esters thereof, comprising first converting a 2-piperazinecarboxylic acid of formula:
• ##STR14## or a salt thereof, in the form of its enantiomers or its enantiomer mixtures, into an N-acyl derivative of formula:
• ##STR15## where R.sub.1 and R.sub.4 are as defined above, then cyclizing the N-acyl-derivative of the formula III in a second stage in the presence of a halogenating agent to form a piperazinecarboxylic anhydride of formula:
• ##STR16## where R.sub.1 is as defined above, and finally reacting the piperazinecarboxylic anhydride of formula IV with an amine of formula:
• ##STR17## where R.sub.2 and R.sub.3 are as defined above, to give the end product of formula I.