A Facile, Generally Applicable Synthesis of N‐Acetyldehydro‐α‐amino Acids

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By Franz Effenberger* and n o m a s Beisswenger Dehydroamino acids are used as educts for the synthesis of a-amino acids. Thus, a series of amino acids could be prepared e .g . by addition of acidic compounds such as malonic acid derivatives, indole, imidazole, and thiols to dehydroamino acids]. Of particular synthetic significance is the enantioselective hydrogenation of N-acyldehydroamino acids or -amino acid esters in the presence of optically active metal complexes as catalysts which produce, respectively, amino acids or amino acid esters in very high optical yield[']. Until now the known syntheses of dehydroamino acids are either involved, not generally applicable, or rely upon amino acids as starting materials[']. The recently postulated intermediary 2-iminopropionic acid ester was trapped, by careful acylation, as the N-acetyldehydroalanine ester in the alkoxide-catalyzed preparation of pyruvic acid ester from 2-azidopropionic acid ester[']. We have now obtained N-acetyl-2 and/or N, N-diacetyldehydro-a-amino acid esters 3 by warming the 2-azidocarboxylic acid esters 1 in acetic anhydride or acetic anhydride/acetic acid in presence of catalytic amounts of rhenium heptasulfide (Table 1). The azidocarboxylic acid esters 1 are almost quantitatively accessible from the corresponding a-halogen derivatives via azide exchange by a phase-transfer reaction with Aliquat 336 as catalyst. Exclusive formation of 2 can be achieved using a smaller AczO/ AcOH ratio, advantageously in presence of HCI. Work-up