Amino-Esters and Related Compounds

Typical a-amino esters only undergo racemization slowly, but methods for accelerating this process have been devised137 381. Temporary conversion of the amine to an imine lowers the pKa of the substrate, such that racemization becomes faster. 

A series of a-aminoesters has been hydrolyzed to a-amino acids using alcalase in the presence of pyridoxal 5-phosphate[40). During the course of these reactions, the amino acids precipitated from the reaction mixture, thereby protecting them from racemisation. The method was used to prepare enantiomerically enriched phenylalanine, leucine, tryptophan and norvaline with high selectivity (Fig. 9-19). 

A related ammonolysis of an amino ester has been reported using either pyridoxal or salicylaldehyde as the racemizing agent1411. The amino ester undergoes racemization more quickly than the amino amide, and an effective dynamic resolution could be achieved at -20 °C. 

Pre-formed imino-esters have also been used as substrates for dynamic kinetic resolution reactions[42). The free amino acid precipitated from the reaction mixture as the reaction proceeded. 

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Amino acids, amino esters, and related compounds, a-Amino acids (Continued)... 

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