Synthesis of Isotopically Labeled, Enantiomerically Pure Compounds

2 Synthesis of Isotopically Labeled, Enantiomerically Pure Compounds 

A relatively large number of different enzymes have been used to prepare enantiomerically pure isotopically labeled compounds. 

Use of aminotransferases Aminotransferases are pyridoxal 5 -phosphate-dependent enzymes that catalyze the reversible transfer of an amino group from an a-amino acid to an a-ketoacid in a two-step process. Of the many transaminases, aspartate aminotransferase (EC 2.6.1.1) is synthetically most useful since spontaneous decarboxylation of the generated oxaloacetate to pyruvate shifts the equilibrium to the product side. 

Glutamic-pyruvic transaminase from porcine heart (EC 2.6.1.2) was used for the synthesis of (5)-[4- C]glutamic acid (18) from labeled 2-oxo-glutarate (17) and (5)-alanine (16) as an amino group donor . 

The combination of transaminases with amino acid oxidases opens oppormnities for selectively inverting chiral centers. An example of such an operation is in the synthesis of (5)-[l- C]valine (20). Racemic [l- C]valine, produced by Strecker synthesis from isobutyraldehyde, K CN and ammonia in 83% yield, was treated with (R)-amino oxidase and branched-amino acid amino transferase in order to invert the (R)-enantiomer into the (5)-enantiomer via the a-ketoacid 19. Addition of a substantial excess of glutamic acid drove the aminotransferase reaction to completion, giving (5)-[ l- C] valine in 75% overall yield from K CN on a 30 mmol scale. Due to the low substrate specificity of the enzymes, it is anticipated that this approach will also be effective for most standard amino acids.  

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