Synthesis of each stereoisomer of [3-2H1]phenylalanine and evaluation of the stereochemical course of the reaction of (R)-phenylalanine with (S)-phenylalanine ammonia-lyase

Abstract

The four stereoisomers of [3-²H₁] phenylalanine have been prepared, each as a single enantiomer in ca. 98% diastereoisomeric excess and with ca. 99% deuterium incorporation, by side-chain bromination of phenylalanine derivatives, followed by deuteriolysis of each of the diastereoisomeric product bromides with deuterium over 5% palladium-on-carbon. The latter reactions proceeded with retention of configuration. (2R, 3S)-[3-²H₁]Phenylalanine reacted with (S)-phenylalanine ammonia-lyase to give [3-²H₁]-trans-cinnamic acid, with 92% deuterium incorporation, while the (2R, 3R)-stereoisomer of the deuteriated phenylalanine gave [3-²H₁]-trans-cinnamic acid with 27% deuterium incorporation. These results indicate that reaction of (R)-phenylalanine with the enzyme involves mainly loss of the 3-pro-R hydrogen and ammonia, in an antiperiplanar elimination process analogous to that previously reported for (S)-phenylalanine, while a minor pathway for reaction of (R)-phenylalanine is either isomerization to (S)-phenylalanine, before elimination, or snnperiplanar elimination.