The biocatalytic synthesis of optically pure L-and D-unnatural phenylalanines with high synthetic value is continuously providing new green synthetic procedures. Recently, by mapping the binding pocket of phenylalanine ammonia-lyase from Petroselinum crispum (PcPAL), we developed tailored variants to use as biocatalysts for the synthesis of several phenylalanine analogues [1–3].

L-p-Br-phenylalanine L-1b is an important intermediate for the production of several valuable biarylalanines [4], whilst D-p-CH3-phenylalanine D-1a has been successfully incorporated into Pin1 inhibitors [5] and into anti-inflammatory formyl-peptide receptor 1 antagonists [6]. The efficient PcPAL-mediated synthesis of these products can be achieved by performing the corresponding ammonia elimination and ammonia addition reactions (Scheme 5.6) using Escherichia coli Rosetta cells, harbouring the plasmid carrying the gene of the tailored PcPAL I460V variant, as whole-cell biocatalysts. Within the kinetic resolution-type ammonia elimination reaction, the substrate is the racemic p-CH3-phenylalanine rac-1a, from which the L-enantiomer L-1a is dehydroaminated to p-CH3-cinnamic acid 2a, yielding the unreacted D-p-CH3-phenylalanine D-1a in a maximal theoretical yield of 50%. Meanwhile, the asymmetric synthetic route of the ammonia addition reaction, using p-Br-cinnamic acid 2b as substrate, provides L-p-Br-phenylalanine L-1b in 100% theoretical yield.