Exploring the Scopeof an α/β-Aminomutasefor the Amination of Cinnamate Epoxides to Arylserines and Arylisoserines

Biocatalytic process-development continues to advance toward discovering alternative transformation reactions to synthesize fine chemicals. Here, a 5-methylidene-3,5-dihydro-4H-imidazol-4-one (MIO)-dependent phenylalanine aminomutase from Taxus canadensis (TcPAM) was repurposed to irreversibly biocatalyze an intermolecular amine transfer reaction that converted ring-substituted trans-cinnamate epoxide racemates to their corresponding arylserines. From among 12 substrates, the aminomutase ring opened 3'-Cl-cinnamate epoxide to 3'-Cl-phenylserine 140 times faster than it opened the 4'-Cl-isomer, which was turned over slowest among all epoxides tested. GC/MS analysis of chiral auxiliary derivatives of the biocatalyzed phenylserine analogues showed that the TcPAM-transamination reaction opened the epoxides enantio- and diastereoselectively. Each product mixture contained (2S)+(2R)-anti (erythro) and (2S)+(2R)-syn (threo) pairs with the anti-isomers predominating (similar to 90:10 dr). Integrating the vicinal proton signals in the H-1 NMR spectrum of the enzyme-catalyzed phenylserines and calculating the chemical shift difference (AS) between the anti and syn proton signals confirmed the diastereomeric ratios and relative stereochemistries. Application of a (Delta delta)-threonine aldolase from E. coli further established the absolute stereochemistry of the chiral derivatives of the diastereomeric enzymatically derived products. The 2R:2S ratio for the biocatalyzed anti-isomers was highest (88:12) for 3'-NO2-phenylserine and lowest (66:34) for 4'-F-phenylserine. This showed that the stereospecificity of TcPAM is in part directed by the substituent-type on the cinnamate epoxide analogue. The catalyst also converted each cinnamate epoxide analogue to its corresponding isoserine, highlighting a biocatalytic route to arylisoserines, which play a key role in building the pharmacophore seen in anticancer and protease inhibitor drugs.