One Pot Cascade Synthesis of Mono- and Di-Substituted Piperidines and Pyrrolidines using Carboxylic Acid Reductase (CAR), ω-Transaminase (ω-TA) and Imine Reductase (IRED) Biocatalysts

Access to enantiomerically pure chiral mono- and disubstituted piperidines and pyrrolidines has been achieved using a biocatalytic cascade involving carboxylic acid reductase (CAR), ω-transaminase (ω-TA), and imine reductase (IRED) enzymes. Starting from keto acids or keto aldehydes, substituted piperidine or pyrrolidine frameworks can be generated in high conversion, ee, and de in one pot, with each biocatalyst exhibiting chemo-, regio-, and/or stereoselectivity during catalysis. The study also includes a systematic investigation of the effect of the position of a methyl group ring substituent on the IRED-catalyzed reduction of a chiral imine. Analysis of the selectivity observed in these reactions revealed an interesting balance between substrate versus enzyme control; the configurations of the products obtained were rationalized on the basis of minimizing 1,3- or 1,2-steric interactions with incoming NADPH.