Chemo-enzymatic flow synthesis of chiral piperidine scaffolds

Chiral amines are important building blocks in 40–45% of small molecule pharmaceuticals, as well as in numerous industrially important fine chemicals and agrochemicals. Furthermore, environmental regulations and the increasing demand for enantiopure compounds as high-value products for different sectors require the integration of traditional chemical synthetic methods with greener (bio)catalytic approaches. Since nitrogen-containing heterocycles represent a privileged structure in many APIs, in this work we focused our attention on the asymmetric synthesis of enantiopure piperidines as high-value scaffolds for the preparation of different alkaloids. A pyridoxal 5'-phosphate (PLP)-dependent transaminase (ATA-117) was selected and immobilized to perform a stereoselective transamination, followed by a spontaneous intramolecular aza-Michael reaction (IMAMR, Scheme 1), to synthesize natural (-)- pinidinone. First, two chemical steps were performed in batch to prepare the desired substrate 1, namely an oxidation reaction followed by a Wittig olefination using commercially available ylides. Then, after expressing and purifying the enzyme, various trials were conducted to immobilize the (R)-selective biocatalyst. Eupergit®C was chosen as the carrier for the covalent immobilization of ATA-117 to enhance its operational stability and reusability. Afterwards, the reaction was optimized in a continuous flow system evaluating substrate concentration, isopropylamine equivalents, reaction temperature, residence time, type and amount of cosolvent. The protocol was extended to different substrates to isolate various 2,6-disubstituted chiral piperidines.