Chemo-enzymatic asymmetric flow synthesis of piperidine as pharmaceutically relevant 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 necessitate 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 synthesis of different alkaloids4,5. 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), to synthesize natural (-)-pinidinone (5a). First, two chemical steps were performed in batch to prepare the desired substrate, i.e., an oxidation reaction and a Wittig reaction 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. The transamination reaction was then optimized in a continuous flow system, achieving full conversion of the substrate into the two desired diastereoisomers, adopting HEPES buffer 100 mM pH 7.5 as flow stream. Parameters such as substrate concentration, isopropylamine equivalents, reaction temperature, residence time, type and amount of cosolvent were evaluated. Then, the protocol was extended to different substrates to isolate differently 2,6-disubstituted chiral piperidines.