Stereoselective, catalytic strategies for the in-flow synthesis of APIs performed in micro- and (3D-printed) meso- reactors

In the last few years, continuous-flow systems1 have become very popular as a powerful tool for performing synthesis of organic molecules. In this context, we explored the use of different micro- and mesoreactors to perform stereoselective organocatalyzed reactions under continuous flow conditions for the synthesis of Active Pharmaceutical Ingredients2 (Scheme 1). In these studies, we employed and compared traditional glass microreactors with different PEEK (Polyetheretherketone) and PTFE (Polytetrafluoroethylene) home-made micro-and mesoreactors. Scheme 1 The same approach was applied to the synthesis in flow of chiral 1,2-amino alcohols displaying biological activities such as Norephedrine and Metaraminol using 3D-printed reactors (scheme 2).3 These devices were designed, fabricated from different materials (PLA, HIPS, NYLON), and used in a catalytic stereoselective Henry reaction. The use of readily prepared and tuneable 3D-printed reactors allowed for a rapid screening of devices with different sizes, shapes and channel dimensions, aimed at the identification of the best performing reactor set up. The optimized process afforded products in high yields, moderate diasteroselectivity and up to 90% e.e. through two-steps, all-in-flow sequence that involves, after the nitroaldol reaction, a continuous flow hydrogenation without any intermediates purification or solvent switching operations.