Continuous-flow catalytic, stereoselective synthesis in micro- and meso- reactors

Continuous-flow systems[1] have recently emerged as a powerful tool for performing chemical transformations, since they ensure some advantages over traditional batch procedures, such as faster reactions, safer reactions, quick reaction optimization and easy scale-up. However, the application of continuous-flow systems to the stereoselective synthesis of chiral multifunctional molecules is still underdeveloped. In this context, we explored the use of different micro- and mesoreactors to perform stereoselective organocatalyzed reactions under continuous flow conditions. These studies were performed employing commercial traditional glass microreactors but also with PEEK (Polyetheretherketone) and PTFE (Polytetrafluoroethylene) HPLC tubing-based mesoreactors. Different in continuo organocatalytic synthesis of Active Pharmaceutical Ingredients (APIs) have been developed. The synthesis of a direct precursor of (S)-Baclofen in flow was realized by performing a metal-free stereoselective addition of activated nucleophiles to β-nitrostyrenes promoted by a bifunctional Takemoto type catalyst.[2] Optimization of the experimental setup gave excellent enantioselectivities (up to 85% ee) and higher productivities if compared to the flask syntheses. The potential of this flow chemistry approach was demonstrated by the successful synthesis of other APIs such as (S)-Pregabalin intermediate and (S)-Warfarin [3]. Moreover, continuous flow synthetic methodologies can also be easily combined to other enabling technologies, such as 3D-printing. This technology allows chemists to build devices with high precision and well-defined architecture.[4] The combination of these two technologies in organic synthesis opens new and intriguing possibilities such as the fabrication of ad hoc designed-reactors. In this context, an in-house designed and 3D-printed reactionware device was realized and employed for the catalytic in flow synthesis of a Metaraminol, with good diastereo- and enantioselectivities.