a b s t r a c tRecent advances in the field of continuous flow chemistry allow the multistep preparation of complex molecules such as APIs (Active Pharmaceutical Ingredients) in a telescoped manner. Numerous examples of laboratory-scale applications are described, which are pointing towards novel manufacturing processes of pharmaceutical compounds, in accordance with recent regulatory, economical and quality guidances. The chemical and technical knowledge gained during these studies is considerable; nevertheless, connecting several individual chemical transformations and the attached analytics and purification holds hidden traps. In this review, we summarize innovative solutions for these challenges, in order to benefit chemists aiming to exploit flow chemistry systems for the synthesis of biologically active molecules.

Immobilization of transaminases creates promising biocatalysts for production of chiral amines in batch or continuous-flow mode reactions. E. coli cells containing overexpressed transaminases of various selectivities and hollow silica microspheres as supporting agent were immobilized by an improved sol-gel process to produce immobilized transaminase biocatalysts with suitable stability and mechanical properties for continuous-flow applications. The immobilized cell-based transaminase biocatalyst proved to be durable and easy-to-use in kinetic resolution of four racemic amines 1a–d. The batch and continuous-flow mode kinetic resolutions with transaminase biocatalyst of opposite stereopreference provided access to both enantiomers of the corresponding amines. By using the most suitable immobilized transaminase biocatalysts, this study describes the first transaminase-based approach for the production of both pure enantiomers of 1-(3,4-dimethoxyphenyl)ethan-1-amine 1d.

A continuous-flow system consisting of two sequential microreactors was developed for the synthesis of nonsymmetrically substituted ureas starting from tert-butoxycarbonyl protected amines. Short reaction times could be achieved under mild conditions. In-line FT-IR analytical technique was used to monitor the reaction, including the formation of the isocyanate intermediate, thus allowing optimization of the reagent ratios. The mechanistic role of the applied base was also clarified. The setup was successfully utilized for the synthesis of several urea derivatives including the active pharmaceutical ingredient cariprazine.

Immobilised whole-cell (R)-transaminases (TAs) enabled synthesis of either (R)- or (S)-enantiomers of drug-like amines from prochiral ketones or from racemic amines, respectively, in >95% ee.

In this work, we report a practical method for alkylation of saturated heterocycles with chloroacetone yielding prochiral heterocyclic ketones, including previously not described molecules. The desired building blocks were obtained with high yields in hydrochloric salt forms, without the need for chromatographic purification.