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.

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Continuous‐flow multistep synthesis is combined with quasi‐continuous final‐product purification to produce pure products from crude reaction mixtures. In the nucleophilic aromatic substitution of 2,4‐difluoronitrobenzene with morpholine followed by a heterogeneous catalytic hydrogenation, the desired monosubstituted product can be continuously separated from the co‐ and by‐products in a purity of over 99 % by coupling a flow reactor sequence to a multiple dual‐mode (MDM) centrifugal partition chromatography (CPC) device. This purification technique has many advantages over HPLC, such as higher resolution and no need for column replacement or silica recycling, and it does not suffer from irreversible adsorption.

Centrifugal partition chromatography (CPC) is an emerging separation technology in pharmaceutical and natural product purifications, reaching recently industrial-scale solutions. Because both the stationary and mobile phases are liquids in CPC, the solvent consumption of such separation processes is considered significant. Thus, automation is highly required in the preparation and recycling of the solvent mixtures for economical (i.e., continuous) CPC operation. Here, we report a feasible solution for this industrial issue, including the concept, algorithm, and instrumentation of solvent system handling. For majority of ternary solvent systems used in CPC separations, the linear correlation between the density and composition of the phases was recognized and utilized during the continuous solvent recycling. Hence, an efficient density-based composition adjustment algorithm was established in a mixer-settler unit using Coriolis flow meters for precise density monitoring of the upper and lower phases of the biphasic liquid system (BLS). In addition, a complete cascade consisting of a buffer, waste, a recycler (evaporator), and mixer-settler units was designed around the industrial-scale CPC device. The proof of concept was demonstrated by sequential industrial-scale CPC separations of a binary model mixture in the n-hexane/methanol/water (5/4/1, v/v/v) solvent system, followed by the purification of a crude steroid active pharmaceutical ingredient performed in the methyl isobutyl ketone/acetone/water (2/2/1, v/v/v) solvent system. The reproducibility of chromatographic performance and the productivity of the mixer-settler and evaporator units were satisfactory, enabling a robust and continuous operation of the developed separation process.

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