Development and Process Intensification of an Efficient Flow–Cascade Reaction Sequence in the Synthesis of Afizagabar

Aromatic nitration and catalytic hydrogenation are among the most dangerous reactions in the chemical industry. The traditional, batchwise pilot plant manufacturing process of a key intermediate of our drug candidate afizagabar (S44819) involved these kinds of transformations (besides a Dakin–West-type reaction, a ring closure, and a keto reduction step). To mitigate some of the hazards associated with this sequence, a flow chemical approach was developed. First, a flow–cascade process was elaborated, which furnished the product with a throughput of 1.52 g/h with an HPLC purity of 95.6%. The bottleneck of the procedure in terms of output was the heterogeneous catalytic hydrogenation; therefore, our subsequent process intensification efforts primarily concentrated on this step. Finally, application of higher concentrations and an upscaled hydrogenation reactor combined with the corresponding adjustment of parameters of further reaction steps resulted in an efficient process with an effective product yield of 11.95 g/h and an increased HPLC purity (97.1%). The 4-step uninterrupted process described here is based on a newly developed heterogeneous flow reactor system and a custom-made liquid–liquid extractor, providing an instructive case study on handling hazardous processes in a safe and efficient way.