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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.

A safe and metal-free process using ethyl glycinate hydrochloride as the starting material has been developed for the preparation of ethyl 5-acetyl-1H-pyrazole-3-carboxylate, a key intermediate for the synthesis of potential blockbuster drug substance darolutamide. In the key step, the toxic and explosive intermediate, ethyl diazoacetate was generated and used in situ. Reaction parameters were optimized for both the batchwise and the continuous-flow variant of the synthesis. In the next step, alkaline hydrolysis of the ester led to 5-acetyl-1H-pyrazole-3-carboxylic acid, which can not only be used as a darolutamide intermediate, but it can also be considered as a valuable building block for other types of organic and medicinal chemistry transformations.

A new, efficient process for the synthesis of 5-acetyl-1H-pyrazole-3-carboxylic acid, a versatile building block and the key intermediate of darolutamide, using diethyl pyrazole-3,5-dicarboxylate as the starting material is described. Contrary to the synthetic routes known from the literature, this procedure does not apply explosive diazo reagents, moreover it is simple and safe, thereby suitable for scale-up.