A Practical and General Amidation Method from Isocyanates Enabled by Flow Technology

Amide Bonds Meet Flow Chemistry: A Journey into Methodologies and Sustainable Evolution

Formation of amide bonds is of immanent importance in organic and synthetic medicinal chemistry. Its presence in "traditional" small-molecule active pharmaceutical ingredients, in linear or cyclic oligo- and polypeptidic actives, including pseudopeptides, has led to the development of dedicated synthetic approaches for the formation of amide bonds starting from, if necessary, suitably protected amino acids. While the use of solid supported reagents is common in traditional peptide synthesis, similar approaches targeting amide bond formation in continuous-flow mode took off more significantly, after a first publication in 2006, only a couple of years ago. Most efforts rely upon the transition of traditional approaches in flow mode, or the combination of solid-phase peptide synthesis principles with flow chemistry, and advantages are mainly seen in improving space-time yields. This Review summarizes and compares the various approaches in terms of basic amide formation, peptide synthesis, and pseudopeptide generation, describing the technological approaches and the advantages that were generated by the specific flow approaches. A final discussion highlights potential future needs and perspectives in terms of greener and more sustainable syntheses.

A Modular, Argon-Driven Flow Platform for Natural Product Synthesis and Late-Stage Transformations

A modular flow platform for natural product synthesis was designed. To access different reaction setups with a maximum of flexibility, interchangeable 3D-printed components serve as backbone. By switching from liquid- to gas-driven flow, reagent and solvent waste is minimized, which translates into an advantageous sustainability profile. To enable inert conditions, "Schlenk-in-flow" techniques for the safe handling of oxygen- and moisture sensitive reagents were developed. Adopting these techniques, reproducible transformations in natural product synthesis were achieved.

Continuous Flow Synthesis of ACE Inhibitors From N-Substituted l-Alanine Derivatives

A strategy for the continuous flow synthesis of angiotensin converting enzyme (ACE) inhibitors is described. An optimization effort guided by in situ IR analysis resulted in a general amide coupling approach facilitated by N-carboxyanhydride (NCA) activation that was further characterized by reaction kinetics analysis in batch. The three-step continuous process was demonstrated by synthesizing 8 different ACE inhibitors in up to 88 % yield with throughputs in the range of ≈0.5 g h^-1 , all while avoiding both isolation of reactive intermediates and process intensive reaction conditions. The process was further developed by preparing enalapril, a World Health Organization (WHO) essential medicine, in an industrially relevant flow platform that scaled throughput to ≈1 g h^-1 .