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A novel flow-based approach for the preparation of benzimidazol-2-one ( 1 ) scaffold by the 1,1'-carbonyldiimidazole (CDI)-promoted cyclocarbonylation of o -phenylenediamine ( 2 ) is reported. Starting from a preliminary batch screening, the model reaction was successfully translated under flow conditions and optimised by means of design of experiment (DoE). The method allowed the efficient preparation of this privileged scaffold and to set up a general protocol for the multigram-scale preparation in high yield, purity, and productivity, and was successfully applied for the multigram flow synthesis of N -(2-chlorobenzyl)-5-cyano-benzimidazol-2-one, which is a key synthon for hit-to-lead explorations in our anti-inflammatory drug discovery program.