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An optimized kilogram‐scale synthetic route for pinoxaden was successfully developed. The key intermediate dimethyl 2‐(2,6‐diethyl‐4‐methylphenyl)malonate (19) was prepared from 2,6‐diethyl‐4‐methylaniline through diazotization reaction, Suzuki cross‐coupling reaction, and alcoholysis reaction in 59% yield over three steps in 97.3% purity. 1,4,5‐Oxadiazepane dihydrobromide (11) was obtained from diethylene glycol through esterification, cyclization, deprotection, and salification in 64% yield over three steps. Pinoxaden was given by aminolysis of 19 with 11, followed by acylation by pivaloyl chloride in 64% yield over two steps in 99.9% purity. Purification methods for the intermediates and the final product were developed.