A Comprehensive Study on the Synthesis of 1‐Methyl Mebendazole (USP Mebendazole Related Compound D)

Abstract 1‐Methyl mebendazole (or mebendazole related compound D, USP‐NF 2025) stands out as the only impurity to be included as a reference standard in the mebendazole quality analysis. Due to its close relative retention time to that of mebendazole, this compound serves as a reliable control for evaluating peak resolution in HPLC chromatograms and ensuring the validity of the method. It can be synthesized through a single N‐methylation of mebendazole; however, no direct data on the synthesis of this compound has been published, whilst purchasing a few milligrams is costly. This paper presents, for the first time, the synthesis of mebendazole related compound D, yielding a purity of approximately 99%. We also disclose the formation of a tautomeric isomer of this compound, along with an optimized and validated HPLC chromatographic condition that achieves excellent resolution of the isomeric mixture, whereas the recommended method in the relevant USP monograph showed no separation. The relative retention times of mebendazole, isomers 1 and 2 were determined to be 0.8, 0.9, and 1.7, respectively, compared to 1.0 of the target compound. Data on PXRD, NMR, HR‐MS, and IR are provided and serve as a useful reference for in‐house synthesis of mebendazole related compound D.