Simple and robust method for the synthesis of Metopimazine by utilising smile’s rearrangement. Elimination of genotoxic impurities via derivatization

A simple, efficient and scalable methodology has been developed for the synthesis of Metopimazine, a dopamine D₂–receptor antagonist. Starting from nitrophenylthio substituted methylsulfonyl benzenamine subjected to simple N–formylation using formic acid followed by base–catalysed cyclization resulted the formation of 2–(methylsulfonyl)–10H–phenothiazine. This method does not involve the conventional peracid oxidation that exemplifies the formation of several oxidized species that are tedious to separate. The later derivative is proven effective in the synthesis of title active pharmaceutical ingredient via N–chloroproylation followed by treating piperidine–4–carboxamide resulted in the formation of Metopimazine in high yield. The potential genotoxic impurities such as nitroaromatic derivatives were effectively removed by reduction using simple zinc dust formic acid treatment, which effectually converts nitro derivatives into amine hydrochlorides that are water soluble. Similarly, the N–alkylation of phenothiazine derivative forms potential impurities such as the formation of N–allyl and propyl bridged dimer derivatives that are removed by effective recrystallization. This method has been scaled up and evidenced no compromise with the yield and hence can be considered as industry ready.