Development of a Scalable Electrophilic Amination Protocol for the Multi-kg Production of 5-Methyl-2-pyridinesulfonamide: A Regulatory Starting Material of Endothelin Receptor Antagonist Clazosentan

5-Methyl-2-pyridinesulfonamide is a regulatory starting material of endothelin receptor antagonist clazosentan. The original route to the key sulfonamide relied on the textbook conversion of the corresponding thiophenol to the intermediate sulfonyl chloride followed by its quenching with aqueous ammonia. However, this route suffered from a wide range of issues such as a low overall yield (29%), challenging aqueous workups and isolations, and the formation of a genotoxic benzyl chloride impurity. Therefore, we developed a conceptually novel production route for 5-methyl-2-pyridinesulfonamide. The new process relied on selectively oxidizing the thiophenol to the intermediate sulfinate salt followed by an electrophilic amination of the nucleophilic sulfinate sulfur-atom with hydroxylamine-O-sulfonic acid (HOSA). This oxidation/electrophilic amination sequence worked as a "one-pot" procedure by simply adding HOSA to the reaction mixture after complete oxidation of the thiophenol with 70% aq. t-BuOOH. The process was extensively optimized with regard to the oxidation step, increasing the stability of HOSA in the reaction mixture, and the final isolation of 5-methyl-2-pyridinesulfonamide. The new process was performed on a 22 kg scale, delivering the desired product as a white solid in 69% overall yield and excellent purity (>99.9% a/a).