Process Development and Crystallization in Oiling-Out System of a Novel Topical Antiandrogen

An efficient route to (S)-N-(2-bromo-6-methoxypyridin-4-yl)-2-hydroxy-2,4-dimethylpentanamide 1, a new topical antiandrogen, is described. The target compound has been manufactured on kilogram scale with an overall yield of 25% (HPLC purity 98.8% and >99% ee) from citrazinic acid. The key amide coupling between aminopyridine 4 and alpha-hydroxy-acid 6 was performed using a temporary protecting group to facilitate the acyl chloride formation. Aminopyridine 4 was manufactured from commercially available citrazinic acid via dibromide formation using phosphorus(V) oxybromide followed by mono SNAr reaction with sodium methoxide and a final Hofmann rearrangement. Enantiopure alpha-hydroxy-acid 6 was obtained using an enantioselective cyanosilylation followed by salt resolution with (S)-alpha-methyl benzylamine. The absolute configuration of compound 1 was determined with anomalous scattering and the final crystallization of API was performed after seeding a liquid-liquid mixture below the monotectic temperature and afforded a crystalline powder presenting a desert rose shape clusters.