3,4-Diaminopyridine (I) is condensed with 2-methoxy-4-methylthiobenzoic acid (II) in the presence of phosphorus oxychloride (with or without pyridine cosolvent) to provide the sulfide LY-137150 (III), which is selectively oxidized to LY-175326 by low-temperature reaction with 3-chloroperoxybenzoic acid. The sulfone LY-16325 (IV) is obtained by oxidation of (III) with 40% peracetic acid in methanol.