Enantioselective Synthesis of Brinzolamide (AL-4862), a New Topical Carbonic Anhydrase Inhibitor. The “DCAT Route” to Thiophenesulfonamides

A large scale synthesis of the topical carbonic anhydrase inhibitors AL-4623A (13a·HCl) and AL-4862 (13b) from 3-acetyl-2,5-dichlorothiophene (“DCAT”, 1) is described. Reaction of 1 with NaSBn gave thioether 2, which was converted via sulfenyl chloride 3 and sulfenamide 5 to sulfonamide 6. Bromination of 6 gave bromo ketone 7, which upon reduction with (+)-B-chlorodiisopinocampheylborane and cyclization of the resulting bromohydrin produced S thieno[3,2-e]-1,2-thiazine 8a (96% ee) after chromatography. Treatment of 8a in THF with n-BuLi at −70 °C resulted in Li−Cl exchange. Reaction of the thienyllithium with SO2 and hydroxylamine O-sulfonic acid afforded bis-sulfonamide 11a. Protection of 11a as the acetimidate 12a, followed by tosylation and amination, gave R amine 13a. The synthesis of 13b proceeded via primary sulfonamide 16, which was brominated, reduced, and cyclized to give S thieno[3,2-e]-1,2-thiazine 18 (>98% ee). By virtue of the ionizable NH, 18 was separable from reduction byproducts by base e...