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 degrees 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 acetinidate 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 extraction. Alkylation of 18 with 3-bromopropyl methyl ether afforded 8b, which was converted as above, via 11b, to AL-4862 (13b), These procedures provided multihundred gram lots of 13a and 13b.