Aromatic heteroannulation via metalation-cyclization of N-acyl-2-chlorobenzenesulfonamides and N-acylbenzenesulfonamides

N-Acyl-2-chlorobenzenesulfonamides 7a-d undergo competitive metal-halogen exchange and or alpha-deprotonation reactions when treated sequentially with sodium hydride and n-butyllithium. The o-lithio intermediates derived from metal-halogen exchange and ortho-deprotonation undergo cyclization to afford 3-substituted 1,2-benzisothiazole 1,1-dioxides 10a-d and 3-substituted 7-chloro-1,2-benzisothiazole 1,1-dioxides 14a-d, respectively. Reaction time-temperature data show evidence for the slow conversion of the lateral dianion of N-acetyl-2-chlorobenzenesulfonamide (7a) to the corresponding N,ortho-dialkali salt. 1,2-Benzisothiazole 1,1-dioxides 14a-d were obtained in good yield and free from products resulting from metal-halogen exchange by treatment of sulfonamides 7a-d with 2 equiv of LDA in THF. In the presence of 2 equiv of LDA, N-acylbenzenesulfonamides devoid of or containing only weakly acidic alpha-hydrogens undergo alpha-deprotonation-cyclization to afford the respective 1,2-benzoisothiazole 1,1-dioxides.