A Convenient Synthesis of 2H-1,2,4-Benzothiadiazine 1,1-Dioxides from 2-Aminobenzenesulfonamide and Carboxylic Acids by using Polyphosphoric Acid Trimethylsilyl Ester as Condensing Agent - Imai, Y; Mochizuki, A; Kakimoto, M (Synthesis, 1983)