A mixture consisting of phthalimide and formaldehyde contained in water was heated to give 2-(hydroxymethyl) isoindolin-1,3-dione (N-methylol phthalimide) (I), which on reaction with p-toluenesulphonic acid in presence of conc. sulphuric acid furnished 3-[(1,3-dioxoisoindolin-2-yl)methyl]-4-methyl benzenesulphonic acid (II). Reaction of (II) with benzoin in the presence of conc. sulphuric acid yielded 2-[(6-methyl-3,4-diphenyl benzo[c][1,2] oxathiin-5-yl) methyl] isoindolin-1,3-dioxo-1,1-dioxide (III) which on reaction with a primary amine in pyridine gave the target compounds i.e. 2-[(6-methyl-2-aryl-3,4-diphenyl-2H-benzo[e][1,2] thiazin-5-yl) methyl] isoindolin-1,3-dioxo-1,1-dioxides (IV a-d) in the yields varying from 50 to 65%.