Synthesis of 3,4-dihydro-2,l-benzothiazine 2,2-dioxide derivatives

An improved synthesis of 3,4-dihydro-2,l-benzothiazine 2,2-dioxide was reported by Togo and co-workers using photochemical conditions OOJOC8391 OOJOC926 . Treatment of A-alkyl 2-(aryl)ethanesulfonamides 18 with (diacetoxyiodo)arenes under irradiation with a tungsten lamp at 20-30 °C afforded 2,1-benzothiazines 19 and 20. Chemical yields and selectivities were dependent upon the choice of solvents and the reactant s substituents 18 (Table 1). When THF and EtOH were used as solvents, the reactions failed to give the cyclized products, since their a-hydrogen was abstracted by the intermediate sulfonamidyl radical. Compound 20 was obtained as a major product when 1,2-dichloroethane was employed as a solvent. In contrast, in the case of EtOAc as solvent, compound 19 was obtained as the major product. 

In 2003, Togo and co-workers described a radical cyclization and ionic cyclization onto the aromatic rings of 2-(aryl)ethanesulfonamides 21 to produce 3,4-dihydro-2,l-benzothiazine 2,2-dioxides with polymer-supported hypervalent iodine reagents in good yields 03ARK11 . 

---