Cycloaddition. XVIII. Electrophilic Addition of Chlorosulfonyl Isocyanate to Ketones. Reaction with Aromatic Ketones

Electrophilic addition of chlorosulfonyl isocyanate (CSI) to “aromatic” ketones 1 (R¹COCH₂R², R¹ and/or R² = aryl) was found to be a synthetically versatile reaction, producing 2H-1,3-oxazine-2,4(3H)-diones (6) when dichloromethane was the solvent and 1,2,3-oxathiazin-4(3H)-one 2,2-dioxides (4) as well (if R′ = aryl) in diethyl ether solvent. A number of additional products are observed in low yield. An examination of substituent effects and nmr studies indicates that the determining factor in the oxathiazineroxazine ratio is the position of the keto-enol equilibrium in the intermediate N-chlorosulfonyl-β-ketocarboxamide, 2. After the initial formation of 2, the reaction takes one of the following routes. (1) Asecondelectrophilic addition to the enol can occur to produce a malonamide derivative 23, as evidenced by the isolation of 23a and 23f under appropriate conditions. In solution, 23 readily eliminates CSI to regenerate 2. This sequence can serve as a synthetic route from ketones to malonamides and malononitriles. (2) CSI can act as a Lewis acid upon 2, abstracting a chloride to eventually produce an N-sulfonylamine 15 which then cyclizes to 4. (3) Reaction of CSI with the enol form of 2 can produce the enol carbamate 25. Cyclization of this intermediate with loss of sulfamoyl chloride then leads to 6. Evidence supporting the proposed overall mechanism is provided.