Synthesis and reactions of lactam sulfonium salts with a sulfonio bridgehead. Part 1. 4,4a,5,6-Tetrahydro-5-oxo-1H-thiopyrano[1,2-a]-1,4-be nzothiazinium perchlorates
2'-Vinyl-2H-1, 4-benzothiazin-3(4H)-one-2-spirocyclopropanes (1) were irradiated with a tungsten lamp at room temperature in the presence of a catalytic amount of diphenyl dichalcogenide to provide 1, 2-dioxolane derivatives (3) in good yields. Diphenyl diselenide was more effective than diphenyl disulfide as a radical source. The photochemical [3+2] cycloaddition of 1b with electron-deficient alkenes proceeded smoothly under reflux in benzene to give spiro-cyclopentanes (5). Spiro-cyclopentenes (6) were formed by the photochemical [3+2] cycloaddition of 1b with alkynes.
Photochemical thiylation of 2'-vinyl-2H-benzothiazine-2-spirocyclopropan-3(4H)-ones (1) to form allyl sulfides (3) was examined. Although the reactions proceeded with complete regioselectivity because of the high stabilizing ability of the capto-dative substituents, geometrical selectivity of the olefinic moiety was dependent on the substituents on the cyclopropane ring. The conformation of 1 probably plays an important role in the addition step of the thiyl radical to the double bond.