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.
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
Tricyclic benzothiazinium salts 3 are prepared by
[2++4] polar cycloaddition of thionium intermediates 2A,
generated from the corresponding α-chloro sulfides 2, and dienes
in the presence of silver perchlorate. Ring transformation of
benzothiazinium salts 3 with a reducing agent such as Mg,
NaBH4 and Zn–AcOH or with a base, furnishes
spiro-vinylcyclopropane derivatives 4 in moderate to high yields.
Electrolysis of 3a at -1.4 V vs. SCE in acetonitrile also
affords vinylcyclopropane 4a (60%). These results indicate that both
ionic and radical mechanisms may account for the vinylcyclopropane
formation, although it is unclear as to the nature of the radical
intermediate. The stereochemistry of 4a was determined by X-ray analysis
showing that sulfur and the vinyl group are cis-orientated.
Ten-membered lactam sulfides 6 are obtained as the major product of
SmI2 reduction of 3.
Reactions of 2'-vinyl-2H-benzothiazine-2-spirocyclopropan-3(4H)-ones (1) with several proton acids were examined. Reactions of 1 with HCl and HBr predominantly gave (Z)-allyl halide derivatives (2). In the cases of HClO4 and HBF4 4, 4a, 5, 6-tetrahydro-5-oxo-1H-thiopyrano[1, 2-a]-1, 4-benzothiazinium salts (3) were isolated. Allyl halides (2) were also obtained by treatment of the salts (3) with HCl and HBr.