1.2- Benzisothiazoles rearrangements

There are many related examples which are now known as the general Dimroth rearrangement. For example, 3-ethylamino-l,2-benzisothiazole (419) is in equilibrium in aqueous solution with the 2-ethyl-3-imino isomer (420) <72AHCf 14)43). Dimroth rearrangements are known in the 1,2,4-thiadiazole series (421- 422), and in the 1,3,4-thiadiazole series as products of reactions of halogeno-l,3,4-thiadiazoles see Section 4.02.3.9.1 <68AHC(9)165). For a similar example in the 1,2,3,4-thiatriazole series, see Section 4.02.3.1.9. 

An unusual thermal rearrangement of 3-allyloxy-l,2-benzisothiazole 1,1-dioxides 4 was described by Cristiano et al. Heating in a non-polar solvent, such as toluene at 85° C gave the A-allyl derivative 5 which was proposed to have formed via a [3,3] sigmatropic shift. In polar solvents, at similar temperatures, the compound... 

Products isolated from the thermal fragmentation of A-arylbenzamide oximes and A-arylbenzamide O-phenylsulfonyl oximes have been accounted for by invoking a free-radical mechanism which is initiated by the preferential homolysis of the N-O bond." Time-resolved IR spectroscopy has revealed that photolysis of A, A -diphenyl-l,5-dihydroxy-9,10-anthraquinone diimine affords acridine-condensed aromatic products via excited-state intramolecular proton transfer." The absolute and relative rates of thermal rearrangements of substituted benzyl isocyanides have been measured,and it has been found that the relative rates are independent of temperature and exhibit excellent Hammett correlations. Thionitrosoarene (25), thought to be generated by desulfurization of the stable A-thiosulfinylaniline (24), has been established" " as an intermediate in the formation of 3,3a-dihydro-2,l-benzisothiazole (26) from o-alkylthionitrosoarene (24). 

A reaction of rather less general application, but one which produces valuable intermediates, is the treatment of dithiosalicylamide and its A-substituted derivatives with phosphorus pentachloride. Thus, A.A-diethyldithiosalicylamide (20) yields 3-chloro-2-ethyl-l,2-benz-isothiazolium chloride (21).14-16 This salt (21) is a versatile compound for example, heating it with diethylamine gives 3-diethylamino-l,2-benzisothiazole (22)16, treatment with base yields 2-ethyl-l,2-benzisothiazolinone (23), and reaction with ammonia, followed by hydrochloric acid, affords 3-ethylamino-l,2-benzisothiazole (24).14 This final reaction involves an interesting rearrangement which will be discussed in more detail in Sections II, C, 2 and II, C, 3. 

Displacement reactions of 3-chloro-l,2-benzisothiazole (26) have already been mentioned in Section II,A, 8.18,19 22 24 In some cases this displacement is accompanied by a rearrangement. Boshagen has shown that treatment of the above-mentioned compound (26) with thioacetic acid yields an A-acyl-3H-l,2-benzodithiole (39).31 This rearrangement is analogous to that described in Section II, C, 2 above. 

Allyloxy-l,2-benzisothiazole A,A-dioxides undergo thermal rearrangement forming products of both a sigmatropic [3,3]-shift and of a [l,3]-shift (see Section 4.05.7.5). 

Isothiazoles are reported to yield the lactam (101) on reaction with diphenyl ketene <85BSB149>. Ethyne dicarboxylate esters add to 2,1-benzisothiazole to generate quinoline esters (102), and benzyne reacts to yield aeridine <83H(20)489, 88JCS(Pl)2l4l>. 2-Methyl-2,l-benzisothiazolin-3-thione with acetylene esters forms 2-iminobenzoquinone methides (103), which dimerize to give diazocines (104) or further react with the ethyne ester to yield diadducts (105). The initial adduct formed with phenylethyne and 2-methyl-2,l-benzisothiazolin-3-thione rearranges to produce l,2-dithiole[3,26]... 

The 3-alkylamino-l,2-benzisothiazole 2-oxide (198) like pyridine A-oxides is reduced with phosphorus trichloride or thionyl chloride to the deoxy compound (199) ( = 0). The A-oxide (198) rearranges in methanol to the 3-hydroxylamine compound (200) and in formic acid to the sulfoxide... 

Nucleophilic displacement of the chlorine atom of 3-chloro-1,2-benzisothiazole has proved to be a popular procedure. Boeshagen and Geiger34 have continued their earlier work on nitrogen nucleophiles, and now include carbon, oxygen, and sulfur nucleophiles.35 In some cases, rearrangements occur, as in the formation of 3-amino-2-acylbenzo[6]thiophenes (20) from reaction of 21 with methyl ketones. Similar results are obtained from the reaction of other carbon nucleophiles, and it has been suggested that attack may be either at the 3-carbon or the sulfur atom.36 The reaction of 3-chloro-1,2-benzisothiazole (8) with the anion of ethyl cyanoacetate, for example,... 

Heating the oxime of 2-mercaptoacetophenones with PPA at 120-I30"C gives mainly a benzothiazole, but as a Beckmann rearrangement may occur to some extent during the reaction, some benzisothiazole may also be formed. 

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