Thiazolium salts rearrangement

However, depending on the nature of the initial heterocycle, rearrangements are possible. Alkylation of thiazole to form the thiazolium salt (390) and generation of the ylide (391) with triethylamine in the presence of DMAD gave not (392) but the isomeric product (393) by the rearrangement indicated (76JOC187). Rearrangements of these types are described in Chapters 4.07 and 4.19. 

The precursor 514 has been shown to undergo anodic oxidation to produce the 5-amino-l,2-thiazolium salt 515, which rearranged to the pyrrole 516 upon treatment with triethylamine (Scheme 68) <1995JPR310>. The mechanistic aspects of this approach have also been discussed <1996J(P1)2339>. Cycloaddition of nitrones with acetylenes has been used to generate A -isoxazolines, which underwent thermal rearrangement to pyrroles fused to the isoquinoline framework <1996T12049>. 

A proposed reaction mechanism is shown in Scheme 130. It involves the addition of a neutral carbene/zwitterionic species 514 (generated in situ from the exposure of thiazolium salt to DBU) to an acylsilane <2001JOC5124, 1996TL8241>. This nucleophilic addition initiates a 1,2-silyl group migration (Brook rearrangement)... 

The versatile desulfurization/rearrangement of 1,2-thiazolium salts 117 to pyrroles 119 is believed to proceed via intermediate 2//-l,3-thiazines 118 [93AG797, 93AG(E)712] (Scheme 32). 

Reactions of Thiazolium Salts. - Base-induced attack of thiazolium salts and rearrangement of subsequent 2-hydroxy-A -thiazoline was studied by u.v. kinetic spectroscopy (two consecutive irreversible steps).The first step was the nucleophilic attack of OH on C-2 of (103 R = H or Me), affording the thiazoline (104), with measured third-order rate constants. The second step was the nucleophilic cyclization of the thiolate (105) to form the thietans (106) (Scheme 9). 

Quaternary thiazolium salts that possess a 2-chloroethyl group, e.g. (76), are rapidly rearranged by aqueous base to thietan derivatives, e.g. (77), presumably via an intermediate bicyclic sulphonium salt. ... 

Compounds in which other heterocyclic rings (pyrazoles, isoxazoles, imidazoles ) are fused to thiopyrans have been described. The interesting thiazolium salts (56) were prepared by allowing the betaines (55) to react with 4-thioxothiazolidin-2-ones. Application of the Fischer indole synthesis and the Friedlander reaction to tetrahydrothiopyran-3-one led to the exported indolo-and quinolino-derivatives. Ring closure of (57) under acidic conditions afforded the rearranged product (58). Syntheses of benzothiopyrano[4,3,2-flfe]-quinazolines (59) from 1-amino-thioxanthones and of their S- and 7V-oxides have been described. ... 

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