Thiazoles, mesoionic

In the first chapter, devoted to thiazole itself, specific emphasis has been given to the structure and mechanistic aspects of the reactivity of the molecule most of the theoretical methods and physical techniques available to date have been applied in the study of thiazole and its derivatives, and the results are discussed in detail The chapter devoted to methods of synthesis is especially detailed and traces the way for the preparation of any monocyclic thiazole derivative. Three chapters concern the non-tautomeric functional derivatives, and two are devoted to amino-, hydroxy- and mercaptothiazoles these chapters constitute the core of the book. All discussion of chemical properties is complemented by tables in which all the known derivatives are inventoried and characterized by their usual physical properties. This information should be of particular value to organic chemists in identifying natural or Synthetic thiazoles. Two brief chapters concern mesoionic thiazoles and selenazoles. Finally, an important chapter is devoted to cyanine dyes derived from thiazolium salts, completing some classical reviews on the subject and discussing recent developments in the studies of the reaction mechanisms involved in their synthesis. 

As apparent from the contributing resonance structures, both mesoionic systems contain an azomethinylide contribution, accounting for the reaction with representative dipolarenophiles to give cycioadducts such as 3 or 4 (Scheme 4). The cydoadditions and extrmsion reactions of the adducts have been the mam object of investigation. since previous reviews on me.soionic thiazoles (2.9V Results appearing since 1969 and before June 1976 are reported for each type of compound in this chapter. Tables VIIRl-5 contain all mesoionic thiazoles described before June 1976. 

Besides being useful precursors to pyrroles pyridine-2-ones -4-ones, -4-thiones. and -4-imines 4-alkylidene-dihydropyridines thiophenes 1,2,4-triazoles thiapyrane-2-thiones, isoquinoline-3-ones isoben-zothiophenes and 4-mercaptoimidazolium hydroxide inner salts, mesoionic thiazoles are potentially useful in the construction of molecules with herbicidic (39). central nerve stimulating, and antiinflammatory properties (40,41). Application in dye synthesis has likewise been reported (42). 

Mesoionic thiazol-4-one, see 4-Hydroxy-thiazolium hydroxide inner salts Methanol, action on thiazolium salts, 32 Methine dyes, nonionic, nomenclature of, 27... 

Cava and Saris591 refluxed the mesoionic selenazole 5 for a week with DM AD in benzene, and obtained 65% of the pyridone 6. This may be compared with the analogous mesoionic thiazole 49 (Section X,E). The easy loss of selenium from 6 contrasts with the loss of phenyl isocyanate from 49. 

The photochemical rearrangement of the mesoionic thiazole (33) provides an original synthesis of the 4-methylthio-derivatives of the A-4-thiazoline-2-one (34) (Scheme 15) (33). 

Thiolactam (67) reacted with the epoxide (68) to afford the mesoionic thiazole (69) in 78% yield (Equation (13)) <90HCA763>. 

Benzothiazole 3-oxide reacts with some dipolarophiles yielding 2-substituted ben-zothiazoles (Scheme 53) (70CPB1176). 1,4-Cycloadditions of dipolarophiles are observed with mesoionic thiazoles. The 4- and 5-hydroxythiazolylium hydroxide inner salts (95) and (98) react with dipolarophiles, such as electron-deficient alkynes, to give cycloadducts which react further by extrusion either of sulfur (96) or of a simple molecule (97, 99 Scheme 54). An example is given for the reaction of DMAD with 4-hydroxy-2,3-diphenylthiazolylium... 

Reactions of N-monosubstituted thioureas 146 and a-haloacyl halides 147, in the presence of triethylamine, give mesoionic thiazol-3-ium-4-olates 148 (Scheme 54) <1996M1251>. 

Reaction of 2,3-diphenylcyclopropenethione and the mesoionic thiazole, 2-aryl-3-phenyl-thiazolium-4-olate, in benzene yielded the 1 1 cycloadducts, 8-thia-6-azabicyclo[3.2.1]oct-2-ene-4-thiones 8. ... 

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