Thiazoles, 4-hydroxy-, tautomerism

Bei der sauren katalysierten Hydrolyse von a-Thiocyanat-ketonen entstehen in dieser Weise glatt 2-Hydroxy-l, 3-thiazole bzw. tautomere 2-Oxo-2,3-dihydro-l,3-thiazole ... 

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. 

The 4- and 5-hydroxy-imidazoles, -oxazoles and -thiazoles (499, 501) and 4-hydroxy-pyrazoles, -isoxazoles and -isothiazoles (503) cannot tautomerize to an aromatic carbonyl form. However, tautomerism similar to that which occurs in hydroxy-furans, -thiophenes and -pyrroles is possible (499 500 503 504 501 502), as well as a zwitterionic... 

Thiazole, 2-acetylamino-4-methyl-alkylation, 6, 256 Thiazole, 2-acylamino-4-hydroxy-synthesis, 6, 297 Thiazole, 5-alkoxy-cleavage, 6, 289 synthesis, 6, 302 Thiazole, 2-alkyl-A7-alkylation, 6, 253 hydrogen exchange, 6, 276 methylation, 6, 253 quatemization, 6, 253-254 reactions, S, 88 Thiazole, 4-alkyl-A7-alkylation, 6, 253 methylation, 6, 253 quatemization, 6, 253-254 Thiazole, 5-alkyl-A7-alkylation, 6, 253 methylation, 6, 253 Thiazole, 2-alkylamino-tautomerism, 6, 248 Thiazole, 4-alkyl-2,5-dimethyl-quatemization, 6, 253-254 Thiazole, 2-alkylthio-reactions, S, 103 rearrangement, 5, 103 6, 291 Thiazole, 3-allyl-4-hydroxy-2-imino-synthesis, 6, 297 Thiazole, 2-allyloxy-rearrangement, 6, 289 Thiazole, 2-amino-diazo coupling, 6, 257 nitration, 6, 255... 

Thiazole, 2-( D-galactopentaacetoxypentyl)-4-methyl-synthesis, 6, 295 Thiazole, 2-hydrazi nosynthesis, 6, 297 Thiazole, 2-hydroxy-reactions, 6, 285-286 synthesis, 6, 298, 299 tautomerism, 5, 99 6, 247, 269 Thiazole, 4-hydroxy-reactions, 5, 101 6, 287-288 synthesis, 6, 303 tautomerism, 6, 248, 269 Thiazole, 2-hydroxybenzyl-biosynthesis, 1, 96 Thiazole, 2-hydroxy-4-methyl-arylation, 6, 256 Thiazole, 2-iodo-photolysis, 6, 244 Thiazole, 2-isobutyl-occurrence, 6, 327... 

Thiazolidine-2-thione, 4-hydroxy-synthesis, 6, 314 Thiazolidinethiones tautomerism, 6, 273 1,2,4-thiazoles from, 5, 776 Thiazolidine-2-thiones reactions... 

Investigations on the tautomerism of fused heterocyclic moieties favor the lactam form of the imidazole substructure of 16 (65CPB473) and the thiolactam tautomer in thiazoles 192 and 197 [62BCJ1998 64BCJ1526 95PS(101)167], but the hydroxy form in pyrazole 183 [82IJC(B)765]. 

Thiazoles substituted in the 2-, 4- or 5-position with an amino, hydroxy or mercapto group are in tautomeric equilibrium with the corresponding imino-, oxo- or thioxo-thiazo-lines (Scheme 57). A similar protomerism has been established for (4-phenylthiazol-2-yl)acetone (109 Scheme 58). 

Thiazoles bearing hydroxy, thio, or amino groups at C-2, C, or C-5 are in tautomeric equilibrium with the corresponding oxo, thioxo, or imino thiazolines. Thiazoles bearing more than one of these groups also display similar prototropic tautomerism, although with some restrictions. 

Some derivatives of both thiazole and benzothiazole have been studied by IR spectroscopy. In particular, an extensive study has been carried out with thiazole-2-carboxylic acids and the corresponding carboxylate ions <88Mi 306-01 >. The infrared spectra of 2,3-disubstituted 1,3-thia-zolidin-4-ones have been studied and the majority of the absorption bands assigned <93PS(78)223>. The tautomerism of thiazoles substituted in 2- and 4-position by amino, thio, and hydroxy groups was examined by infrared spectroscopy (85JPR25l> (see Section 3.06.4.4). 

Thiazoles and their derivatives bearing amino, hydroxy, or thio groups in the 2-position can exist in at least two tautomeric forms,... 

Substituted thiazoles at C-2 or C-4 with hydroxy, thio, and amino groups can be represented by five tautomeric forms (26a-e) (Scheme 1). However, form (26e) has never been observed <85JPR25l>. The aminothiazoles exist predominantly in the amino form when the substituent is in position two (26b, Z = NH). On the other hand, when the substituent is in position four the amino form is preferred, although there are exceptions to this rule (for instance, in the case Y = Z = NHCOMe). The hydroxy and thio derivatives predominantly exist in the oxo and thioxo forms (26a, Y, Z = O, S). The main evidence of these arguments came from the C NMR spectra of the thioxo derivatives and from the IR spectra of the oxo and amino derivatives. 

The reactivity of this class of compounds is discussed either in Section 3.06.5, whether the transformation occurs at the aromatic ring level or in Section 3.06.7, when the protomeric group (hydroxy, thio, or amino) is responsible for the reactivity. Thiazoles substituted by two of these groups also present prototropic tautomerism although with some restrictions (see Section 3.06.4). The reactivity of those compounds is discussed in Section 3.06.5 and Section 3.06.7, in a similar fashion to the monosubstituted derivatives. 

Thioglycolic acid reacts with active methylene compounds such as (288) to give 4-hydroxy thiazoles (289). The spectral analysis shows that the predominant tautomeric form is (289b) both in solid state and DMSO solution (Scheme 73) <92T9295>. 

Die Nitrosierung ist auf elektronenreiche 1,3-Thiazole beschrankt, die in 2-Position eine Amino-, Hydroxy- oder Mercapto-Gruppe tragen (vgl. Bd. X/3, S. 58). Die 5-Nitroso-Produkte liegen haufig als Oxim-Tautomere vor. 

Numerous compounds of this series exist in their tautomeric form as 2-amino(or hydroxy or mercapto)thiazoles or as substituted thiazolidines, and are dealt with under those headings. Examples of true 4-thiazolines, containing an iV-substituent in their heterocyclic ring, are less numerous, and their literature is correspondingly less extensive. 

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