Thiazole Thiazolium salts

The amino group is expected to have decreased reactivity in thiazolium salts. 2-Amino-4,5-trimethylene thiazole (224) heated in diluted HQ at 80°C, however, gives the product 227a (463). The probable mechanism is shown in Scheme 139. This mechanism suggests a retro-Hantzsch ... 

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. 

II. General Synthetic Methods for Thiazole and Thiazolium Salts... 

This volume is intended to present a comprehensive description of the chemistry of thiazole and its monocyclic derivatives, based on the chemical literature up to December, 1976. It is not concerned with polycyclic thiazoles, such as benzo- or naphthothiazole, nor with hydrogenated derivatives, such as thiazolines or thiazolidines later volumes in this series are devoted to these derivatives. The chemistry of thiamine has also been excluded from the present volume because of the enormous amount of literature corresponding to the subject and is developed in another volume. On the other hand, a discussion of selenazole and its monocyclic derivatives has been included, and particular emphasis has been given to the cyanine dyes derived from thiazolium salts. 

The first chapter discusses thiazole itself, its structure, and its phyacal and chemical properties. The second chapter describes the general synthetic methods for thiazoles and thiazolium salts. 

Chapters III to VII discuss the general properties of thiazoles having hydrocarbon and functional substituents, respectively. A special chapter (Chapter VIII) is devoted to mcso-ionic thiazoles, and Chapter IX describes the thiazolium salts and their numerous cyanine dyes derivatives. The last chapter concerns the monocyclic selenazoles. 

The only reduction investigated more recently on thiazole derivatives concerns the action of sodium borohydride upon thiazolium salts chosen as model molecules for thiamine (478-480). 

Thiazolium salts can be obtained successfully by a modification of the Hantzsch s thiazole synthesis. This method is particularly valuable for those thiazolium compounds in which the substituent on the ring nitrogen cannot be introduced by direct alkylation, for example, aryl or heteroaryl thiazolium salts (Scheme 42). 

Thiazolium salts with alkyl (103, 722), arylalkyl (116), aryl (305), or heteroaryl (96) substituents on the nitrogen have been also prepared by this procedure. As in the thiazole series, N-substituted thioamides can be formed directly in the reaction mixture from phosphorus pentasulfide and N-substituted amides (127). These methods are important in the synthesis of thiamine 102 (vitamin Bj) (Scheme 45). 

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