Selenazoles, 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. 

Selenazole, 2-aryl-4-chloromethyl-hydrolysis, 6, 344 Selenazole, 2-diethylamino-nitration, 6, 341 Selenazole, 2,4-dimethyl-oxidation, 6, 341 Selenazole, 2-hydrazino-oxidation, 6, 342 Selenazoles, 6, 339-347 electrophilic substitution, 6, 340 hetero fused synthesis, 6, 344 mass spectra, 6, 340 mesoionic... 

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. 

Anhydro-4-hydroxy-2,3,5-triphenyl-l,3-selenazolium hydroxide (65) is prepared by a multistep reaction selenobenzanilide (63) reacts with a -bromophenylacetic acid to give the a -seleno add (64) this acid readily cyclizes to the mesoionic selenazole (65), which upon reaction with DMAD affords an unstable adduct which by extrusion of selenium gives the pyridone diester (66 Scheme 23) (75CC617). 

In independent work, a variety of selenoamides and 1,2-bielectrophiles such as a-bromophenylacetyl chloride and 2-bromo-2-ethoxycarbonylacetyl chloride gave mesoionic selenazoles (65). The adduct formed from the mesoionic selenazole (67) and phenyl isocyanate leads to anhydro-4-mercapto-2-p-methoxyphenyl-6-oxo-l,3,5-triphenylpyrimidinium hydroxide (68 R = p-OMeC6H4). This constitutes the first example of conversion of a... 

As with selenoesters, selenoamides are excellent reagents for the synthesis of selenium-containing heterocycles. Anhydro-2,3,5-triphenyl-4-hydroxyselenazolium hydroxide (96), as the first mesoionic selenium heterocycle,selenophene derivatives (97) " and 1,3-selenazoles (98) have been synthesized as shown in Scheme 33 and equations (38) and (39). 

The cycloaddition of mesoionic thiazoles and selenazoles was explained using second-order perturbation theory, limited to frontier orbitals. Only a detailed study of the sulfur case was reported, but an understanding of the selenium model may be gained <82T2129>. 

Cycloaddition of a mesoionic selenazole with an alkyne forms a bicyclic intermediate that undergoes deselenization to form pyridones. This is in contrast to the analogous thiazole which can undergo a retro-Diels-Alder to liberate the thiophene <82T2129>. The reaction was also discussed in CHEC-I <75CC617, 77JOC1644). 

Synthesis of condensed mesoionic selenazoles (Scheme 12) can be carried out by cyclization of a pyridine moiety with a nitrile. Reduction of diselenide (52) was followed by alkylation with chloroacetonitrile. The ring closes directly to form the 3-aminoselenazolo[3,2-a]pyridinium hydroxide inner salt (16) <83UC(B)328>. 

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