Five-membered heterocycles with two heteroatom

Considering the formation of saturated five-membered heterocycles with two heteroatoms, it is worth to note the possibility to prepare 1,3-dioxolanes, dithiane, oxathianes 148 [93] and dioxolanones 149 [94] by condensation of the corresponding carbonyl compounds under microwave irradiation in acid medium (Scheme 52). The reaction, which is very useful for the protection of carbonyl compounds or for the preparation of useful synthetic intermediates, has also been carried out under batch conditions over Montmorillonite KIO clay in more than 150 g scale, using a 1 L quartz reactor [95]. 

Five-membered heterocycles with two heteroatoms have the jr-electron deficiency of Y-type heteroatoms compensated by the jr-electron excessive character of the X-type atoms therefore, this category includes some of the most stable heterocycles. For example, NMR spectral data and chemical behavior (e.g., resistance to oxidation by potassium permanganate) suggest that pyrazole and imidazole have delocaliza-... 

Fig. 3.7 Five-membered heterocycles with two heteroatoms obtained by sol-id-phase chemistry.

Oximes and their derivatives are widely used in organic synthesis. A number of reviews are devoted to the chemistry and biological activity of oximes and their derivatives The synthesis, reactions and biological activity of oximes containing a heterocyclic substituent, e.g. furan and thiophene", indole and isatin, pyridine, pyrrole, quinoline and five-membered heterocycles with two heteroatoms have been reviewed. 

Five-membered heterocycles with two heteroatoms connected in 2-position also form two step redox systems of the Weitz type which may be described as hetero-tetrasubstituted ethylenes on the level of RED . The great significance of tetrathia (selena) flilvalenes 17 as a basis for organic metals has already been discussed ... 

Thus far only eight different types of five-membered heterocycles with two heteroatoms, one of these being tellurium, are known. Of special interest are those containing S,Se, or Te as the second heteroatom in the ring. Such compounds afford electron-donor components of charge-transfer complexes or radical cation salts, exhibiting properties of the so-called organic metals. 

DIMEDONE-ANNULATED FIVE-MEMBERED HETEROCYCLES WITH TWO HETEROATOMS... 

The reactivity of five membered heterocycles with two heteroatoms as dienes with at least one nitrogen for Diels-Alder reactions is also very low. In fact, there is not much experimental data in this area of research, except for addition of dienophiles to oxazole, better known as the Kondrateva reaction [57]. The main reason for their low reactivity is high heterocycle aromaticity delocalization of molecular x-orbitals that should be part of the cycloaddition reaction. That can be explained from FMO energy difierences between aromatic heterocycles as well as by bond order uniformity of heterocycles with two heteroatoms... 

Summary of the general chemistry of five-membered heterocycles with two heteroatoms ... 

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