Five-membered heterocycles with two

Use of mesoionic ring systems for the synthesis of five-membered heterocycles with two or more heteroatoms is relatively restricted because of the few readily accessible systems containing two heteroatoms in the 1,3-dipole. They are particularly suited for the unambiguous synthesis of pyrazoles as the azomethine imine is contained as a masked 1,3-dipole in the sydnone system. An attractive feature of their use is that the precursor to the mesoionic system may be used in the presence of the cyclodehydration agent and the dipolarophile, avoiding the necessity for isolating the mesoionic system. 

Unusual heterocyclic systems can be obtained by photodimerizations and for five-membered heterocycles with two or more heteroatoms such dimerizations need be effected on their ring-fused derivatives. Cyclobutanes are usually obtained as in the photodimerization of the s-triazolo[4,3-a]pyridine (540) to the head-to-head dimer (541). These thermally labile photodimers were formed by dimerization of the 5,6-double bond in one molecule with the 7,8-double bond in another (77T1247). Irradiation of the bis( 1,2,4-triazolo[4,3-a]pyridyl)ethane (542) at 300 nm gave the CK0ifused cyclobutane dimer (543). At 254 nm the cage-like structure (544) was formed (77T1253). 

Utilization of carbenes in the synthesis of five-membered heterocycles with two or more heteroatoms has not been featured prominently in the synthetic strategies developed for these ring systems. The following illustrations show their considerable promise. 

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.

A series of five-membered heterocycles with two and three heteroatoms were synthesized. 4-Hydroxyisothiazoles 57 were prepared from a-amino ketones with sulfur monochloride (1968BCJ959). Polar solvents, especially N,Af-dimethylfor-mamide, were preferable (Scheme 28). In a similar reaction of 1-amino-l-phenyl-2-propanone with sulfur monochloride 5-chlorinated isothiazole 58 was obtained in high yield. 

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. 

Five-membered heterocycles with two vicinal chalcogen atoms in the ring system can be used as stable precursors for sulfur as well as for selenium-containing hetero-1,3-dienes in cycloaddition reactions. Consequently, 3//-1,2,4-thiaselenazoles have been used for the in situ formation of 4,4-bis(trifluoromethyl)-l-thia-3-azabuta-1,3-dienes, which exist at room temperature only as 4,4-bis(trifluoromethyl)-2//-l,3-thiazetes. This strategy was applied to the synthesis of the first stable selenophosphorane from bis(trifluoromethyl)-substituted 3//-diselenazol and 2-methoxy-1,3,2-dioxaphospholan [78AG(E)774] (Scheme 83). 

The formation of a five-membered heterocycle with two new cr bonds almost always indicates a [3 + 2] cycloaddition. 

Five-membered heterocycles with two vicinal oxo groups 04MI7. 

Five-membered heterocycles with two adjacent heteroatoms with at least one boron atom were discussed very sparsely in the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) <84CHEC-l(l)637>. Only the class of 1,2-azaborolines was described a little more comprehensively. Other types such as 1,2-azaborolidines, the 1,2-oxaborolanes, and the 1,2-oxaboroles were completely neglected. For that reason it seems appropriate to discuss these heterocycles in more detail. This requires partial consideration of literature before 1982. 

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

Tautomerism of five-membered heterocycles with two or more heteroatoms ... 

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