Of heterocycles: five-membered rings with

The Tautomerism of Heterocycles Five-Membered Rings with Two or More Heteroatoms... 

Friedrichsen, W., Traulsen, T., Elguero, J., Katritzky, A. R., Tautomerism of Heterocycles Five-Membered Rings with One Heteroatom, 76, 85. 

From this point of view, both the pyranoses and the furanoses may participate in the mutarotation reaction, and both a- and /3-d 1uco-pyranose will have similar mutarotation constants. We may expect a heterocyclic five-membered ring with two adjacent cis hydroxyl groups to have a A of about 1000 units for 0.5 M solutions. In the case of D-glucose it is between 93 and 80 hence, the concentration of a-D-glucofuranose is probably less than 10 per cent. This quantity is sufficient, however, to explain the formation of derivatives of the furanoses. 

Fig. 5.10 Iron sandwich complexes, which are structurally related to ferrocene but contain either one or two planar heterocyclic five-membered rings (with the names of the principal investigators and the date of discovery)...

Dicarborative telomerization has also been successfully applied to intramolecular cyclization reactions. Various tetraenes with enamines, indole, nitroalkanes or carbonyl-stabilized carba-nions give good yields of carbocyclic or heterocyclic five-membered rings, with predominantly the trans arrangement of the two different unsaturated side chains14,15. 

Many versatile approaches to the construction of fused heterocyclic systems (6 5 6) with ring junction heteroatoms have been reported. More general reactions which can be used for synthesis of derivatives of several tricyclic systems, and transformations which have potential for use in the preparation of a series of substituted compounds, are discussed in this section. Formation of the five-membered ring is presented first because it is a conceptually simple approach. It should be noted, however, that the addition of a fused six-membered ring to a bicyclic component offers much more versatility in the construction of a (6 5 6) system. Each subsection below starts with intramolecular cyclization of an isolated intermediate product. Reactions which follow are one-pot intermolecular cyclizations. 

A lot of methods are available for the synthesis of this heterocycle, and most of them rely on the formation of the five-membered ring. In this section, only the methodologies of reasonable scope will be reported. The most classical method involves the cyclocondensation of 2-aminopyridine with an a-halo carbonyl compound. Due to the broad availability of the required substrates and the efficiency of this cyclocondensation, it continues to be the method of choice to prepare this heterocycle. New examples highlighting the generality of this reaction are collected in Table 14. 

The recently synthesized phosphatriazolo[l,5- ]pyridines 4 can also participate in reactions involving the ring opening of the five-membered ring <1995ZNB558>. When the unsubstituted phospha-heterocycle is treated with aqueous acetonitrile, a hydrolysis occurs and the open-chained phosphenic amide 47 can be obtained in acceptable yield (Scheme 13). 

Three systems were pyrazoles are condensed to five-membered rings with one heteroatom exist (300-302). For pyrrolopyrazoles, a fourth iso-merer (303) is also possible. When the pyrazole ring is fused to a heterocyclic system in which the heteroatom is tetravalent, several isomeric structures can be drawn. Derivatives of 300-303 can be prepared either... 

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