Of heterocycles: five-membered rings with one heteroatom

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

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... 

The reactivity of alkyl groups on five-membered rings with one heteroatom is similar to that of alkyl groups on benzenoid rings. Because of the high reactivity of the heterocyclic nuclei, specific reactions... 

Bird CW, Cheeseman GWH (1984) Structure of five-membered rings with one heteroatom. In Katritzky AR, Rees CW (eds) Comprehensive heterocyclic chemistry. Pergamon, Oxford, chap 3.01, p 1... 

C.W. Bird, G.W. H. Cheeseman, Synthesis of Five-membered Rings with One Heteroatom in Comprehensive Heterocyclic Chemistry , A. R. Katritzky, C.W. Rees (eds.) Vol. 4 (1984), Pergamon Press, Oxford p. 89-153. 

N-Substituents in hetarylazomethine ligands are represented by three-and five-membered rings with one heteroatom as well as azole and azine heterocycles. They may not always participate in the formation of metallacycles and thus play the role of coordination-active or inactive constituents of the hetarylazomethine ligands. A great majority of coordination compounds includes mononuclear complexes, although di- and oligonuclear structures are also known. 

Mathey F (2002) In Maas G, Regitz M (eds) Science of synthesis hetarenes and related ring systems fully unsaturated small-ring heterocycles and monocyclic five-membered eetarenes with one heteroatom, vol 9. Thieme, Stuttgart, p 553... 

The 1,3- and 1,2-azoles, five-membered rings with two heteroatoms, present a fascinating combination of heteroatom types - in all cases, one heteroatom must be of the five-membered heterocycle (pyrrole, thiophene, furan) type and one of the imine type, as in pyridine imidazole with two nitrogen atoms illustrates this best. Contributor 39 is a particularly favourable one. 

In this large group of heterocycles, ring strain is of little or no importance. Ring-opening reactions are, therefore, rarer than in three- and four-membered heterocydes. The crudal consideration is rather whether a compound can be regarded as a heteroarene or whether it has to be classified as a heterocycloalkene (see p. 2). Various aromatidty criteria apply to heteroarenes, and, as a consequence, different opinions have been expressed on this matter [1]. As will be shown by means of examples of the various systems, the nature and number of heteroatoms are the critical factors. The parent compound of the five-membered heterocydes with one oxygen atom is furan. 

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. 

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