Ring compounds heterocyclic five-membered rings

Prototropic Tautomerism of Heteroaromatic Compounds III, Five-Member Rings and One Hetero Atom A. R. Katritzky and J. M. Lagowski, Adv. Heterocycl. Chem., 1963, 2, 1-81. 

Halogenation of heterocycles five-membered rings, 57, 291 fused to other aromatic and heteroaromatic rings, 59, 245 six- and seven-membered rings, 58, 271 of heterocyclic compounds, 7, 1 Hammett equation, applications to... 

The compounds with five-membered rings containing three or four heteroatoms including at least one phosphorus were first systematically reviewed in Chapter 4.22 of CHEC-II(1996) by Schmidpeter <1996CHEC-II(4)771>. In CHEC(1984), only few examples of 2//4,2,3-diazaphospholcs were mentioned in the general chapter on phosphorus heterocycles <1984CHEC(1)493>. 

The best known mesoionic compounds have five-membered rings, and initially it was advocated by Baker, Ollis, Ramsden and other authors that only five-membered heterocycles which cannot be satisfactorily represented by any one covalent or ionic structure possessing a sextet of TT-electrons in association with the five atoms comprising the ring may be called "mesoionic". Here, following Katritzky, mesoionic means a mesomeric betaine. The first such compounds to be discovered were sydnones, followed by miinchnones and then by diazolones. In all these compounds the Z-type atom is part of a carbonyl group, and two Y-type atom chains separate two odd-numbered chains of X- and Z-atom chains. Only the main resonance structures are displayed in formulas (Figure 11). 

Solely the photolysis of 1 with the heterocyclic, five-membered ring compounds fliran, thiophene and selenophene furnished other products instead of the respective siliranes (see below). The reactions of tetra-t-butyldisilene 3 with alkenes gave rise to a more complex palette of products. In the cases of cyclopentene and cyclohexene, no reactions with the alkene were observed. Instead, compounds 24, 25, 27, and 28 were formed these results can be rationalized in terms of a dimerization of the disilene with / elimination of isobutene 26 and subsequent reactions of the disilene and the silylene with 26 [7, 8],... 

Oxazolines Heterocyclic compounds containing five-membered rings in which one carbon is replaced with an oxygen atom and another with a nitrogen atom. Oxazolines are colorless liquids soluble in organic solvents and water. Used as intermediates, e.g., in synthesis of surfactants. 

Relative Reactivities oe Five- and Six-Membehed Ring Heterocycles Five-membered ring compound (I) Six-membered ring compound (II) i/ n Reference... 

Iminodihydrooxathiazole-2-ones 140 react with phenyl isocyanate with addition to the 0=N bond to give a polar adduct 141 which rearranges to the oxazolidineimine 142. This reaction amounts to the transformation of one heterocyclic five-membered ring compound into another one . 

Details of bond lengths and bond angles for all the X-ray structures of heterocyclic compounds through 1970 are listed in Physical Methods in Heterocyclic Chemistry , volume 5. This compilation contains many examples for five-membered rings containing two heteroatoms, particularly pyrazoles, imidazoles, Isoxazoles, oxazoles, isothlazoles, thlazoles, 1,2-dlthloles and 1,3-dlthloles. Further examples of more recent measurements on these heterocyclic compounds can be found in the monograph chapters. 

Phosphorus heterocyclic compounds, 1, 493-538 five-membered ring systems, 1, 513-523 nomenclature, 1, 496 six-membered ring systems, 1, 497-513 Photoaromatization oxirenes from, 7, 125-126 Photobleaching chromenes in, 3, 880 Photochemical reactions heterocyclic compound synthesis from, 5, 159 reviews, 1, 56 heterocyclic compounds reviews, 1, 71, 72... 

Dipolar cycloaddUions. Interest in 1,3-dipolar cycloadditions increased dramatically during the past 20 years, largely because of the pioneering studies of Huisgen [7, 2] The versatility of this class of pericychc reactions in the synthesis of five-membered-ring heterocyclic compounds is comparable with that of the Diels-Alder reaction in the synthesis of six-membered-ring carbocyclic systems (equation 1)... 

Conditions more accessible to rate measurements and, possibly, more favorable to a normal course of substitution (Section II, A) make the use of activated compounds such as nitro- and aza-sub-stituted derivatives desirable. In both cases, the activated halogeno-substituted five-membered ring heterocycle is also more reactive than a similarly activated six-membered ring compound. This... 

Relatively little data are available on sulfonamido derivatives of heterocyclic systems with five-membered rings. The tautomeric equilibrium between structures 222 and 223 has been shown to favor the imine form by about 10 1 by comparison of the ultraviolet spectrum of the parent compound with those of both methylated forms." ... 

N-heterocyclic compounds containing six-membered rings (pyridine and analogues) behave as excellent -acceptors and in turn they provide a rather soft site for metal ion coordination. The 7r-excessive five-membered pyrazole is a poorer 7r-acceptor and a better 7r-donor and it acts as relatively hard donor site. Inclusion of six- and five-membered N-heterocycles like pyridine and pyrazole in one ligand system leads to very attractive coordination chemistry with variations of the electronic properties.555 The insertion of a spacer (e.g., methylene groups) between two heterocycles, which breaks any electronic communication, makes the coordination properties even more manifold. 

Grubbs and coworkers [238] used the ROM/RCM to prepare novel oxa- and aza-heterocyclic compounds, using their catalyst 6/3-15 (Scheme 6/3.9 see also Table 6/3.1). As an example, 6/3-35 gave 6/3-36, by which the more reactive terminal alkene moiety reacts first and the resulting alkylidene opens the five-membered ring. In a similar reaction, namely a domino enyne process, fused bicyclic ring systems were formed. In this case the catalyst also reacts preferentially with the terminal alkene moiety. 

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