Tautomeric structures

Examples of the remaining potential 3,4-dihydroxy heterocycles are presently restricted to furan and thiophene. Although the parent 3,4-dihydroxyfuran apparently exists as the dioxo tautomer (86), derivatives bearing 2-alkyl or 2,5-dialkyl substituents prefer the keto-enol structure (87) (71T3839, 73HCA1882). The thiophene analogues also prefer the tautomeric structure (87), except in the case of the 2,5-diethoxycarbonyl derivative which has the fully aromatic structure (88) (71T3839). 

Applications of MO methods to such diverse problems as aromaticity, tautomeric structure, dipole moments, and UV, NMR and PE spectroscopy are discussed in various monograph chapters. 

In this method, photons of an energy well in excess of the ionization potential are directed onto a molecule. The photoelectron spectrum which results allows assessment of the energies of filled orbitals in the molecule, and thus provides a characterization of a molecule. Comparisons between photoelectron spectra of related compounds give structural information, for example, on the tautomeric structure of a compound by comparison of its spectrum with those of models of each of the fixed forms. 

Complex tautomerism for azoles with heteroatoms in the 1,2-positions occurs for pyrazoles which are not substituted on nitrogen. Scheme 10 shows the four important tautomeric structures (148)-(151) for 3-methylpyrazolin-5-one, and (152) and (153) as examples of other possible structures. A detailed investigation of this system disclosed that in aqueous solution (polar medium) the importance of the tautomers is (149) > (151) (150) or (148), whereas in cyclohexane solution (non-polar medium) (151) > (148) (149) or (150). 

Since the electrophilic reagent attacks the multiply-bonded nitrogen atom, as shown for (68) and (69), the orientation of the reaction product is related to the tautomeric structure of the starting material. However, any conclusion regarding tautomeric equilibria from chemical reactivity can be misleading since a minor component can react preferentially and then be continually replenished by isomerization of the major component. 

The two NH2 stretching vibrations of aminopyrazoles follow the Bellamy-Williams relation [r-jlNH) = 345.5-0.876r <,s(NH)] (76AHC(Sl)l) establishing the amino tautomeric structure of (91). Azidopyrazoles also show the Vs and Vas bands of the azido group. The... 

For consistency, structures are drawn as the isoquinoline-1,4-diol throughout regardless of tautomer drawn in the original reference. The actual tautomeric structure is dependent on the pH of the solution. See Caswell, L. R. Campbell, R. D. J. Org. Chem. 1961, 26,4175. 

A detailed investigation of the tautomeric structure of 6-azauracil was carried out by Jona and Gut and by Horak and Gut. They measured the UV and IR spectra and compared similar systems and their derivatives in which the lactam or lactim configuration was fixed by A - or 0-substitution (as will be seen later no 0-alkyl deriva-... 

Tautomerism of types iii-vi occurs less frequently than of types i and ii. As a general rule, both tautomeric structures in types Hi and iv, but neither structure in type v, and only one in type vi, will be aromatic. Type v does not fall strictly within the scope of this review, but it is included because both of these structures can be tautomeric with a third form which is aromatic e.g., 12 and 13 can be in equilibrium with 11. 

Interconversion between two tautomeric structures can occur via discrete cationic or anionic intermediates (scheme 24, where T is an anion capable of reacting with a proton at a minimum of two distinct sites). Alternatively, interconversion can occur by simultaneous loss and gain of different protons (scheme 25, w here T has the same definition as in scheme 24). These mechanisms are well established for acyclic compounds, but they have been much less thoroughly investigated for heteroaromatic systems. The rate of interconversion of two tautomers is greatest when both of the alternative atoms to which the mobile proton can be attached arc hetero atoms, and isolation of the separate isomers is usually impossible in this case. If one of the alternative atoms involved in the tautomerization is carbon, the rate of interconversion is somewhat slower, but still fast. When both of the atoms in question are carbon, however, interconversion is... 

In general, physical methods have been used to study tautomerism more successfully than chemical methods, and, of the physical methods, those involving measurements of basicities and ultraviolet spectra are the most important, followed by those involving measurement of infrared and proton resonance spectra. An attempt is made here to delineate the scope and to indicate the advantages and disadvantages of the various methods. A short review by Mason of the application of spectroscopic methods appeared in 1955. Recently a set of reviews on the applications of physical methods to heterocyclic chemistry has appeared, which treats incidentally the determination of tautomeric structure. 

Three types of tautomeric structures must be considered for acyl-... 

The tautomeric structure leads to ambiguities in the nomenclature of compounds in this series. Thus, 5-methyl- and 6-methylbenzo-furoxan denote two different molecules which, because of their interconversion, cannot be isolated separately at normal temperatures. Throughout this review, when we intend to refer to the ambiguous mixture, we shall use the system employing the lowest numbers. The above methyl derivative, for example, will be described as 6-methyl-benzofuroxan regardless of the form adopted in the crystal. When a... 

Many tautomeric structures for azoloquinolines without a bridgehead hetero atom can be drawn, the most likely of them for X = O, S, Se, NH oxazoloquino-lines, thiazoloquinolines, selenazoloquinolines and imidazoquinolines are shown in Scheme 1. 

As early as 1895, Knorr realized that three types of derivatives could be formed for this type of compound, corresponding to the three tautomeric structures. Other early investigators assigned an 0X0 structure to these compounds on the basis of tenuous chemical evidence, while still others discussed the tautomerism without reaching any definite conclusions. ... 

Many computational studies in heterocyclic chemistry deal with proton transfer reactions between different tautomeric structures. Activation energies of these reactions obtained from quantum chemical calculations need further corrections, since tunneling effects may lower the effective barriers considerably. These effects can either be estimated by simple models or computed more precisely via the determination of the transmission coefficients within the framework of variational transition state calculations [92CPC235, 93JA2408]. 

Three tautomeric structures (1,2-dihydro 27, 1,4-dihydro 28, and 3,4-dihydro 29) are possible for N-unsubstituted dihydroquinolines, which retain the aromatic benzene ring, and two structures 27 and 28 for N-substituted derivatives. No information on the tautomerism of such dihydroquinolines and stability of the possible tautomers was available when the earlier review (76AHCS1) was published. 

Coelenterazine (A) is oxidized into dehydrocoelenterazine (D) by MnC>2 in a mixed solvent of ethanol and ether (Inoue et al., 1977b). Dehydrocoelenterazine (C26H19O3N3) can be obtained as dark red crystals. It does not have the capability of chemiluminescence. The ultraviolet absorption spectrum (Fig. 5.6) shows its absorption maxima at 425 nm (e 24,400) and 536 nm (g 12,600) in ethanol. An addition of NaOH significantly increases the 536 nm peak at the expense of the 425 nm peak. Dehydrocoelenterazine can take a tautomeric structure of quinone type (not shown), in which the phenolic proton on the 2-substituent is shifted onto the N(7) of the imida-zopyrazinone ring. Dehydrocoelenterazine can be readily reduced to... 

Formazans can be characterized as tautomeric structures 178 and 179. Schiele proposed that, in some cases at least, formazans are better represented bv the delocalized structure 180 implying coplanarity among substituents. 302... 

Triphenylformazan can be isolated in two forms the red form is assigned to the tautomeric structure 178 or 179 and the yellow form is assigned to the trans-anti geometry 181.303,304 This assignment is supported by IR, NMR, X-ray diffraction as well as kinetic studies.245,305-315... 

Heterocyclic amino compounds including, for instance, 2-aminothiazoles or 2-amino-benzothiazoles, which contain a nitrogen in the ring capable of forming tautomeric structures, yielded the corresponding amides, although in general in lower than usual yield 11381... 

The presence of tautomeric structures in compound 32 has been determined from ultraviolet (UV) spectra in ethanol. Compound 32 showed two absorption bands (due to its tautomeric forms 32a and 32b) at 288.6 and 241.0 nm attributed to an n-7i and 7I-7I bands, respectively <1997IJB269>. Previous studies are discussed in CHEC-II(1996) <1996CHEC-II(8)713>. 

Treatment of bis(tolylimidoyl) dichloride 358 with 3-aminotriazole (that can exist as two tautomeric structures) affords, after refluxing in THF in the presence of triethylamine, a separable mixture of 5,6-bis-(4-tolylimino)-5//-... 

A Russian team, when investigating the ring-closure reaction to some tiazolo[3,2- ][l,2,4]triazinones 45 and 46, carried out AMI calculations for these molecules <2000ZOB650>. With the help of the quantum-chemical results, infrared (IR) spectra of these compounds have been calculated and compared with the experimentally observed spectra. Thus, clear differentiation is possible between the two tautomeric structures. 

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