Tautomeric interconversion

Tautomeric interconversions of heterocyclic compounds Algebraic characterization of the thione-thiol prototropic tautomerism... 

Tautomeric interconversions of 1,3-diimino-isoindolenine and its nitro and amino derivatives have been studied theoretically by the AMI method (97MI9). 

No tautomeric interconversions have been observed between 4//-pyrans (1, X = O) and 2//-pyrans (2, X = O), 4//-thiopyrans (1, X = S) and 2H-thiopyrans (2, X = S), or their benzo-fused derivatives. Under normal conditions, all these compounds exist as sole isomers that are initially formed, both in solution and in the solid state. The only tautomeric equilibrium reported was that between the two possible 2H isomers of the unsymmetrical sulfone 3 (77TL1149). 

However, no tautomeric interconversions between 1,2-dihydro- and 1,4-dihydropyridine analogs have been observed, probably because a high energy barrier is inherent to this transformation. 

No data on tautomerism of dihydropyiimidines were available at the time of the early summary (76AHCS1), but much has been done since then. The results of tautomeric studies carried out during the period between 1976 and 1984 were reviewed comprehensively in [85AHC(38)l,pp. 63-77]. Later, Weis and vanderPlas published an excellent review on the synthesis, structure, and tautomerism of dihydropyrimidines [86H(24)1433], where the tautomeric interconversions of these compounds were discussed in detail. In a more recent review on dihydropyrimidines (94MI1), the question of tautomerism in partially hydrogenated pyrimidines was also included. 

In tautomeric equilibria of some functionalized pyrimidine derivatives, such as isocytosine 52 (R = H) [77ZN(C)894] or pseudocytidine 52 (R = furanosyl) (99MI1), the potentially tautomeric oxo and amino groups are practically not involved, and only annular tautomeric interconversions N(1)H N(3)H are ob-... 

Four possible tautomeric forms, e.g., 1,2- (62), 2,3- (63), 1,4- (64), and 2,5- (65), exist for unsubstituted dihydropyrazines however, information on the tautomeric interconversions and stabilities of these forms is sparse. It was demonstrated that, of the four tautomers, 1,4-dihydropyrazine 64 is the least stable (unsubstituted... 

Representatives of all three types of isomeric 1,3-oxazines 82-84 (X = O) and 1,3-thiazines 82-84 (X = S) and of the corresponding partially hydrogenated derivatives are known. However, no tautomeric interconversions between these structures have been observed. 

Just a few examples of oxatriazine systems capable of existing in different tautomeric forms, such as 133 [87H(26)2199], have been reported. However, according to their NMR and IR spectra, these compounds exist as preformed, and no annular tautomeric interconversions in oxatriazine series have yet been observed. 

Hydroxyaminobenzo-furan and -thiophene (32a X = O, S) are the unstable enam-ine tautomers of the corresponding oximes (32b). Kinetics of the tautomeric interconversions have been measured, yielding tautomeric constants the latter have been compared with the corresponding keto-enol constants. The enamines are ca 40 times less stable, relative to the oximes, than are the enols, relative to the ketones. The minor tautomers are ca 100 times more stable (relative to the major) for the benzothiophene system. 

In the second group of ring-chain tautomeric interconversions, an open-chain system is transformed into a cyclic system through the intramolecular reversible addition of a functional group to a polar multiple bond lA IB 2A 2B 3A 3B and 4A 4B. The book (I) and this article deal with... 

Ring-chain tautomeric interconversions proceeding by intramolecular reversible addition reactions to the C=0 group (Scheme 1) have been well studied, particularly with respect to the 3- and 4-oxocarboxylic acids containing five- or six-membered rings, respectively. Relatively few new investigations have appeared in the literature. 

The tautomeric equilibrium between IH- and 2//-formycin (7-amino-3)8-D-ribofuranosyl-l//-pyrazolo[4,3-d]pyrimidine) has a constant ratio N(2)H/N(1)H = 0.2 and an enthalpy difference estimated at 1 kcal mol". The tautomeric interconversion is catalyzed by (3 x 10 Msec" ) and by OH" (5 X 10 M" sec" ). No other catalytic pathway such as water catalysis or tautomerization via tautomeric cations contributes significantly to the interconversion. Protonation on the pyrazole ring of formycin does not occur significantly (80JA3897). 

Knowledge of the composition of sugars in solution is fundamental to carbohydrate chemistry. The physical and chemical properties of the sugars in solution depend on the proportions of their various forms and their biological properties may also show such dependence. Enzymes that utilize these sugars as substrates may not be able to use each of the forms. Where only a single form is utilized, the other forms may either be converted into the reactive form or may function as inhibitors. The latter is especially important if the reactive form is present in very low proportion at equilibrium. It is possible that the substrate form is utilized faster than it is generated from the other forms the observed rate of the reaction is then that of the tautomeric interconversion.4... 

The equilibrium constant for a tautomeric interconversion is simply the ratio of the mole fractions of the two forms for example, the ratio of enol to oxo forms of acetone12 in water at 25°C is 6.0 x 10 9, while that for isobutyraldehyde is 1.3 x ICE4. The ratio of 2-hydroxypyridine to 2-pyridone is about 10 3 in water but increases to 0.6 in a hydrocarbon solvent and to 2.5 in the vapor phase.13 14 The ratio of dipolar ion to uncharged pyridoxine (Eq. 2-5) is 4 at 25°C in water.15 The ratios of tautomers B, C, and D to the tautomer A of uracil (Eq. 2-4) are small, but it is difficult to measure them quantitatively.16 These tautomeric ratios are defined for given overall states of protonation (see Eq. 6-82). The constants are independent of pH but will change if the overall state of protonation of the molecule is changed. They may also be altered by... 

Cremer and colleagues concluded the 76 is non- or only very weakly, bishomoaromat-ic183. However, the transition state for the valence tautomeric interconversions of 75 and 76 was found to possess all the characteristics of homoaromatic electron delocalization. 

Zwanenburg and his group [105] have addressed the question of the tautomeric interconversion of aliphatic sulfines into vinyl sulfenic acids. They have synthesised dithioesters bearing an a asymmetric carbon centre. Oxidation with mCPBA did produce chiral sulfines, which did not racemise except when a phenyl group was attached to the a carbon. 

Luminescence spectra of formycin B (88b), its aglycone (88c) and various N-methyl derivatives at room temperature and at 77K indicated that they consist of two tautomeric species (88) and (89) both of which emit at 300 nm at 77K. They can be distinguished by location of emission maxima <82MI 712-02). Photolysis induced tautomerization <82Mi 712-02). The tautomeric equilibrium between (88c) and (89c) has a constant ratio [Ar(2)-H/A(l)-H = 0.2] and the energy difference in ethanol is estimated to be 1 kcal mol-1. The tautomeric interconversion is catalyzed by H+ (3 x 109 M-1 sec-1) and by OH- (5 x 105 M-1 sec-1). No other catalytic pathway such as water catalysis or... 

Dynamic Vibronic Problem of tbe Valence-Tautomeric Interconversion in Cobalt Compounds... 

Ring/chain tautomerism The tautomeric interconversion between the closed ring form and the open chain form of a molecule, e.g. in sugars, the interconversion between the closed ring hemiacetal and the open chain aldehyde. 

Molecular designs of tautomeric interconversions of heterocycles 02IZV197. 

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