Tautomeric isomerization

A peculiar case of fast and reversible (i.e., tautomeric) isomerizations initiated thermally or photochemically is represented by triarylimidazole dimers 349a formed by oxidation of triaryUmidazoles (66JA3825 90MI2) and diarylhetarylimidazoles (Ar = Het) (Scheme 140) [97MI553]. 

This gives tautomeric mixtures119 when the tert-butyl group is removed. The methyl ether has been used to obtain 3-hydroxy-2-carbonyl derivatives in the selenophene series.120 The unsubstituted 2-hydroxyselenophene system has been prepared by hydrogen peroxide oxidation of 2-selenophene-boronic acid.121 However, in the 5-methyl-substituted system deboronation became such an important side reaction that 5-methyl-2-hydroxyselenophene had to be prepared by acid-catalyzed dealkylation of 5-methyl-2-fert-butoxy-selenophene. Both 2-hydroxy- and 5-methyl-2-hydroxyselenophene exist mainly as 3-selenolene-2-ones (93) and for the 5-methyl derivative it was possible to isolate the / ,y-unsaturated form (92) and follow the tautomeric isomerization. The activation parameters thus obtained were compared with those for the corresponding furan and thiophene systems. 

Cycloadditions have been carried out to 37/-indoles (222, 223) (125,126), N-arylmaleimides (224) (127,128), l,2), -azaphospholes (225) (129), 5(47/)-oxazo-lones (226) (130), and 4,5-dihydrooxazoles (230) (131). The primary cycloadducts from the reaction of oxazolones (e.g., 226 with diaryl nitrile imines), derived from tetrazoles in refluxing anisole, do not survive. They appear to lose carbon dioxide and undergo a dimerization-fragmentation sequence to give the triazole 228 and the diarylethene 229 as the isolated products (130). In cases where the two aryl substituents on the oxazole are not the same, then, due to tautomerism, isomeric mixtures of products are obtained. 

Sequential keto-enol tautomeric isomerization (Eq. 7) could convert 118 to the tetrahydropterin derivative, the structure of which is widely accepted as (6R)-pyruvoyltetrahydropterin (124). Contrary to the importance of 124 in the biosynthesis of tetrahydrobiopterin (43) (Sect. 5.1), thus far, only a few... 

The 1,4-dihydro quinoxaline is formed as the first product in reduction of 2-phenylquinoxaline with chromium(II) ethylenediaminetetraacetate or zinc-amalgam at pH > 7 however, a tautomeric isomerization to the thermodynamically more stable 1,2-dihydro isomer occurs. 

Tautomerism Isomerism in which isomers, called tautomers, are readily interconvertible. Keto-enol tautomerism is an example... 

The valence tautomeric isomerism of oligomethylene clamped 1 //-azepines, illustrated in Equation (2), shows a marked dependence upon the A -substituent and the value of n. For example, where n = 3 and R = COOMe, the closed tautomer (14) is preferred whereas, in contrast, for the next homologue the open tautomer (15) is preferred. In the case of R = H, when, n = 3 the closed tautomer predominates, which is reversed when = 5. When n = 4, the two tautomers exist in a 3 7 equilibrium <89CB1765>. 

SEARCH. With the current molecule from DRAW or ATTACH as a query, this menu provides for novelty search or substructure search operations over a MACCS database (Figure 2). The search algorithms treat stereochemistry, parent status, tautomerism, isomerism, formula, etc. This menu relies on the Chemical Database facility of MACCS-II among others. 

The presence of the solvent is known to have proven influences in such a variety of chemical equilibria acid-base, tautomerism, isomerization, association, dissociation, conformational, rotationaL condensation reactions, phase-transfer processes, etc., that its detailed analysis is outside die reach of a text such as this. We will limit ourselves to analyz-... 

In most applications the ability of the system to undergo essentially endless recycling is an important factor. Thus, the degradation response time impacts heavily on the effectiveness of the system. In this regard, the valence bond tautomeric isomerizations would appear to possess the most promising properties, while those mechanisms that involve fragmentation open the... 

Tautomerism, an equilibrium involving two or more isomeric structures accomplished via migration of an atom or a small group within a molecule [1—4], has been attracting scientific interest from a fundamental as well as a practical point of view for more than a century [5, 6]. The tautomeric isomerization is a phenomenon traditionally related to solutions of compounds, where the tautomeric compounds exist in different tautomeric forms that usually interconvert rapidly. In the case of slower interconversion, the specific tautomers can be identified by spectroscopic methods, such as nuclear magnetic resonance (NMR) spectroscopy (Scheme 13.1) [8]. The abundance of a particular tautomer can be controlled by tuning the reaction conditions (proticity, dielectric constants, temperature, and pH of the solution) [9-11]. Numerous studies have dealt with the control of tautomeric systems and their corresponding properties, such as fluorescence [12], photo- [13], or thermochromism [14] and the bioavailability of proper tautomeric forms [15]. 

According to the same authors [30], tautomeric isomerization proceeded through the migration of a proton from the moiety of a profen molecule to another in aqueous alcoholic medium. A suppression of the oscillatory transenantiomerization of profens was observed storing them in dichloromethane, owing to their much less pronounced electrolytic dissociation. 

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