Tautomers feature

The best binding to DNA is obtained with the first tautomer featuring a = 60.0 and being attached to the ATAT segment. 

Clearly, in the case of (66) two amide tautomers (72) and (73) are possible, but if both hydroxyl protons tautomerize to the nitrogen atoms one amide bond then becomes formally cross-conjugated and its normal resonance stabilization is not developed (c/. 74). Indeed, part of the driving force for the reactions may come from this feature, since once the cycloaddition (of 72 or 73) has occurred the double bond shift results in an intermediate imidic acid which should rapidly tautomerize. In addition, literature precedent suggests that betaines such as (74) may also be present and clearly this opens avenues for alternative mechanistic pathways. 

S/D (Single/Double bonds) query bond features added if tautomers can be built from sample structure. 

Quinolizine exists as a mixture of up to three species, namely the 2H-, 4H- and 9a/7-tautomers (17 and 18, respectively). This tautomeric equilibrium has been extensively studied in the case of tetramethyl quinolizine-1,2,3,4-tetracarboxylate, where, as previously mentioned, the 4/7-tautomer 17 has been shown to be thermodynamically more stable than the 9/7-species 18 through ab initio Hartree-Fock calculations. An interesting feature of these compounds is that the rate of the interconversion of the 9a//-tautomer 18 into the more stable 4//-cornpound 17 depends on the position of substituents in ring B <2003JST651>. 

Silyl Nitronates The characteristic features of the behavior of SENAs in [3+ 2]-addition reactions (75, 133, 175-177, 185, 186, 189, 201a, 203, 205, 206, 216, 355-362c) are virtually identical to those of acyclic alkyl nitronates considered in the previous section. As mentioned above, minor but more reactive Z-tautomers of SENAs derived from primary AN can be detected by cycloaddition reactions (see Section 3.3.4.1 and Scheme 3.128). 

The MS of some nitro derivatives of imidazole-4(5)-carboxaldehyde have been studied100. Based on the compounds 15-17, several new features caused by the interaction between the nitro group and adjacent substitutents were disclosed. It should be noted that for the compounds 16 and 17, there is a rapid equilibrium between the two possible tautomers resulting in equivalence of the 4 and 5 positions. 

Fig. 5.5 Putative interactions of the cocrystallized selective Cox2 ligand SC-558 with its active site, assuming a neutral sulfonamide group - the state used to assign pharmacophore feature flags by the used software. Dotted lines stand for hydrogen bonds, the other residues being responsible for hydrophobic contacts. From a physicochemical point of view, an ionized SO2 NH involved in a salt bridge with Arg 513 and hydrogen bonding to the other tautomer of His 90 would make more sense.

We believe that detailed analysis of absorption, fluorescence and fluorescence excitation spectra permits us to investigate the main spectroscopic features of individual NH-tautomers in porphyrins with non-symmetrical substitution. Then, obtained information... 

The tautomeric forms 13-15 would be responsible for a relatively strong signal at 2050 cm", characteristic for 1-lithioacetylenic moiety Eurthermore, it is hard to account for the strong and relatively broad band at 1800 cm" from either species 13 or 14. AU of the spectral features seem to iudicate the tautomeric structure 16 as the most probable, with species 15 uot fully excluded as a possibility. However, the simplicity of the spectral pattern iudicates that oue structure predominates in case of a mixture of tautomers". ... 

The Mills-Nixon hypothesis that small ring annelation on benzene would induce bond fixation (bond alternation) by trapping out one Kekul6 tautomer is a casualty of early twentieth century structural chemistry. Due to a lack of direct methods for analyzing molecular structure, structural postulates of that time were often supported by an analysis of product distributions. An experimental observable such as product selectivity or isomer count was correlated to an unobservable structural feature derived on the basis of a chemical model. Classical successes of this method are van t Hoff s proof of the tetrahedral carbon atom and Fischer s proof for the configuration of sugars. In the case of Mills and Nixon, however, the paradigm broke down. 

The gross structural features, presence of a tetramic acid and E-decenoyl side chain, could be inferred from NMR studies. Methanolysis (HCl/MeOH) of 47 and pentane extraction of the quenched reaction mixture gave two compounds that were determined to be the methyl esters of decenoic acid and N-(2-decenoyl)leucine. The nature of the 3-acyl tetramic acid was deduced from the identification of 48 and 49 in the aqueous portion of the methanolysis reaction mixture following treatment with trifluoroacetic acid anhydride. The unusual C-C bond fragmentation under acidic conditions, and the structure of the antibiotic was confirmed by synthesis of racemic 47 [86]. The configuration at the lone chiral centre was established as R by chiral GC. The carbon NMR spectrum of 47 indicated an equilibrium between three tautomers in which the A2-pyrrolin-4-one form is preferred (60%) and the two internal tautomers (50, 51) make equal contributions (20% each). 

An interesting feature was discovered by Sharma and co-workers494,495 in the crystal structure of isocytosine. Two tautomers of isocytosine (42 and 43) exist in an exact 1 1 ratio in the crystal. The tautomers are hydrogen-bonded to each other in a manner analogous to that proposed by Watson and Crick496,497 for the guanine-cytosine pair in DNA. It is worth underlining that the base pair of isocytosine was not obtained by expedient cocrystallization of the two components. It seems therefore probable that both forms 42 and 43 of isocytosine are of approximately equal stability and are present in comparable amounts in solution. 

The basicity of the amine nitrogen appears to be an important factor for an effective asymmetric induction. Phenyl substituents on the nitrogen atom greatly retard the reaction rate. Thus, /V-phenyl- and IV.lV-diphenylgeranylamine are inert at 40°C and 24 h reaction time. Few characteristic features are worth noting. If an allylamine is secondary, the product is the corresponding imine, a more stable valence tautomer of the enamine, which cannot be detected in the reaction mixture. The exclusive formation of an ( )-enamine regardless of the double-... 

Tautomer K is a resonance-stabilized structure as shown however, tautomer E has a principal resonance contributor which is an aromatic species. The need to separate charges decreases the stability somewhat, but the aromatic system is a large stabilizing feature. Thus E is of much lower energy than K and the equilibrium between K and E is shifted to E. 

An important factor in realizing such potential is that of prototropic tautomerism and, in particular, which tautomer is present in solution immediately prior to the formation of the metal-ligand bond576. A common feature of all nitrogen-containing heterocyclic thiones is thione (157c)-thiol (157b) tautomerism. 

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