Tautomerization effects

The final log P model was obtained by correlahng a total of 94 parameters with the logP values through MLR analysis. Correchon fragments idenhfied by the CASE program can be classified into four categories (i) tautomerization effects,... 

G. Lhoest, R. K. Verbeeck, N. Maton, P. Muthelet, D. Latinne, The in vivo Immunosuppressive Activity of the C15-Demethylated Metabolite of FK-506 Is Governed by Ring- and Open-Chain Tautomerism Effects , J. Pharmacol. Exp. Ther. 1995, 274, 622 -626. 

So it appears that some action by the rest of the molecule is exerted on the atom X. The magnitude of this interaction is much greater than what can be expected by inductive or tautomeric effects, so it is reasonable to assume that some bonding exists between X and one of the sulfur atoms. G. Guillouzo 4 was first to stress this fact, pointing... 

The behavior of thioatnides (70, Table 8) is influenced by tautomeric effects. In the case of bis-aminomcthylation of 2-imidazolidinothionc, - for extunplc, the mobile H atom can be selectively substituted in both the forms by reaction with secondary di-alkylamincs, or primary arylamines, to give, respectively, N,N- (A attack) or N,S- (B attack) bis-Mannich bases. 

Matters become more complex still in neutral conditions because now tautomeric effects can be superimposed upon steric and electronic effects. In an 5e2 process 2-methyl-4-phenylimidazole is methylated by dimethyl sulfate to give a mixture of the 4-phenyl and... 

In contrast to resonance effects where delocalization of n-electrons occur, tautomeric effects lead to isomeric structures, which differ significantly in the relative positions of their atoms. Tautomerism always involves formation and dissection a-bonds in the course of a transformation together with a change in geometry. A... 

C- and N- nitroso compounds may exist as two Z and E (major) isomers due to the tautomeric effect which restricts the rotation about the C-N and N-N bonds. Thus CTI can occur via free rotation of the tautomeric form, doublet inversion etc. [29,14]. In this respect, C-nitroso compounds are the tautomeric form of the corresponding oximes (CH-N=0 —> C=N-OH). The energy barrier is usually between 12 and 27 kcal mol-1 [32], however, many factors may influence CTI, such as substituents, solvents, and pH [33]. For example, alkyl-substituted C- and N-nitroso compounds usually exist as a mixture of Z and E isomers [33], whereas acylnitroso [34] and nitrosocarbamate [35] species have an exclusive E relative stereochemistry due to oxygen-oxygen electronic repulsion. 

The other parent compound indazole is a white solid at room temperature with a melting point of 147-149 C. It does not have the tautomerization effect due to the restrain of the benzene ring. 

The cations of quinazolines that have a strong —I substituent (e.g. Cl, NO2) are almost completely hydrated, and those with a weaker — I, or with a +/, substituent (e.g. OMe, OH, NH2, Me) may contain a small proportion (about 10-25%) of the anhydrous cation. Two important exceptions are now discussed. Substituents in the 4-position, whether — / or + /, have a strong dehydrating effect (cf. Section II, C). Again, when a strongly tautomeric (+T) substituent (sometimes called a mesomeric substituent) is placed pam (i.e. in position 7) to the carbon atom C-4, the tendency to hydrate is strongly reduced. This example of action at a distance was demonstrated in 7-methoxy-, 7-hydroxy, and 7-amino-quinazoline. The tautomeric effect, which is absent or barely perceptible in the 5-, 6-, and 8- (but present in the... 

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