Covalent hydration determination

The physical properties of the pyridopyrimidines closely resemble those of their nearest A-heteroeyclie neighbors the quinazolines and the pteridines. Thus, in common with the pteridines, the presence of groups capable of hydrogen-bonding markedly raises the melting point and lowers the solubility. - The acid dissociation constants (pif a values) and ultraviolet absorption spectra of all four parent pyridopyrimidines have been determined by Armarego in a comprehensive study of covalent hydration in these heterocyclic systems. The importance of these techniques in the study of covalent hydration, and... 

P3Timidines, and pyrido[4,3-< ]pyrimidines. The experimentally determined values have been used for studies of covalent hydration, structural assignments, - and tautomerism. -... 

It is a simple matter to determine an ionization constant and also to predict its magnitude. When these values do not agree, and if ringopening has been carefully excluded, the likelihood of covalent hydration must be considered. Equilibria encountered during the determination of the ionization constant of a hydrating heteroaromatic base are shown in the following diagram. Similar equilibria exist for... 

Rate and equilibrium constant data, including substituent and isotope effects, for the reaction of [Pt(bpy)2]2+ with hydroxide, are all consistent with, and interpreted in terms of, reversible addition of the hydroxide to the coordinated 2,2 -bipyridyl (397). Equilibrium constants for addition of hydroxide to a series of platinum(II)-diimine cations [Pt(diimine)2]2+, the diimines being 2,2 -bipyridyl, 2,2 -bipyrazine, 3,3 -bipyridazine, and 2,2 -bipyrimidine, suggest that hydroxide adds at the 6 position of the coordinated ligand (398). Support for this covalent hydration mechanism for hydroxide attack at coordinated diimines comes from crystal structure determinations of binuclear mixed valence copper(I)/copper(II) complexes of 2-hydroxylated 1,10-phenanthroline and 2,2 -bipyridyl (399). 

The high activating power of the furoxan ring in nucleophilic addition has also been observed by Bailey et al.2 9 in 7-nitro-l,2,5-oxadiazolo[3,4-c]-pyridine 3-oxide, a nitropyrido[3,4-c]furoxan that easily undergoes covalent addition of water by nucleophilic attack at the position para to the nitro group. The structure of the covalent hydrate is supported by elemental analysis, osmometric molecular weight determination, and H-NMR spectra in DMSO-[Pg.429]

The ready reversibility of such spectral changes to the spectrum of the cation upon acidification is an important test to rule out irreversible chemical reactions. In general, spectral techniques similar to those extensively used16,19 for the determination of the site of covalent hydration in a heteroaromatic molecule are also applicable to the determination of the site of nucleophilic addition in pseudobase formation. 

Yet another very important method for diagnosing covalent hydration requires an examination of the NMR spectra of the anhydrous and the hydrated species. The C chemical shifts of the anhydrous neutral species of quinazoline and its hydrated cation have been determined and are in Table I. The deshielding of the C-4 resonance is characteristic for the change from an sp to an sp carbon atom upon hydration. Other heterocycles that are known to undergo covalent hydration showed similar changes in spectra. ... 

The covalent addition of water to C=N in an N—0=N system to form a stable hydrate is rare in heterocyclic chemistry. Two examples are known in the quinazoline series, and these are 2-methyl- and 2-phenyltetra-zolo[l,5-c]quinazoline. In these compounds water addition across the 3,4 double bond is not possible because of ring fusion. When these were treated with hydroxides, the hydrates (7 R = Me and Ph) were isolated and characterized. - Undoubtedly such hydrates must be involved as intermediates in the syntheses or hydrolytic degradation of quinazolines in which the C-2, N-3 bond is made or broken. Indirect evidence that a 1,2-covalent hydrate was a necessary intermediate in the bromination of quinazolin-4-one came from judicious kinetic studies. The kinetic order, acidity dependence or rates, inverse dependence of rates on bromide ion, and the relative reactivities of quinazolin-4(3//)-one, 3-methylquinazolin-4-one and l,3-dimethyl-4-oxoquinazolinium perchlorate were consistent with a mechanism in which the rate-determining step was attack of molecular bromine on the 1,2-covalent hydrate, i.e., 8 -> 9. ... 

The review on the electrolysis of JV-heterocyclic compounds by Lund in 1970 included a discussion on the polarographic behavior of quinazoline. The reduction of quinazoline was complicated by covalent hydration in acidic solution, because the hydrated species were not easily reduced. The anhydrous species in alkaline medium were reduced stepwise to dihydro and then to tetrahydroquinazoline, and the dihydro radical intermediate was capable of dimerization. The protonation rates of N-heterocycles in aqueous solution could be determined by polarographic techniques. The rates for quinazoline, and pyrimidine, however, were too fast for measurement which was consistent with predictions from quantum-chemical calculations. ... 

The prediction of the values of such shifts has been considered in closely related N-heteroaromatic systems.109 Ultraviolet absorption maxima have been determined for pyrido[2,3-djpyrimidines,8 10,18,33, 4i— 4,56, no-112 pyrido[3,2-d]pyrimidines,10 16,33,79 81 pyrido[3,4-rf] pyrimidines,33,91 andpyrido[4,3-d]pyrimidines.38 The experimentally determined values have been used for studies of covalent hydration, structural assignments,9,10,41,44 and tautomerism.10,118... 

Is any reasonable mechanism consistent with the data The answer lies in an observation of a probable isotope effect in a coupled nonenzymic phenomenon. The double-isotope fractionation method does not enter into the analysis. The keto group of glyoxalate is actually present as a covalent hydrate to the extent of about 99% of the total glyoxalate concentration (27). However, the ketone is the form that will react in the enzymic process and the concentration of ketone determines the rate of reaction and binding to the enzyme. The equilibrium between ketone and hydrate is not catalyzed by the enzyme and as a result the isotope effect on this equilibrium will appear in the measured kinetic isotope effects. Of course, the extent of this equilibrium will not be affected by deutera-tion of the methyl group of acetyl-CoA. Therefore, the observed HVIK) is not an indication of kinetically significant carbon-carbon bond formation but of a preequilibrium hydration, a process that is independent of the enzyme. The value for HV/K) of 1.0037 is consistent with measured equilibrium isotope effects in related molecules (23). Therefore, the deuteration of acetyl-CoA has no effect on the observed kinetic because that value in fact is due to a preequilib-... 

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