8-Azapurines covalent hydration

Covalent hydration has been demonstrated in the following families of compounds 1,6-naphthyridines, quinazolines, quinazoline. 3-oxides, four families of l,3,x-triazanapththalenes, both l,4,x-triazanaphthalenes, pteridines and some other tetraazanaphthalenes, and 8-azapurines these compounds are discussed in that order. In general, for any particular compound (e.g. 6-hydroxypteridine) the highest ratio of the hydrated to the anhydrous species follows the order cation > neutral species > anion. In some cases, however, anion formation is possible only when the species are hydrated, e.g. pteridine cf. 21 and N-methyl-hydroxypteridines (Section III, E, 1, d). Table V in ref. 10 should be consulted for the extent of hydration in the substances discussed here. 

Albert, A., 4-Amino-1,2,3-triazoles, 40, 129 The Chemistry of 8-Azapurines( 1,2,3-Triazolo[4,5-d]pyrimidines), 39, 117 Annelation of a Pyrimidine Ring to an Existing Ring, 32, 1 Covalent Hydration in Nitrogen Heterocycles, 20, 117. 

Penta-azaindenes (8-Azapurines). Part I. Covalent Hydration. Syntheses and Reactions. 

The ionization of 8-azapurines, which has not previously been reviewed, is complicated in many cases by covalent hydration, a phenomenon discovered by the present author in the pteridine and quinazoline series in 1952, and twice reviewed in this series. - This occurrence of covalent hydration, which does not exist to any detectable degree in the purine series, - indicates the increased n deficiency conferred by the additional doubly bonded 8-nitrogen atom. 

The absence of covalent hydration [65AHC(4)1 76AHC(20)117] enables the ionization constants and UV spectra of 1,2,3-triazoles to be interpreted more readily than those of 8-azapurines (1,2,3-triazolopyrimidines), which exhibit this phenomenon strongly [86AHC(39)117]. Table I presents the... 

Apparently the first published use of PMR to study hydration was the report that the proton in the 4-position of quinazoline moved upfield (t 0.63 — 3.58) after the molecule was converted into the cation by aqueous acid.10 Normally a downfield shift would be expected on ionization, and this reversal of direction afforded a further and convincing proof of the covalent character of hydration, as well as demonstrating the value of this technique in determining the position of hydration. Contemporaneous application of PMR to the hydration of 8-azapurine (6) cations established the scope and value of this technique.11... 

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