Covalent hydration occurrence

The spectra of protonated polyaza heterocycles are frequently complicated by the occurrence of covalent hydration. This is more common with polycyclic systems, e.g. pteridine. 

Increased interest is added to the topic of this review by the discovery of several examples of covalent hydration in nature although no systematic investigation has been made, an increasing alertness to its possible occurrence can be detected among natural product workers. Some naturally occurring pteridines, such as xanthopterin,67 are covalently hydrated. The hydration of uracil and of the coenzyme diphosphopyridine nucleotide (DPNH) were mentioned in Section IV. 

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 anomalous behavior of quinazoline was first discovered by Albert et who made the surprising observation that 4-methyl-quinazoline 2.5) was a weaker base than quinazoline (pA 3.5). Mason then observed that the ultraviolet spectrum of the quinazoline cation was abnormal but that the spectrum of 4-methylquin-azoline was normal (see Fig. 2). These anomalies led to the suggestion that water adds covalently to the cation of quinazoline to give 12 (R = H). The occurrence and position of hydration were confirmed by a detailed study of the ultraviolet and infrared spectra of the anhydrous and hydrated hydrochlorides and by mild oxidation of the cation to 4(3 )-quinazolinone. Using the rapid-reaction technique (the continuous-flow method), the spectrum of the unstable... 

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