Examples of Covalent Hydration in New Ring Systems

This section will deal with nuclei not known to undergo hydration when the previous review33 was written. 

Not until 1967 was covalent hydration found in a single-ring aromatic nucleus, but several examples are now known. The cation of 5-nitro-pyrimidine (also its 2-methyl and 2-benzyl derivatives) adds a molecule 

The acidic properties of l-methyl-4-dimethylamino-5-nitro-2-oxo-pyrimidine (p/fa 9.04), a molecule with no ionizable proton, have been traced to the structure 30, which is the anion of a hydrate,50 The acidic properties (pKa 10.57) of the corresponding primary amine, 5-nitro-cytosine,51 probably depend on a similar hydration. In solution, 1,3-dimethyl-5-nitrouracil adds water across the 5,6 (C=C) double bond at pH 9.5, but not at neutrality.52 

4-Triazine furnishes the species 31 when a solution in trifluoro-acetic acid is titrated with water. The new upheld signals thus produced in PMR spectra are not seen when the hydrogen atom on C-5 is replaced by a methyl group. The hydration is reversible by basification.53 Acidification of an aqueous solution of the anhydrous sodium salt of 3-hydroxy-l,2,4-triazine precipitated a hydrate of the neutral species, and elemental analysis combined with PMR (r5 4.62) indicated structure 32. This substance was not dehydrated during sublimation or heating in distilling toluene.54 

8-Azapurine (6) is the parent of a series in which most members with a free 6-position undergo 1,6-hydration (see Table I, and pp. 120, 128). No shift of the position of the added water molecule has ever been 

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