Amino-pyridines, acylation basicity

In their acidity, basicity, and the directive influence exerted on electrophilic substitution reactions in benzenoid nuclei, acylamino groups show properties which are intermediate between those of free amino and hydroxyl groups, and, therefore, it is at first surprising to find that the tautomeric behavior of acylaminopyridines closely resembles that of the aminopyridines instead of being intermediate between that of the amino- and hydroxy-pyridines. The basicities of the acylaminopyridines are, indeed, closer to those of the methoxy-pyridines than to those of the aminopyridines, the position of the tautomeric equilibrium being determined by the fact that the acyl-iminopyridones are strong bases like the iminopyridones and unlike the pyridones themselves. Thus, relative to the conversion of an... 

However, like other methods of test to be discussed in the following section, the salt formation with pyridine hydrohalides in the control of completed amino acylation of amino groups unfortunately interferes in the peptide synthesis with masking functions labile to acid [133] and — as already mentioned — with other basic centres on polymer like guanidyl, indolyl, and imidazolyl residues. 

The synthesis of aminopyridine A-oxides usually occurs under acidic conditions in organic media and generally requires protection of the amino group by acylation reaction and then a final deprotection [47]. By using oxone in water under neutral or basic conditions at room temperature, the reaction can be carried out directly on aminopyridine and the A-oxide is isolated in good yield [48]. The chemoselectivity depends on the position of the amino group in the pyridine ring. 

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