Amino-1,3-azoles, basicity

The diazotization of heteroaromatic amines is basically analogous to that of aromatic amines. Among the five-membered systems the amino-azoles (pyrroles, diazoles, triazoles, tetrazoles, oxazoles, isooxazoles, thia-, selena-, and dithiazoles) have all been diazotized. In general, diazotization in dilute mineral acid is possible, but diazotization in concentrated sulfuric acid (nitrosylsulfuric acid, see Sec. 2.2) or in organic solvents using an ester of nitrous acid (ethyl or isopentyl nitrite) is often preferable. Amino derivatives of aromatic heterocycles without ring nitrogen (furan and thiophene) can also be diazotized. 

Substituted isoxazoles, pyrazoles and isothiazoles can exist in two tautomeric forms (139, 140 Z = 0, N or S Table 37). Amino compounds exist as such as expected, and so do the hydroxy compounds under most conditions. The stability of the OH forms of these 3-hydroxy-l,2-azoles is explained by the weakened basicity of the ring nitrogen atom in the 2-position due to the adjacent heteroatom at the 1-position and the oxygen substituent at the 3-position. This concentration of electron-withdrawing groups near the basic nitrogen atom causes these compounds to exist mainly in the OH form. 

From the point of view of reactivity, there is little difference between 2-amino-selenazoles and aryl- or alkyl-2-aminoselen azoles, except that the A -arvl derivatives are generally less basic and that their salts are more easily hydrolyzed. 

The azoles (oxazole, imidazole, and thiazole) are five-membered aromatic heterocycles that have two heteroatoms in the ring. One of the heteroatoms in each of these heterocycles is an sp2-hybridized nitrogen that contributes one electron to the 6n aromatic system and has a basic nonbonded lone pair. The other heteroatom (oxygen, nitrogen, or sulfur) contributes two electrons to the 6n system. The imidazole skeleton is present in the amino acid histidine. The thiazole ring occurs in thiamin (vitamin B. ... 

Reactions of 2-formyl-galactal, presented as an unsaturated sugar derivative with push-pull functionalization, with guanidinium and amidinium salts, respectively were carried out imder basic conditions to furnish the substituted 5-( 1,2,4-tri-O-benzyl-D-lyxo- l,2,3,4-tetrahydroxy-butyl)pyrimidines 92 (Fig. 16). Treatment of the 2-formyl pentose glycals with 2-aminobenzimid-azole and 3-amino-1,2,4-triazole, respectively afforded 3-(l,2,4-tri-0-benz-yl-D-lyxo-l,2,3,4-tetrahydroxy-butyl)benzo[4,5]imidazo[l,2-a]pyrimidine 94... 

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