5-Nitropyridine-2-sulfonic acid, a new precursor for 2,5-disubstituted pyridines
Oxidation of 5-hydroxyaminopyridine-2-sulfonic acid (1) to 5-nitropyridine-2-sulfonic acid (2) may be performed selectively by SPB in acetic acid, bleach in water or potassium permanganate in water, with the last as the preferred one. The structure of 2 was verified by X-ray crystallography. The crude products proved difficult to purify efficiently due to inorganic impurities, but substitution of the sulfonyl group in 2 with methanol yielded 2-methoxy-5nitropyridine (6) in 44, 42 and 57 %
... l yield from 1, respectively from the three methods. The reaction of 1 with SPC afforded 5,5'-azoxypyridine-2,2'-disulfonic acid (3) which, upon treatment with methanol gave 5,5'-azoxypyridine-2,2'-dimethoxypyridine (5) in 57 % yield from 1. as the final product. 4 In continuation with this we wished to investigate the versatility of 1 for preparation of other 2,5-substituted pyridines by substituting the sulfonic acid with other nucleophiles. In order to improve the nucleofuge character of the sulfonic acid it was desirable to oxidize the hydroxylamino group of 1 to the corresponding 5-nitropyridine-2-sulfonic acid (2). By this it would also be possible by standard transformations of the nitro group to introduce a variety of substituents into the 5-position of the pyridine ring. Only one brief reference to compound 2 was found in the litterature. In an attempt to convert 2-thiol-5-nitropyridine to 2amino-5-nitropyridine with ammoniacal potassium permanganate minor amounts of potassium-5-nitropyridine-2-sulfonate were obtained. No yields or analytical data were reported. 5 The litterature offers numerous reagents for oxidations of amines, hydroxylamines and nitroso compounds to the corresponding nitro compounds. Unfortunatly, such oxidations are often complicated by condensation between unreacted hydroxylamine and the in situ formed nitroso compound, to form an azoxy product 6 as illustrated for 1 in Scheme 1.