2- hydroxypyridine-N-oxide

Nitration of 2-substituted-3-hydroxypyridine N-oxides (9.94) gave —50% yield of 4-nitro derivatives, as also did 2,6-dimethyl-3-hydroxypyr-idine N-oxide (70BAU2440). 

Benzyl-3-hydroxypyridine W-oxide (9.105) is iodinated in the 4- and 6-positions (cf. other reactions described in, Section 6) (74BAU2023), as is 3-hydroxypyridine N-oxide (66RC1875). 

Azo-coupling of 5-benzyl-3-hydroxypyridine N-oxide (9.105) goes only into the 2-position, probably because of steric hindrance to 6-substitution (74BAU2023). 

Cationic complexes are formed with 1,3-diketones, 2-hydroxypyridine N-oxide and tropolones (325). For example, tris(acetylacetonato)silicon (IV) chlo-... 

Methyl fluorosulfonate 2,6-Dicyanopyridines from 3-a-hydroxypyridine N-oxides... 

Figure 3. Information hierarchy in the SMD structure of 2-Hydroxypyridine-N-oxide given by Figure 2.

Oxidation of N-acyl-2-aminoinethylflirans leads to an intermediate that can be converted by treatment with acid to a 3-hydroxypyridine [180, 181]. Such a transformation is used as one stage in a synthesis of pyridoxine [181,182]. 

Hydroxypyridines have been obtained by direct photolysis of several N- oxides, but only one example exists in the quinoline series. Reaction involves the intermediacy of... 

Cycloaddition of nitriles with dipropargyl ethers. A new synthesis of pyridoxine (4, vitamin BJ is based on the (2 + 2 + 2 cyclo.iddition of acetonitrile with dipropargyl ethers catalyzed by Cp,Co or Cp(CO),Co to torn the pyridine derivative I. Subsequent steps involve rearrangement of the N-oxide of 1 to the 3-hydroxypyridine 2 with acetic anhydride. The final step involves the known cleavage of the dihydrofuranc ring. 

Hydroxypyridine (201) itself possesses latent 1,3-dipolar character because of tautomerism involving 1-protiopyridinium 3-oxide (202). Aprotic diazotization of anthranilic acid in the presence of 201 gives two heterocyclic products [196 (20%) and 203 (23%)] which were isolated in separate experiments run under almost identical conditions.103,105 Formation of the bis-adduct 196 must involve cycloaddition of benzyne to 202 and N-phenylation and there is some evidence from related additions to 2//-phthalazin-1 -one (208) that the steps occur in this order.3 7b Formation of the isocoumarin structure 203 apparently involves electrophilic substitution of 201 by the benzyne precursor 5, followed by lactonization. From 3-hydroxy-6-methylpyridine compounds analogous to 196 and 203 were also obtained (10 and 29%, respectively). 3-Hydroxyquinoline afforded only the corresponding isocoumarin 204 (20%) whereas 4-hydroxyisoquinoline gave 4-phenoxyisoquinoline (12%) and the bis-adduct 205 (12%) with benzyne.103,105... 

The gas-phase equilibrium between 2-hydroxypyridine and 2-pyridone favours the hydroxy-form, but in the equilibrium between 2-hydroxypyridine iV-oxide and N-hydroxy-2-pyridone, the major tautomer is the hydroxy-pyridone. Bicyclic adducts between 2-pyridones and dimethyl acetylene-dicarboxylate, unobtainable at atmospheric pressure, have been obtained at 10—15 kbar. A novel route to iV-hydroxy-2-pyridone involves the trimethyl-silylation of 2-pyridone followed by oxidation of the resulting 2-(trimethyl-silyloxy)pyridine with the DMF complex of molybdenum pentoxide. p-Nitro-phenols (45) and nitro-acetamides (46) are formed from the reaction of 3,5-dinitro-2-pyridones (43) with the sodium salts of /3-keto-esters (44) (Scheme 20). ... 

Nitropyridine and 4-hydroxypyridine-l-oxide in boiling methanol react to give 7V-(4 -pyridyloxy)-4(l/0pyridone (MI-371), which rearranges to 3,3 -(4,4 -dihydroxy)bipyridyl on heating at 100 in benzene or dioxane. Evidence supports a homolytic fission of the N—0 bond in XII-371 to give a biradical intermediate. ... 

Her examined the pH-tltration behavior of silicic acid in the presence of 2-hydroxypyridine 1-oxide by titrating 16 mA/ (1000 ppm SiO ) silicic acid silica from pH 10.5 to 3.0 in the presence and absence of a 43 mA/ concentration of the N-oxide. At no point did the titration curves differ, indicating that no complex had formed. In another experiment, a solution of Si(OH) containing 100 ppm as SiOj was mixed with a 200-fold excess of the above -oxidc at pH 6.15 and aged for a few hours. Tests with molybdic acid showed that the reaction rate with silica monomer was the same as a control, indicating either that no complex was formed at this pH or that it dissociated very rapidly. However, the rate of dissolution of monomer from colloidal silica particles at pH 1.4 was apparently doubled in the presence of a 20 mA/ concentration of the yV-oxide, indicating some type of interaction at low pH. -... 

Reszka, K. J. and Chignell, C. F., Photochemistry of 2-mercaptopyridines. 3. EPR study of photoproduction of hydroxyl radicals by N-hydroxypyridine-2-thione using 5,5-dimethyl-l-pyrroline N-oxide in aqueous-solutions Photochem. Photobiol, 61, 269, 1995. 

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