27808-53-3

Pyridofuroxan ([1,2,5]oxodiazolo[3,4-b]pyridine 1-oxide) undergoes isomerization between the N1-oxide and N3-oxide forms which can be observed by the 1H, 13C and 15N NMR spectroscopy but not by 14N and 17O NMR at ambient and low temperatures. The rearrangement becomes slower at low temperatures and at 233 K 1H NMR signals for the two structures become observable. 1H, 13C and 15N chemical shifts and 1H-1H, 13C-1H and 13C-13C coupling constants are used to characterize both forms in the equilibrium mixture. From the 1H NMR integrals at 233 K equilibrium constants are calculated. Protonation studies using trifluoroacetic acid as a solvent showed the favoured site of protonation to be the pyridine N4 nitrogen atom. DFT shielding calculations are reported for the 13C, 15N and 17O nuclei which support the assignments given. From the point of view of structural changes, 1JCC data for 8-nitrotetrazolo[1,5-a]pyridine and o-nitroaminopyridine as precursors of the pyridofuroxans are given for comparison purposes. X-ray diffraction data on 5-methoxypyridofuroxan support the structural results obtained from the NMR investigations. Copyright

Published reports describe the oxidative cyclization of suitable ortho-substituted arenamines to form such products as 2,1-benzisoxazoles, benzofurazan 1-oxides and benzotriazoles, by using bis(acetato-O)phenyliodine at room temperature.However, the reactions are often inconveniently slow.We now report attempts to achieve short reaction times with more powerful iodine(III) oxidants.These often failed to give cyclic products, but the results enable us to argue that the reaction competing with cyclization involves the arenaminyl cation ArN+H.When such cations are predicted to be relatively unstable, the parent arenamine can be rapidly cyclized in high yield by oxidation with bis(acetato-O)phenyliodine in boiling benzene.

Although there are literature reports of anomalous behaviour on pyrolysis of ortho-nitroazidopyridines and ortho-nitroazidoquinolines, we find that these compounds cyclize to the expected furazan 1-oxides in near-quantitative yields, and with a high degree of neighbouring group participation by the nitro group.Kinetic studies in decalin solution reveal the following rate enhancements by the ortho-nitro group: 3-azido-2-nitropyridine, 466 at least; 4-azido-3-nitropyridine, 5400; 4-azido-3,5-dinitropyridine, 640; 4-azido-3-nitroquinoline, 82.4; 5-azido-6-nitroquinoline,27.6; 2-nitro-1-azidonaphthalene, 12.7.The unstable furazan oxide, oxadiazolopyridine 1-oxide, has now been characterized. Oxadiazolopyridine 1-oxide decomposed in decalin at 145 deg to give products indicative of a nitrene intermediate.

Two parallel reactions, viz., cyclization of 2-nitro-3-azidopyridine to pyrido-[2,3-c]furoxan, which is aminated in the 6 position of the pyridine ring with opening of the furoxan ring, and reduction of the azido group to form 2-nitro-3-aminopyridine, occur in the reaction of 2-nitro-3-azidopyridine with amines. It was established that pyrido[2,3-c]furoxan reacts with amines in aqueous media in the 1-oxide form.