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Furoxan rearrangement of some pyridofuroxan derivatives studied by 1H, 13C, 14N, 15N and 17O NMR spectroscopy

✍ Scribed by P. Cmoch; B. Kamieński; K. Kamieńska-Trela; L. Stefaniak; G. A. Webb

Publisher
John Wiley and Sons
Year
2000
Tongue
English
Weight
107 KB
Volume
13
Category
Article
ISSN
0894-3230

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✦ Synopsis

Pyridofuroxan ([1,2,5]oxodiazolo [3,4-b]pyridine 1-oxide) undergoes isomerization between the N1oxide and N3-oxide forms which can be observed by the 1 H, 13 C and 15 N NMR spectroscopy but not by 14 N and 17 O NMR at ambient and low temperatures. The rearrangement becomes slower at low temperatures and at 233 K 1 H NMR signals for the two structures become observable. 1 H, 13 C and 15 N chemical shifts and 1 H-1 H, 13 C-1 H and 13 C-13 C coupling constants are used to characterize both forms in the equilibrium mixture. From the 1 H 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 13 C, 15 N and 17 O nuclei which support the assignments given. From the point of view of structural changes, 1 J CC 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.

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