Osazones chelated structures

Analogous to the formazans obtained by loosening the chelate ring in the above manner, the sugar osazones of supposedly chelate structure were found to undergo reaction in alkaline alcohol and to give n-glucose phenylosazone formazan in dark-violet needles. Mild acetylation produces the black tetraacetate of this compound. 

Further evidence for the chelated structure of osazones was obtained by physical methods,317 such as polarography318 and ultraviolet3 7,319 327 and 1H NMR307 312326 spectroscopy, which clearly showed that the two NH groups (attached to C-l and C-2) were not in identical environments. Similarly, 5N N M R spectroscopy revealed significant differences in the chemical shifts of the two H-coupled nitrogen atoms, attributable to chelation.328,329... 

Scheme 36. The starting and ending chelated structures of osazones in basic media. ...

Because of the chelated structure of sugar osazones, the osazone formazan 181, obtained by coupling diazotized [14C]aniline to a saccharide phenylosazone (180) loses 42%) of the label (as aniline) upon conversion to an osolriazole (182), whereas the formazan of an unchelated osazone, such as pyruvaldehyde osazone loses exactly half of the radioactivity as aniline during osolriazole formation (Scheme 40).168... 

It will be seen that the cleavage of the hypothetical hydrazo derivatives to the hypothetical imino compounds obviates the necessity of assuming direct oxidation by phenylhydrazine. Fieser and Fieser inquire why osazone formation ceases with the introduction of two phenylhydrazine residues, and seek to explain this fact by the postulation of chelate tautomeric ring structures XLIII or to the existence of a resonance... 

In order to find the reason for the failure of formazan reaction, a model experiment was set up using 2-quinolinecarboxaldehyde phenylhydrazone whose structure, if the nitrogen of the quinoline be considered, is like that of a phenylosazone, with the six-membered chelate ring in it as proposed by Fieser and Fieser for sugar osazones. 

On the basis of these experimental facts and the above considerations concerning the structure of n-glucose phenylosazone, the chelate ring form VI of Fieser and Fieser appears to describe the finer structure of the mixed osazone A. 

The structure of inosose bis(phenylhydrazones) resembles that of saccharide osazones in that the bishydrazone residues in both are chelated that is, bridged by an imino group (the other NH group is unchelated). 

In similar studies on the structures of osazones, but using 100 Mc./sec. n.m.r. spectroscopy, a long-range coupling between the C-1 proton and the non-chelated imino-proton was observed, and was confirmed by double resonance and deuteration techniques. 

Hi) Bicycle Dianhydro-Osazones.—This third type of anhydroosazone was first prepared by Percival when deacetylating a hexulose phenyl-osazone tetraacetate with sodium hydroxide. Two enantiomeric compounds are otainable from hexose precursors, one from D-hexoses and the other from L-hexoses.166 Tricyclic structures were first assigned to these compounds, but were later revised.428 Finally, NMR data provided evidence for a chelated bicyclic structure 231 which possesses one imino proton429 These dianhydroosazones are formed from the same intermediate, 2-phenylazo-l-phenylhydrazono-l-alkenes (229) by conjugate elimination to give 230, followed by nucleophilic attack of 0-6 to afford the product 231 (Scheme 54). 

Further evidence for an acyclic structure for glucosazone is provided by the formation of a formazan after treatment with benzenediazonium chloride 238). The osazone may have a six-membered chelate ring formed by hydrogen bonding of the two hydrazine radicals. (See under Hy-drazones.)... 

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