Structure of Osazones

Fischer s formulation of phenylosazones in the acyclic structure, such as (43), was based on the fact that the same osazone is produced from two epimeric aldoses and the corresponding 2-ketose, as well as from their phenylhydrazones, indicating that, in the osazone, the phenyl-hydrazine residues are attached at C-1 and C-2 of the saccharide chain. As the asymmetry at C-2 is destroyed during the reaction, it is found that, for a given substituted hydrazine, there are eight isomeric hexose osazones (four D and four l isomers) and four pentose osazones (two d and two l). 

Fischer s acyclic structure (43) is supported by a considerable weight of experimental evidence, summarized in the following. 

Acetylation.— Acetylation of n-arabino- and n-li/xo-hexulose phenyl-osazones results in the formation of crystalline tetraacetates. Wolfrom and coworkers have established an analytical technique that differentiates between iV-acetyl and 0-acetyl groups the latter were determined by the procedure of Kunz and Hudson. They were thus able to show that all four acetyl groups are 0-acetyl groups. This discovery eliminated the possibility of cyclic structures, which would have required three 0-acetyl groups and one A-acetyl group. The infrared spectra of osazone acetates also support the acyclic structure, as these compounds show only one carbonyl band, namely, that of the 0-acetyl groups. 

Micheel and Bohm have used this reaction to prove that, during the isomerization of 2,3,4,S-tetra-O-acetyl-aldehydo-D-galactose-l- C 6-p-toluenesulfonate to aldehydo-n-L-galactose heptaacetate (with zinc chlo- 

The nuclear magnetic resonance spectra of osazones and then-acetates (see Figs. 1 and 2) afford additional proof of the acyclic structure of the osazones. Both the acyclic osazone acetates and the free osazone, such as (43), show two imino protons in their nuclear magnetic resonance spectra which disappear on deuteration, and, as cyclization as in (48) would result in the formation of a third imino proton, it may be concluded that the free osazone exists mainly in the acyclic form (43). 

---

Since the discovery of the reaction of phenylhydrazine with sugars to form osazones by Fischer,35 many investigations have utilized this reaction in the identification and separation of sugars, but the precise structures of osazones and their mode of formation are still in doubt.34 There is no doubt, however, that the initial reaction involves the formation of a hydrazone, which then reacts further to yield eventually a f)u-l,2-hydrazone or osazone. 

After additional experimental work, Hardegger and Schreier proposed formula XXII, observing that Fischer s original structure for the osazones was retained in it since exact structures of osazones had not yet been unequivocally established. They also pointed out that there was only indirect... 

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. 

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. ...

The principal reactions of this class of compounds are summarized in Scheme 172. In most of these reactions the reactive nucleophilic center is the terminal NHj group, although the other exocyclic nitrogen may also be involved, as shown by acetylation, which yields 284 and 285. However, the structure of compound 281 is not the one proposed in a recent report (1582) that attributes the attack to the other exocyclic nitrogen. The formation of osazones (287) from sugars, 2-hydrazinothiazoles, and hydrazine has been reported (525, 531). 

D-Galactosido-D-glucosone and D-glucosido-D-galactosone were prepared by Fischer and Armstrong203 by the action of benzaldehyde on the osazones of synthetic disaccharides. Both osones were hydrolyzed by emulsin. There is some doubt as to the precise structure of the parent sugars.206... 

Historically, techniques such as the formation of osazones and the demonstration of fermentation have contributed significantly to the separation and identification of carbohydrates. Observation of the characteristic crystalline structure and melting point of the osazone derivative, prepared by reaction of the monosaccharide with phenylhydrazine, was used in identification. This method is not completely specific, however, because the reaction involves both carbon atoms 1 and 2 with the result that the three hexoses, glucose, fructose and mannose (Figure 9.19), will yield identical osazones owing to their common enediol form. 

The trihydroxypropyl derivative 63 was prepared from dehydro-L-ascorbic acid phenylosazone (62) by opening the lactone ring with warm alkali and acidifying the mixture after a few minutes, whereupon the pyrazolinone 63 separated immediately.63 The structure of this compound was established by degradation,63 and confirmed by a study of its n.m.r. spectrum.64 The p-tolyl-, p-(bromophenyl)-, and p-(iodophenyl)-osazones of dehydro-L-ascorbic acid were also converted into the corresponding l-aryl-4-phenylazo-3-(trihydroxy-propyl)-5-pyrazolinone, and their acetylation and benzoylation products were prepared.64... 

Another type of substituted pyrazole is obtained by refluxing osazones with acetic anhydride during the acetylation, two molecules of water per molecule are removed. The structure of the resulting 5-(D-g/ycero-diacetoxyethyl)-3-formyl-l-phenylpyrazole 2-acetyl-2-phenylhydrazone (71) was determined by deacetylation followed by oxidation with (a) periodate to a pyrazolaldehyde (72), and (b) permanganate to the known l-phenyl-pyrazole-3,5-dicarboxylic acid (73), as well as by a study of its n.m.r. spectrum, which confirmed the pres-... 

The structure of epimelibiose was established58 through periodate oxidation, and by comparison of its osazone with that of melibiose (melting point, mixed melting point, and x-ray diffraction pattern). 

Treatment of truns-[PdCl2(PhCN)2] with biacetyl-bis(N-methyl, N-phenyl)osazone (L) in CH2C12 yields the square-planar osazone complex [PdCl2L], which under suitable conditions loses HC1 to yield the internally metallated complex (23), the structure of which has previously been determined.76,77 Studies of the condensation... 

---