Fructose, osazone

Fructose (V) under similar conditions gives first the phenylhydrazonc (Va) by the direct condensation of the >C 0 group of carbon atom 2 with one molecule of phenylhydrazine. The second molecule of phenylhydrazine then oxidises the primary alcohol group of carbon atom 1 to the -CHO group by removal of two atoms of hydrogen, which as before serve to reduce the phenyl-hydrazine to aniline and ammonia. The compound (Vb) which is thus produced then undergoes direct condensation with the third molecule of phenylhydrazine, giving the osazone of fructose, or fructosazone (Vc). 

Now since the configuration of carbon atoms 3, 4 and 5 of glucose and fructose art identical, it folhjws that glucosazone and fructosazone are identical in all respects. The osazone is formed however more rapidly from fructose than from glucose, and this difference in rate of formation may be used to distinguish the two sugars, provided the reactions are carried out under strictly parallel conditions (pp. 138, 338). 

Osazone formation. Fructose forms an osazone identical with that obtained from glucose. It is, however, usually formed more quickly... 

Glucose and fructose give the same osazone. This will be evident from the following schemes representing the reactions which are believed to occur ... 

Methylphenylosazones. cw-Methylphenylhydrazine does not form osazones with aldoses presumably because the base or more probably the methylphenylhydrazonium ion [CgH5NCH3NH3]+ will oxidise a—CHjOH but not a >CHOH group it readily forms osazones with ketoses, thus providing an excellent reagent for fructose. 

Simple sugars undergo reaction with phenylhydrazine, PhNHNH2, to yield crystalline derivatives called osazones. The reaction is a bit complex, however, as shown by the fact that glucose and fructose yield the same osazone. 

It has recently been reported21 that a mixed osazone of 3,4-di-O-methyl-D-glucose can be converted, by treatment with p-nitrobenzaldehyde, into an osone which reacts with phenylhydrazine to give 3,4-di-O-methyl-D-glucose phenylosazone. Von Lebedev29 claimed to have obtained D-glu-cosone 6-phosphate, isolated as an amorphous lead salt, by the action of hydrochloric acid on the phenylosazone prepared from D-fructose 6-phosphate. 

See p. 298 for information about the production of the osazones of the simple sugars. The osazones of d-glucose, d-fructose, and d-mannose are identical. 

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. 

Problem 22.11 The C -epimeric aldohexoses glucose and mannose give the same osazone as fructose. Write equations and explain the configurational significance. M... 

Since mannose and glucose are C epimers, they are identical at the C, C. C, and C" portions of the molecule, which are unaltered during osazone formation. The chiral C , which differs in each hexose, loses chirality in the osazone and becomes identical for both mannose and glucose. 2-Ketohexoses give osazones in which the C CH,OH is oxidized. Since C, C, C and C of fructose and glucose are identical (Problem 22.8), fructose also gives the same osazone. Identical portions are in the boxes below. 

Problem 22.12 Osazones are converted by PhCHO to 1,2-dicarbonyl compounds called osones. Use this reaction to change glucose to fructose. 

Problem 22.34 (a) Write shorthand structures for the three o-2-ketohexoses other than o-fructose. (b) Which one does not give a meso alditol on reduction (c) Which one gives the same osazone as does D-galactose (Problem 22.22)7 <... 

D,L-Mannitol has been obtained by sodium amalgam reduction of D,L-mannose. The identical hexitol is formed from the formaldehyde polymer, acrose, by conversion through its osazone and osone to D,L-fructose (a-acrose) followed by reduction (83). 

Glucose and fructose (and also mannose) form the same osazone. The osazones are usually yellow, well-defined crystalline compounds and are sparingly soluble in cold water. The characteristic crystalline forms of the osazones of the commonly occurring sugars, when examined under the microscope, may be employed for their identification (Fig. 9.1) the melting or decomposition... 

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