Mannose reaction with phenylhydrazine

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. 

Very little conclusive evidence for the position of the methyl group in 2-methyl-D-mannose is available. Pacsu and Trister4 showed that the sugar reacted with phenylhydrazine in the cold to give a phenylhydrazone but not a phenylosazone. More drastic conditions were required to form the phenylosazone, during which reaction the methyl group was lost with the formation of D-glucosazone. 

Elimination of acetic acid occurs readily from phenylhydrazones of acetylated aldoses. For example, the reaction between 2,3,4,5,6-penta-O-acetyl-aZde%do-D-glucose and phenylhydrazine gives the unsaturated phenylhydrazone, 3,4,5,6-tetra-O-acetyl-l, 2-dideoxy-l -phenylazo-n-ara-6mo-hex-l-enitol (137), which may also be prepared by acetylation of the acyclic phenylhydrazones of n-glucose and D-mannose, or by condensation of 2,3,4,5,6-penta-0-acetyl-aZdeA /do-D-mannose with phenylhydrazine. 2,3,4,5,6-Penta-O-acetyl-D-glucose phenylhydrazone (138) has been... 

For many years the precise structures of some of the products obtained from reaction of sugars with phenylhydrazine have been the subject of much discussion. Reaction of D-mannose phenyl-hydrazone with acetic anhydride in pyridine yields D-ara6fwo-3,4,5,6-tetraacetoxyl-l-phenylazo-1-hexene. The n.m.r. spectrum of this compound clearly establishes its structure. Acyclic structures have similarly been assigned to various phenylhydrazones and osazones. The work of Wolfrom and his co-workers perhaps best illustrates the kind of approach that has been adopted. The n.m.r. spectrum of o-lyxohexose phenylosazone was measured in dimethyl sulphoxide. The three possible structures of the osazone (44, 45, 46) were considered in terms of the spectrum. 

Although D-fructose is not an epimer of D-glucose or D-mannose (D-fructose is a ketohex-ose), all three yield the same phenylosazone. (a) Using Fischer projection formulas, write an equation for the reaction of fructose with phenylhydrazine. (b) What information about the stereochenfistry of D-fructose does this experiment yield ... 

As might be expected, the carbonyl function in aldoses and ketoses undergoes condensation reactions with amine derivatives (Section 17-9). For example, treatment of o-mannose with phenylhydrazine gives the corresponding hydrazone, o-mannose phenylhydrazone. Surprisingly, the reaction does not stop at this stage but can be induced to continue with additional phenylhydrazine (two extra equivalents). The final product is a double phenylhydrazone, also called an osazone (here, phenylosazone). In addition, one equivalent each of benze-namine (aniline), ammonia, and water is generated. 

Since neither the 2V-methyl nor the 2V-benzyl derivative of D-mannose phenylhydrazone showed a color change or the presence of paramagnetic species following treatment with base, it was concluded that the colored radical anion from D-mannose phenylhydrazone must arise following the abstraction of the acidic imino proton by base. Thus, this must be a first reaction step before fission of the phenylhydrazine moiety can occur. 

---