Osazone formation. Epimers

The carbonyl group (and adjacent alcohol) oxidizes with excess phenyl hydrazine (PhNHNH2) to form an osazone (see Figure 16-15). Osazone formation is very important in determining the relationship between various monosaccharides. For example, both D-glucose and D-mannose produce the same osazone, so they re epimers. Epimers differ by only one chiral center, which osazone formation destroys. 

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. 

Because the configuration of the number-2 carbon is lost during osazone formation, C-2 epimers form identical osazones. For example, o-idose and D-gulose, which are C-2 epimers, both form the same osazone. 

In the formation of an osazone, both Cl and C2 are converted to phenylhydrazones. Therefore, a ketose gives the same osazone as its related aldose. Also notice that the stereochemistry at C2 is lost in the phenylhydrazone. Thus, C2 epimers give the same osazone. 

The pair of aldoses obtained from the sequence differ only in configuration about C 2, and hence are epimers. A pair of aldoses can be recognized as epimers not only by tbeir conversion into the same osazone (Sec. 34.7), but also by their formation in the same Kiliani-Fischer synthesis. 

In a study of the deacetylation of acetylated sugar osazones it has been shown that under alkaline conditions and depending upon the substrate, either an external nucleophile fe.a.. Na, MeO , NH3) can be incorporated at C-3 or a 3,6-anhydride can be formed. The 3-methoxy-epimers (49) were thus formed from the L-ervthro-pentose osazone triacetate presumably by formation and 1,4-addition to a phenylazo-ene intermediate." The 2-acetoxy group of aldose diphenylformazan peracetates is also readily displaced. 2-Deoxy-D-arabino-hexose diphenylformazan (50) was thus formed when penta-Q-acetyl-D-glucose or -D-mannose H H diphenylformazan was treated with sodium borohydride." ... 

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