Oxidation of Difructose Anhydrides by Per-iodic Acid

Per-iodic acid oxidation has been used by many investigators in structural studies because this reagent oxidizes selectively the hydroxyls on adjacent carbon atoms. In the course of their studies, McDonald and Jackson found that difructose anhydrides I, II and III and diheterolevulosan reduce per-iodic acid to iodic acid in the following respective molecular ratios 2, 1, 1, and 4 moles of acid per mole of anhydride. 

Difructose anhydride I (XI, page 269) contains two pairs of adjacent carbon atoms to which hydroxyl groups are attached (3,4 and 3, 4 ) thus, it would be expected to react with two moles of per-iodic acid. Difructose anhydride III (XII, page 269) contains only one such pair (3,4) in agreement with its consumption of one mole of acid. Diheterolevulosan (X, page 268) requires four moles of acid, one for each of the following pairs of carbon atoms 3,4 4,5 3, 4 4, 5.  

Difructose anhydride II reacts with one mole of per-iodic acid. Hydrolysis of its methyl derivative indicates that two different trimethyl-D-fructoses are formed whose combined specific rotations in water amount to between 20° and -h 30°. If we assume that this anhydride, like the other two derived from inulin, is made up of D-fructofuranoses, the facts that have been mentioned allow only three possible structures for this anhydride. 

A compound represented by XVIII would yield 1,3,4-trimethyI-D-fructose, a crystalline substance with 51.4° (water), and 

6-trimethyl-D-fructose (now unknown), upon hydrolysis of its hexamethyl derivative. No evidence has been obtained to indicate the presence of this negatively rotating trimethyl-D-fructose and it is established that the mixture of trimethyl-D-fructoses from hexamethyl-di-D-fructose anhydride II rotates in the positive range of 26 to 30°. Unless the 1,3,6-trimethyl-D-fructose should prove to possess the unusually high rotation of about + 100°, which seems at least improbable, structure XVIII can be excluded. 

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