General Properties of 1,6-Anhydrohexopyranoses

Levoglucosan (6) is not fermentable by yeast27,185 (compare Ref. 336), and remains intact on treatment with emulsin,6,27,185 amylases, or mal-tase.185 It does not influence the respiration quotient337 for its obscure role in diabetes, see Refs. 338 and 339. l,6-Anhydro-/3-D-galactopyra-nose (12) is neither a substrate for, nor an inhibitor of, the /3-D-galac-tosidase from Escherichia coli.340 

6- Anhydrohexopyranoses are relatively stable in aqueous alkalis, even on heating,56,57 and this property can be utilized in their separation from free sugars they do not reduce Fehling solution. They are, however, readily hydrolyzed by mineral acids to reducing hexoses, especially when heated, and an equilibrium is established between the hexose and its 1,6-anhydro derivative (see Table I). 

The structure of some 1,6-anhydrohexopyranoses has been demonstrated by classical, methylation analysis,188,282,308 and also follows from the results of their oxidation by periodate99,104,118,122-124,283,288,289,341-344 or lead tetraacetate.345 The incorrect structures originally proposed for levoglucosan28,185,186,188,346,347 were later revised.341,348 

In the crystalline state, the pyranose ring of 6 is flattened4 to such an extent that the distance between the oxygen atoms of the hydroxyl groups on C-2 and C-4 increases from the usual value of 240 pm to 329.9 pm (compare Ref. 358), and the oxygen atom on C-3 recedes to a distance of 300 pm from C-6. No intramolecular hydrogen-bond was observed.4 

In contrast to their per-O-acetylated derivatives, free 1,6-anhydro-hexopyranoses do not have sharp melting-points and, before melting, often sinter or display a tendency to sublime. Under diminished pressure, they distil or sublime without decomposition. 

---