11.10 Determination of Ring Size
The anomeric carbon can be found via methylation of the –OH groups, followed by hydrolysis. In the first step, all the –OH groups are transformed to –OCH3 groups with excess methyl iodide and silver oxide. The hydrolysis of the acetal then forms a hemiacetal in presence of acid. This pyranose structure is in equilibrium with its open-chain form. From the open-chain form we can determine the size of the ring because the anomeric carbon attached –OH group is the one that forms the cyclic hemiacetal (Scheme 7).
Scheme 7
A monosaccharide's ring size can be determined by the oxidation of an acetal of the monosaccharide with excess periodic acid. The products obtained from periodate cleavage of a six-membered ring acetal are different from those obtained from cleavage of a five-membered ring acetal (Scheme 8-9).
Scheme 8
Scheme 9