Idose process

The identification of L-iduronic acid as the major glycuronic acid constituent of heparin proved to be a much slower process than the identification of the amino sugar residue. Although this compound was detected in acid hydrolyzates of heparin116117 and heparin oligosaccharides,118 its yield was usually poor, because of the drastic conditions used for the acid hydrolysis (which are known to lead to extensive destruction of uronic acid).119120 Also, L-iduronic acid escaped detection as L-idose in the hydrolyzates of carboxyl-reduced heparin, probably because L-idose is readily converted into 1,6-anhydro-L-idose under the usual hydrolytic conditions. 

With the synthesis of gulose, one of the two remaining hexoses (the products of chain lengthening of o-xylose) was identified as structure 9. By process of elimination, the other, D-idose, could be assigned structure 10, thus completing the exercise. 

Kraus and Schwinden utilized protecting groups, especially methoxymethyl (MOM), which allowed the pentose xylose to undergo photoinduced 5-hydrogen abstraction, which produces a cyclic acetal. Acid catalysis converts the acetal carbon to an aldehyde, such that the overall process produces two epimeric hexoses, gulose and idose. The same paper provides a short review of other sugar photochemistry. 

---