Factors Affecting Ring Structure and Conformation

Factors Affecting Ring Structure and Conformation 1. Reactions of aldehydo and keto Sugars in Solution 

When the two modifications of a sugar showing a simple mutarotation are known, the equilibrium proportions may be calculated from the equilibrium rotation and the initial optical rotation of the known crystalline modifications. In the event that only one modification is known in the crystalline form, the proportion of the second modification may be obtained from the initial and maximum solubilities of the first, as described on page 18. With sugars that show complex mutarotation, the proportions of the a- and /8-pyranose modifications may be estimated roughly from the extent of the slow mutarotation as compared to the optical rotations of the known a- and 

The tendency of the sugar to form cyclic structures arises from the capacity of the carbonyl group to combine with neighboring hydroxyl groups. When there is no hydroxyl group in a position suitable for an intramolecular reaction to occur, an intermolecular reaction may take place. Thus, glycolaldehyde yields the dimer. 

Examples are the crystalline, dimeric forms of 1,2-0-isopropylidene-a-D-xyio-pentodialdo-l,4-fiiranose and 2,4-O-ethylidene-D-ery-throse.  

Certain dialdehydes obtained by periodate oxidation and lead tetraacetate oxidation form hemialdals. Interconversion of the monomers and dimers in aqueous solution is slow, and the proportions of monomer and dimer depend on the history of the sample. Separation of the dimer of 1,2-O-isopropylidene-a-D-xt/lo-pentodialdo-1,4-furanose from an aqueous solution by crystallization requires a period of several weeks. 

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