Sugars normally exist in cyclic forms with much stereochemistry
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
1142
2025-08-13
349
Sugars normally exist in cyclic forms with much stereochemistry
The most important sugar is glucose. It has a saturated six-membered ring containing oxygen and it is best drawn in a chair conformation with nearly all the substituents equatorial. It can also be drawn as a flat configurational diagram. We have already met one sugar in this chapter, ribose, because it was part of the structure of nucleic acids. This sugar is a five-membered saturated oxygen heterocycle with many OH groups. Indeed, you can defi ne a sugar as an oxygen heterocycle with every carbon atom bearing an oxygen-based functional group— usually OH, but alternatively C=O.
The drawings of glucose and ribose show a number of stereogenic centres, with one centre undefined—an OH group shown with a wavy bond. This is because one centre in both sugars is a hemiacetal and therefore the molecule is in equilibrium with an open-chain hydroxy aldehyde. For glucose, the open-chain form is this.
When the ring closes again, any of the OH groups could cyclize on to the aldehyde but there is no real competition—the six-membered ring is more stable than any of the alternatives (which could have three-, four-, five-, or seven-membered rings—check for yourself). However, with ribose there is a reasonable alternative.
The most important sugars may exist in an open-chain form, as a five-membered oxygen heterocycle (called a furanose, after the five-membered aromatic compound furan) or a six membered oxygen heterocycle (called a pyranose, after the six-membered pyran). Glucose prefers the pyranose structure; ribose prefers the furanose structure.
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