![]() In order to interconvert between Fisher projection (for open-chain structures), Haworth and Chair conformations (for cyclic structures), this table can be very useful:Īnomers of Simple Sugars: Mutarotation of Glucose Examples of four typical pyranose structures are shown below, both as Haworth projections and as the more representative chair conformers. We know that these molecules are actually puckered in a fashion we call a chair conformation. These Haworth formulas are convenient for displaying stereochemical relationships, but do not represent the true shape of the molecules. In β-D-glucopyranose, the OH group on the anomeric carbon points up. By reacting the OH group on the fifth carbon atom with the aldehyde group, the cyclic monosaccharide α-D-glucopyranose (c) is produced. D-Glucose can be represented with a Fischer projection (a) or three dimensionally (b). Any group written to the right in a Fischer projection appears below the plane of the ring in a Haworth projection, and any group written to the left in a Fischer projection appears above the plane in a Haworth projection.Ī similar intramolecular reaction occurs with the glucose molecule, resulting in two possible anomers: α-D-glucopyranose and β-D-glucopyranose:įigure 1: Cyclization of D-Glucose. The structure is simplified to show only the functional groups attached to the carbon atoms. In the D-family, the -CH 2OH always points up. The anomeric carbon is placed on the right with the ring oxygen to the back of the edgewise view. In a Haworth projection or formula, the molecules are drawn as planar hexagons with a darkened edge representing the side facing toward the viewer. These two stereoisomers of a cyclic monosaccharide are known as anomers they differ in structure around the anomeric carbon-that is, the carbon atom that was the carbonyl carbon atom in the straight-chain form.Ĭommonly, the cyclic forms of sugars are depicted using a convention first suggested by Walter N. The structures on the right side, with the OH group on the first carbon atom pointed upward, is the beta (β) form. The structure shown on the left side of Figure 2, with the OH group on the first carbon atom projected downward, represent what is called the alpha (α) form. When a straight-chain monosaccharide, such as any of the structures shown in ribose, forms a cyclic structure, the carbonyl oxygen atom may be pushed either up or down, giving rise to two stereoisomers. ![]() The anomeric carbon atom (colored orange here) is placed on the right. By convention for the D-family, the five-membered furanose ring is drawn in an edgewise projection with the ring oxygen positioned away from the viewer. ![]() Cyclic structures of this kind are termed furanose (five-membered) or pyranose (six-membered), reflecting the ring size relationship to the common heterocyclic compounds furan and pyran shown on the right.įor example, ribose, an important aldopentose, commonly adopts a furanose structure, as shown in the following illustration. These are the most important examples of cyclic hemiacetal formation in monosaccharides. This means that the aldehyde group in aldohexoses react with carbon 5 to format a six-membered ring, while in the case of aldopentoses, the aldehyde group in aldohexoses react with carbon 4 to format a five-membered ring. This carbon is now chiral, and it is called the anomeric carbon. Five and six-membered rings are favored over other ring sizes because of their low angle and eclipsing strain. The resulting structure will be an intramolecular cyclic hemiacetal. The carbonyl carbon (C1) becomes sp3 hybridized, with four different groups attached to it. In a monosaccharide, the carbonyl (C=O) and alcohol group (OH) exist within the same molecule, so they can react forming a cyclic hemiacetal (or hemiketal, in the case of ketoses). Image by Kupirijo at English Wikipedia, CC BY-SA 3.0, via Wikimedia Commons ![]() In organic chemistry, we described the reaction between carbonyl compounds and alcohol to form hemiacetal and hemiketal respectively, ![]()
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