Glucose cyclic hemiacetal structure

Figure 9.5 Cyclic, hemiacetal structures of D-glucose. The reaction between an alcohol and aldehyde group within an aldohexose results in the formation of a hemiacetal. The only stable ring structures are five- or six-membered. Ketohexoses and pentoses also exist as ring structures due to similar internal reactions.

He favored the currently accepted, cyclic hemiacetal structure for the glucosides, but formulated them with the furanoid ring for glucose proposed in 1883 by B. Tollens. W. N. Haworth later established that the glucose residue in such glucosides is present in the form of a pyranoid ring. 

FIGURE 2.18 The cyclic hemiacetal structures of D-glucose and D-fmctose. 

Remember, sugars in solution typically exist as equilibrium mixtures ol open chain plus cyclic hemiacetal structures. For glucose, this equilibrium mixture contains 63.6% /3-pyrano.se, 36.4% o-pyrano.sc, together with just traces of open chain and furanose structures. I hc interconversion of (3 and a anomers is culled nuilamialion. 

These differences show up in the different chemical reactions of D-glucose. Finally, C-1 in the cyclic, hemiacetal structure is a stereogenic center. It has four different groups attached to it (H, OH, OC-5, and C-2) and can therefore exist in two configurations, R or S. Let us consider this last feature in greater detail. 

Monosaccharide structures may be depicted in open-chain forms showing their carbonyl character, or in cyclic hemiacetal or hemiketal forms. Alongside the Fischer projections of glucose, ribose, and fructose shown earlier, we included an alternative... 

As we stated previously, glucose forms some, but not all, of the common carbonyl derivatives. The amount of free aldehyde present in solution is so small that it is not surprising that no hydrogen sulfite derivative forms. With amines, the product is not a Schiff s base but a glucosylamine of cyclic structure analogous to the hemiacetal structure of glucose, Equation 20-3. The Schiff s base is likely to be an intermediate that rapidly cyclizes to the glucosylamine ... 

The sugar fructose is an isomer of glucose. Like glucose, fructose forms a cyclic hemiacetal, but in this case the ring is five membered rather than six membered. Show the structure for the hemiacetal formed from fructose and show a mechanism for its formation in acidic solution. 

Two structures for the sugar glucose are shown on page 858. Interconversion of the open-chain and cyclic hemiacetal forms is catalyzed by either acid or base. 

The structural lability of the cyclic hemiacetals is a consequence of their oxo-cyclo tautomerism, which allows a rapid interconversion of the (2R)-configurated enantiomer into the (2S)-enantiomer and vice versa via the ring-opened oxo form (Fig. (4)). An analogous and well investigated process is the formation of an equilibrium mixture of a-D-glucose and p-D-glucose on dissolution of the pure a- or P-isomer in water by mutarotation. 

The actual structure of monosaccharides such as o-glucose is not what has been shown up to this point. We previously learned that aldehydes and ketones react reversibly with alcohols to form hemiacetals and hemiketals, respectively. (Refer back to Sec. 14.7 to review this reaction.) This reaction takes place intramolecularly with monosacchardes since both OH and C = O are present in the same molecule. The structures of aldoses and ketoses are cyclic hemiacetals and hemiketals, respectively. 

---