Fructose cyclic hemiacetal form

Glycolysis is a ten-step process that begins with isomerization of glucose from its cyclic hemiacetal form to its open-chain aldehyde form—a reverse nucleophilic addition reaction. The aldehyde then undergoes tautomerixa-tion to yield an enol, which undergoes yet another tautomerization to give the ketone fructose. 

Some monosaccharides also exist in a five-mem be red cyclic hemiacetal form called a furanose form. D-Fructose, for instance, exists in water solution as 70% /Tpvranose, 2% a-pyranose, 0.7% open-chain, 23% /3-furanose, and 5% a-furanose. The pyranose form results from addition of the -OH at C6 to the carbonyl group, while the furanose form results from addition of the —OH at C5 to the carbonyl group (Figure 25.5). 

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... 

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. 

Fructose forms a five-membered cyclic hemiacetal. Five-membered rings are usually represented as flat Haworth structures. 

Chitose has often been stated to give 5-(hydroxymethyl)-2-fur-aldehyde readily, yet the yield of 5-(hydromethyl)-2-furaldehyde obtained was 12%, compared to 20-25% from D-fructose and about 1% from aldohexoses. Since chitose exists in the aldehydo form, it should react more readily than aldohexoses, which are almost entirely cyclic hemiacetals. 

The enol (72) may show cis-trans isomerism, and the aldehyde group may exist as the hydrate or hemiacetal. The compounds which have been isolated, 2-methyl ethers, are cis and exist as cyclic hemiacetals. The tautomeric change, (73) to the enol (72), also gives the cis form. However, the trans isomer may be formed by the action of acid on n-fructose, since (rans-hexosulos-3-ene (34) has been isolated from the reaction of acid on D-fructose (see Section IV, 2o p. 194). 

D-glucopyranose is a cyclic hemiacetal, which is an equilibrium with its open-chain form that contains an active aldehyde group. In contrast, the anomeric carbon atoms of glucose and fructose are joined in an a-glycosidic linkage in sucrose. Hence sucrose is not in equilibrium with an active aldehyde or ketone form. 

In monosaccharides (e.g., glucose, fructose), the cyclic ring structure is formed by the intramolecular reaction between the carbonyl and the hydroxyl groups within the same molecule. Glucose (an aldohexose) forms a cyclic hemiacetal by the intramolecular reaction between the aldehyde group of carbon 1 and the hydroxyl group of carbon 5. 

Cyclic hemiacetals and hemiketals are much more stable than the open-chain compounds discussed earlier. In Chapter 7, we ll see that sugars such as glucose and fructose exist predominantly in the form of cyclic hemiacetals and hemiketals. 

Fructose also forms five-membered cyclic hemiacetals. j8-D-Fructofuranose, for example, is found in the disaccharide sucrose (Section 17.7A). 

Other monosaccharides also form five-membered cyclic hemiacetals. Following are the five-membered cyclic hemiacetals of fructose. 

To summarize the mechanistic and structural studies with hex-2-uloses and 2-C-(hydroxymethyl)pentoses performed under the conditions of the Bflik reaction, the following conclusion can be made. Molybdic acid catalyzes two types of interconversions between the sugars shown in Scheme 13. D-Fructose (17), d-sorbose (21) and D-tagatose (29) when treated with the catalyst are subjected to highly stereospecific carbon-skeleton rearrangements to produce thermodynamic equilibrium mixtures with the respective 2-C-(hydroxymethyl)-D-ribose (D-hamamelose, 20), 2-C-(hydroxymethyl)-D-lyxose (30), and 2-C-(hydroxymethyl)-D-xylose (31). (For simplicity, all the sugars are represented in their acyclic non-hydrated forms in spite of which some of their interconversions proceed in the acyclic hydrated structures, while others proceed in cyclic hemiacetal ones. All of the interconversion relationships are schematically represented in the d series, despite the fact that some of them were experimentally performed with the L series.) Probably because of extensive formation of unproductive com-... 

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