Fructose, anomers furanose form

Figure 25.5 Pyranose and furanose forms of fructose in aqueous solution. The two pyra-nose anomers result from addition of the C6 -OH group to the C2 carbonyl the two furanose anomers result from addition of the C5 -OH group to the C2 carbonyl.

Each of these compounds, 53-56, was shown to be a very effective competitive inhibitor of the enzyme with respect to the fructose 1,6-diphosphate, whereas several other analogs, including acyclic structures, had no effect. These and other results suggest that the furanose form of the sugar diphosphate is the active form in the enzymatic reaction (105). More recent studies using rapid quenching techniques and C-nmr measurements have confirmed this hypothesis and indicate that the enzyme uses the a anomer 52 much more rapidly than the 3 anomer 50 and probably uses the a anomer exclusively (106). 

Figure 20-2 Structure and configuration of the o-ketoses from C3 to C6. As with the aldoses (Figure 20-1), the cyclic form is predominantly an oxacyclohexane (pyranose) ring in the free sugar, but the oxacyclopen-tane (furanose) form is shown for fructose because it occurs widely in this form as the disaccharide, sucrose. Only the a anomers are shown (see Section 20-2B).

In the D-fructosides the convention is that the more dextro-rotatory anomer is the a form. Attempts have been made by Boeseken and Couvert,82 Verschuur83 and MacPherson and Percival84 to apply Boese-ken s boric acid method to determine the configuration of D-fructose at C2. The problem is much more complex than for aldoses because there are three hydroxyl groups near the reducing center, and also because the mutarotation of D-fructose involves the conversion of some pyranose to furanose form with the loss of a pair of cis hydroxyls (on C4-C5). This work has been discussed by Boeseken88 but no conclusion has been reached. 

The same procedure can be used to draw the furanose form of D-fructose, the most common ketohexose. Because the carbonyl group is at C2 (instead of Cl, as in the aldoses), the OH group at C5 reacts to form the hemiacetal in the five-membered ring. Two anomers are formed. 

Problem 25.11 Draw both anomers of D-fructose in their furanose forms. 

A five-membered cyclic sugar ring is called a furanose. Fructose prefers a fu-ranose ring system, and is formally named fructofuranose. Like glucose, fructose can cyclize and can form either an alpha anomer or a beta anomer. Notice that,... 

Figure 11.5. Furanose Formation. The open-chain form of fructose cyclizes to a five-membered ring when the C-5 hydroxyl group attacks the C-2 ketone to form an intramolecular hemiketal. Two anomers are possible, but only the a anomer is shown.

Figure 11.6. Ring Structures of Fructose. Fructose can form both five-membered furanose and six-membered pyranose rings. In each case, both a and P anomers are possible.

There are fewer ketoses than there are aldoses because ketoses have one less chiral carbon. The most prevalent of the ketoses are dihydroxyacetone, ribu-lose, xylulose, and fructose (see Figure 4). All four of these sugars are important intermediates in metabolism. Fructose is, along with glucose, part of sucrose or table sugar. Fructose is a ketohexose, and the only one of the four ketoses that can assume a ring structure. Like ribose, fructose forms a five-membered (or furanose) ring and has a and j8 anomers. 

Ans. The previous discussion was an oversimplification. Hemiketal formation in o-fructose and other ketohexoses results in reaction at both C-5 and C-6 to give a mixture of 5- and 6-membered rings. Pure fructose in solution consists of a mixture of the a- and /3-anomers of both furanose and pyranose structures, as well as a very small amount (< 0.2 percent) of the open-chain form. However, all combined forms of o-fructose, e.g., fructose combined with glucose in sucrose, involve only the 5-membered ring. For this reason, most texts simplify matters for the student by ignoring the pyranose structures. The exclusive use of the 5-membered ring in combined forms of o-fructose is not understood. 

Previous assignments of bands characteristic of furanose and pyranose forms (Mathlouthi, Carbohydr. Res., 1980, 7, 225) were shown to be incorrect since these bands co-occur in the spectra of crystalline -D-fructopyranose. Prom the intensity of the carbonyl absorption in fructose solutions (the 0 -deuterated form in D-O) relative to that of diethyl ketone, it was concluded that 0.9 of fructose was present as the open-chain form. From a study of the i.r. spectra of peracetylated aryl glycosides, the value of bands in the 800-1000 cm region for differentiating a- and 6-anoraers has been reassessed. The relative intensities of C-O-C stretching vibrations in the 1000-1100 cm region and a band near 300 cm for -anomers only were suggested as criteria for... 

Anomers also appear in cyclic form as five-membered rings, for instance in the monosaccharides fructose and ribose. These cyclic molecules are named furanoses because of their similarities to the cyclic ether furan. Starting from D-fructose and D-ribose, the cyclization yields anomers a- and p-D-fructofuranose, as well as a-and P-D-ribofuranose, respectively. 

There are obviously many aldose monosaccharides and many ketose monosaccharides, but the discussion will focus only on the d diastereomers, as with the aldoses. The triose is l,3-dihydroxy-2-propanone (41 also called glycerone) and the tetrose is d-glycero-tetrulose (42). Two pentoses are named d-ribulose (43) and d-xylulose (44) and four hexoses are named d-psicose (45), d-fructose (46), d-sor-bose (47), and d-tagatose (48). All of these compounds are ketoses and are further classified according to the number of carbon atoms, as noted in Section 28.1. For example, 42 is a ketotetrose, 43 is a ketopentose, and 45 is a ketohexose. For 43-48, cyclization is possible to form a hemiketal. Just as aldehydes and alcohols react to form a hemiacetal, ketones and alcohols react to form a hemiketal (see Chapter 18, Section, 18.6). For 43, a five-membered ring (a furanose) is formed if the terminal CH2OH unit (in violet) reacts with the ketone carbonyl. The two anomers formed are 49 (a-d-ribulofuranose) and 50 (P-d-ribulofuranose). 

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