Fructose pyranose form

D-fructose, C HijOo. Crystallizes in large needles m.p. 102-104 C. The most eommon ketose sugar. Combined with glucose it occurs as sucrose and rafftnose mixed with glucose it is present in fruit juices, honey and other products inulin and levan are built of fructose residues only. In natural products it is always in the furanose form, but it crystallizes in the pyranose form. It is very soluble in... 

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). 

The enzymatic synthesis of sucrose also throws light on the formation of the furanose form of fructose in the sucrose molecule. The fact that sucrose is directly formed from D-glucose-l-phosphate and D-fructose supports Isbell and Pigman s34 and Gottschalk s85 evidence that the latter monosaccharide occurs in solution in an equilibrium mixture of furanose and pyranose forms. This makes it unnecessary to postulate a special mechanism of stabilization of a five membered (furanose) ring before the formation of compound sugars containing the D-fructose molecule.86... 

Thermolysis of D-fructose in acid solution provides 11 and 2-(2-hydrox-yacetyl)furan (44) as major products. Earlier work had established the presence of 44 in the product mixtures obtained after acid-catalyzed dehydrations of D-glucose and sucrose. Eleven other products were identified in the D-fructose reaction-mixture, including formic acid, acetic acid, 2-furaldehyde, levulinic acid, 2-acetyl-3-hydroxyfuran (isomaltol), and 4-hydroxy-2-(hydroxymethyl)-5-methyl-3(2//)-furanone (59). Acetic acid and formic acid can be formed by an acid-catalyzed decomposition of 2-acetyl-3-hydroxyfuran, whereas levulinic acid is a degradation prod-uct of 11. 2,3-Dihydro-3,5-dihydroxy-6-methyl-4//-pyran-4-one has also been isolated after acid treatment of D-fructose.The pyranone is a dehydration product of the pyranose form of l-deoxy-D-eo f o-2,3-hexodiulose. In aqueous acid seems to be the major reaction product of the pyranone. 

In sucrose, fructose is present as the P anomer. Now, one of these sugars has acted as an alcohol to make a bond to the other sugar. We can look at this in two ways. Either frnctose acts as an alcohol to react with the hemiacetal glucose to form an acetal, or alternatively, glucose is the alcohol that reacts with the hemiketal fructose to form a ketal. In sucrose, the pyranose ring is an acetal, whilst the fnranose ring is a ketal. This all seems rather... 

In another recent study it was concluded that both the py-ranose and the furanose forms of the alternate substrate 5-keto-D-fructose act as substrates. The pyranose form can be compared to glucose lacking hydro l at C-1 and having a second hydroxyl at C-2 (ketohydrate form). The furanose form is comparable to fructose but bears an extra hydroxyl moiety at C-5. Correcting for the amount of 5-keto-D-fructose in the pyranose form (98%), a K value for the furanose form could be calculated that is more tfian an order of magnitude smaller than that for fructose. This suggests that there is a favorable interaction between the enzyme and the additional C-5 hydroxyl. However, since no conclusive data for C-... 

Known to occur in both pyranose and furanose forms all other sugars (except apiose, fructose, and allulose) are normally in the pyranose form. 

FIGURE 7-7 Pyranoses and furanoses. The pyranose forms of o-glucose and the furanose forms of o-fructose are shown here as Haworth perspective formulas. The edges of the ring nearest the reader are represented by bold lines. Hydroxyl groups below the plane of the ring in these Haworth perspectives would appear at the right side of a Fischer projection (compare with Fig. 7-6). Pyran and furan are shown for comparison. 

A Taste of Honey The fructose in honey is mainly in the jS-D-pyranose form. This is one of the sweetest carbohydrates known, about twice as sweet as glucose. The jS-D-furanose form of fructose is much less sweet. The sweetness of honey gradually decreases at a high temperature. Also, high-fructose corn syrup (a commercial product in which much of the glucose in corn syrup is converted to fructose) is used for sweetening cold but not hot drinks. What chemical property of fructose could account for both these observations ... 

D-fhreo-2,5-Hexodiulose ( 5-keto-D-fructose ) is mainly (>95%) in the / -pyranose form (7) in aqueous solution98 furanose forms involving... 

Why do you suppose that the major form of D-fructose in solution is the pyranose form but D-fructose in sucrose is in the furanose form ... 

One further aspect of the 13C NMR data merits comment. In the spectra of D-fructose and D-allulose, the anomeric 13C signals of furanose forms appear downfield from those of pyranose forms. As already noted by Hall and Johnson (21), this same kind of distinction is observed with D-ribose. Broader examination of these spectra and also those of various other aldoses shows that stronger deshielding of furanose 13C nuclei is... 

Fructose is unique among known sugars in being sweeter than sucrose. In solution, fructose can exist as four or five isomers, and the relative sweetness of a solution is dependent upon the equilibrium between the sweeter pyranose isomers and the less sweet furanose isomers, which is in turn dependent on such conditions as pH and temperature. In cold conditions the pyranose form predominates and, therefore, fructose solutions are sweeter (Danisco Sweeteners, 2003). Fructose has a clean, sweet taste it is also synergistic with many bulk and intense sweeteners and is often used at low levels to improve the taste profile of some intense sweeteners. It is very soluble and also relatively hygroscopic, compared with sucrose (Danisco Sweeteners, 2003). 

The mono- (283) and bis-phosphate (284) derivatives of D-threo-2,5-hexodiulose ( 5-keto-D-fructose, 260) were synthesized enzymically and purified by anion-exchange chromatography. The proportions, ring size, and anomeric configuration were determined by 31P and 13C NMR spectroscopy. Compound 283 was found to exist preponderantly (80%) in the /f-pyranose form, with the remainder being present in the 2R.5R-furanose form. Compound 284 assumes two different furanose forms in solution, one ( 80%) being the 2i ,5i -furanose and the other the 2i ,5S -furanose.500... 

Another interesting example of hindered internal rotation is related to the relative stability of the /3-furanose form of D-fructose (12a) in dimethyl sulfoxide solutions,83139-141 in marked contrast to the fact that the /3-pyranose form (13) is by far the predominant tautomer in water. From the, 3C 7, relaxation data83 (see Table HI) of the three major tautomers of D-fructose—a-furanose (12b), jS-furanose (12a), and 0-pyranose (13)—it can be concluded that these tautomers tumble isotropically in both D20 and (CD3 )2SO solutions. The T, values83 for the second-... 

As already mentioned, the ketoses are an important class of sugars. Ketoses or uloses are isomers of the aldoses but with the carbonyl group occurring at a secondary position. In principle, the keto group can be at each position of the sugar chain, but in naturally occurring ketoses the keto group, with a very few exceptions, is normally at the 2-position, d-Fructose is the most abundant ketose and adopts mainly the pyranose form. 

A Taste of Honey The fructose in honey is mainly in the /3-D-pyranose form. This is one of the... 

At equilibrium, D-fructose exists 67.5% in the pyranose form, 31.5% in the furanose form, and 1% in the open, uncyclized form. Draw the pyranose and furanose forms. Explain why D-fructose has more of the uncyclized form present at equilibrium than does D-glucose. 

For the carbohydrates especially, the amount of available crystal structural data decreases sharply with molecular complexity [479]. With the exception of the cyclodextrins, discussed in Part III, Chapter 18, there are less than 40 crystal structure analyses of oligosaccharides, of which less than 10 are trisaccharides, one is a tetrasaccharide, and one a hexasaccharide (Part III, Chap. 18). The majority of the basic monosaccharides that are the subunits of the polysaccharides that occur naturally have been studied for example, the pyranose forms of /7-arabinose, a-xylose, a- and -glucose, / fructose, a-sorbose, a-mannose, a- and -galactose, a-fucose, a-rhamnose, N-acetyl glucosamine, and mannosamine (Box 13.2). How-... 

The aldehyde substrates may be used as racemic mixtures in many cases, as the aldolase catalyzed reactions can concomitantly accomplish kinetic resolution. For example, when DHAP was combined with d- and L-glyceraldehyde in the presence of FDP aldolase, the reaction proceeded 20 times faster with the D-enantiomer. Fuc 1-P aldolase and Rha 1-P aldolase show kinetic preferences (greater than 19/1) for the L-enantiomer of 2-hydroxy-aldehydes. Alternatively, these reactions may be allowed to equilibrate to the more thermodynamically favored products. This thermodynamic approach is particularly useful when the aldol products can cyclize to the pyranose form. Since the reaction is reversible under thermodynamic conditions, the product with the fewest 1,3-diaxial interactions will predominate. This was demonstrated in the formation of 5-deoxy-5-methyl-fructose-l-phosphate as a minor product (Scheme 5.5).20a 25 The major product, which is thermodynamically more stable, arises from the kinetically less reaction acceptor. 

D-Fructose reacts in the /3-pyranose form, to give the 2,3 4,5-bis(phenylboronate)32 (27), and 6-deoxy-a-L-galactose affords32 the stereochemically related 1,2 3,4-diester (28). 

With D-tagatose (D-/yxo-hexulose) and D-manno-heptulose in the CA conformation, the hydroxyl group at C-3 is axial this gives rise to an anomeric effect that favors the a-pyranose form. Hence, it is not surprising that these compounds exist almost exclusively in the a-pyranose form. The equilibrium states for D-fructose (d-arabino-hexulose), turanose, lactulose, and perseulose (h-galacto-heptulose), are, however, quite different. 

The structural formula of glucose contains a ring having six atoms (C5O, pyranose form) and an aldehyde group (aldoses). However, the structural formula of fructose contains a ring containing five atoms (C4O, furanose form) and a ketone group (ketoses) ... 

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