Glucose furanose forms

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... 

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... 

Fig. 2.2 Structures of common ginsenosides. The core damarane structure common to ginsenosides is shown along with the carbon skeletons of the 20(S)-protopanaxadiol and 20(S)-protopanaxatriol ginsenosides. The sugar decorations of the most common ginsenosides are indicated in the table below the structures. Legend Ara(/), arabinose (furanose form) Ara(p), arabinose (pyranose form) Glc, glucose...

It follows that, when we dissolve a sugar such as glucose or ribose in water, we create a mixture of various equilibrating structures. The relative proportions of pyranose and furanose forms, and of their respective anomers for the eight aldohexoses, are shown in Table 12.1. In each case, the proportion of non-cyclic form is very small (<1%). 

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-... 

To accommodate these facts, the earliest mechanisms proposed for degradation of D-fructose assumed that it was present in the furanose form, and that the ring remained intact. It was assumed that the initial reaction was the elimination of water, to form the 1,2-enolic form of 2,5-anhydro-D-mannose, and that further dehydration resulted in 2-furaldehyde. The necessity for D-glucose to isomerize to D-fructose was assumed to account for the much lower reaction-rate of D-glucose. This mechanism does not account for the observation that 2,5-anhydro-D-mannose is less reactive than D-fructose, nor is there any evidence that 2,5-anhydro-D-mannose is present in reacting D-fructose solutions. Nevertheless, similar mechanisms have since been proposed.13-16 Because of the ease of mutarotation of D-fructose... 

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 ... 

In the absence of pyranose forms, the acyclic forms have to compete, in solution, only with the (much less stable) furanose forms, and should therefore be present in much higher proportion than in solutions of glucose and mannose. 5-O-Methyl-D-glucose and -mannose, and 5,6-di-O-methyl-D-glucose give a red color with the Schiff reagent, in contrast to the unsubstituted sugars,116 which do not. 

The protonated amino group, however, has no nucleophilic activity, and does not form a hemiacetal an amino sugar can, therefore, be obtained, as a salt, in an otherwise unfavorable form. For example, 6-amino-6-deoxy-L-sorbose can be isolated as the hydrochloride of a furanose form (22) (presumably a). In alkaline solution, immediate ring-expansion to the pyranose form (23) occurs127 this reaction can be reversed under strongly acidic conditions. 4-Amino-4,6-dideoxy-D-glucose hydrochloride forms a 35 65 mixture of the a- and / -pyranose forms in solution.128... 

Different forms of glucose that result from dissolving glucose in water. At 25°C in water, glucose reaches an equilibrium containing about 0.02% free aldehyde, 38% a-pyranose form ([ce] D= 113), 62% /3-pyranose form ([ajD=9), and less than 0.5% of the furanose forms. The anomeric carbon is shown in color. 

As found typically for glucose and idose, nearly equal proportions of the two furanose forms were observed39 for 5-deoxy-5-fluoro-D-glucose (45 55) and -L-idose (47 53). 

Glycosides - formed between a sugar molecule and an alcohol - are in some sense similar to ethers. Glycosides are formed between the sugar molecule in a ring conformation (pyranose or furanose form) and an alcohol. The example below shows D-glucose (2.31), in equilibrium with p-D-glucopyranose (2.32). 

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