Hemiacetals conformation

Crystallographic data have not yet been reported for one of the four hexuloses, namely, D- or L-psicose, not yet crystallized. Preliminary results are available for /3-D-fructopyranose.18a As expected, the sugar has the 1C (d) conformation. The hemiacetal group on C-2 is not bound to a hydrogen atom, whereas 0-6 is linked to two hydrogen atoms. 

Notice that the eclipsed conformation of D-ribose derived directly from the Fischer projection does not have its C-4 hydroxyl group properly oriented for furanose ring formation. We must redraw it in a conformation that permits the five-membered cyclic hemiacetal to form. This is accomplished by rotation about the C(3)—C(4) bond, taking care that the configuration at C-4 is not changed. 

To rewrite the eclipsed conformation of L-mannose in a way that permits hemiacetal formation between the carbonyl group and the C-5 hydroxyl, C-5 is rotated 120° in the clockwise sense. 

The hemiacetal opens to give an intermediate containing a free aldehyde function. Cyclization of this intermediate can produce either the a or the /I configuration at this center. The axial and equatorial orientations of the anomeric hydroxyl can best be seen by drawing maltose with the pyranose rings in chair conformations. 

T. J. Painter, Preparation and periodate oxidation of C-6-oxycellulose conformational interpretation of hemiacetal stability, Carbohydr. Res., 55 (1977) 95-103. 

When we first investigated processed foods for the occurence of RP s there was no direct method for the determination of their conformation. An RP can exist in the open keto-forra or hemiacetal ring structures derived from it (Figure 1)... 

For each of these diastereotopic hemiacetals there are in principle two different chair conformers, but in each case only one chair conformer is found. The other one would contain too... 

As was the case for cyclohexane derivatives, the chair conformer that has the larger groups equatorial is usually more stable. Therefore, the chair conformers shown in Figure 25.2, which have most or all of the larger substituents equatorial, are more stable than the conformers obtained by ring-flips. The a- and /3-anomers differ only in the stereochemistry of the groups at the hemiacetal carbon. In the a-anomer the hydroxy group on this carbon is axial, and in the /3-anomer it is equatorial. 

Close the ring, and draw the result. Always draw the Haworth projection or chair conformation with the oxygen at the back, right-hand comer, with Cl at the far right. Cl is easily identified because it is the hemiacetal carbon—the only carbon bonded to two oxygens. The hydroxyl group on Cl can be either up or down, as discussed in Section 23-7. 

Draw the chair conformation puckered, as shown in Figure 23-6. The hemiacetal carbon (Cl) is drawn at far right (as the footrest), and the ring oxygen is at the back, right comer. 

Draw the cyclic hemiacetal forms of D-mannose and D-galactose both as chair conformations and as Haworth projections. Mannose is the C2 epimer of glucose, and galactose is the C4 epimer of glucose. 

Allose is the C3 epimer of glucose. Draw the cyclic hemiacetal form of D-allose, first in the chair conformation and then in the Haworth projection. 

Some of the most important saturated oxygen heterocycles are the sugars. Glucose is a cyclic hemiacetal—a pentasubstituted tetrahydropyran if you like—whose major conformation in solution is shown on the right. 

Move away from glucose, and the effect is still there. Here, for example, is the NMR spectrum of this chloro compound. There are now only two possible conformations (no configurational changes are possible because this is not a hemiacetal)—both shown—and from the NMR spectrum you should be able to work out which one this compound has. 

Glucose has all substituents larger than a hydrogen atom in the more roomy equatorial positions, making it the most stable and thus most prevalent monosaccharide. The p anomer is the major isomer at equilibrium, moreover, because the hemiacetal OH group is in the equatorial position, too. Figure 27.7 shows both anomers of D-glucose drawn as chair conformations. 

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