Pyranose forms conformation

Haworth formulas are satisfactory for representing configurational relationships in pyranose forms but are uninformative as to carbohydrate conformations X ray crystal lographic studies of a large number of carbohydrates reveal that the six membered pyra nose ring of D glucose adopts a chair conformation... 

Other aldohexoses behave similarly m adopting chair conformations that permit the CH2OH substituent to occupy an equatorial orientation Normally the CH2OH group is the bulkiest most conformationally demanding substituent m the pyranose form of a hexose... 

Dextrose in solution or in soHd form exists in the pyranose stmctural conformation. In solution, a small amount of the open-chain aldehyde form exists in equiUbrium with the cycHc stmctures (1) and (2). The open-chain form is responsible for the reducing properties of dextrose. 

Draw clear conformational representations of the / -pyranose forms of each of the following carbohydrates ... 

As is apparent from the discussion thus far, fluoro analogs in the furanose form have been studied to a lesser extent than those in the pyranose form, fundamentally because the data they provide are more difficult to interpret, because the conformational mobility of the furanose forms is high (on the n.m.r. scale). A study of 3,6-anhydro-5-de-... 

For ketoses, this simple method of calculation does not give the correct values for the a / -pyranose ratio. Apparently, the conformational interactions have different values in this case it is considered15 that both the st/n-axial and the gauche interactions are larger on the anomeric carbon atom, which carries two substituents, than they are on that in the aldoses. Owing to these larger interactions, the pyranose forms of the ketoses are less stable than those of the aldoses, and, consequently, for the ketoses... 

The position of the pyranose-furanose equilibria in solution has been determined for the four 5-acetamido-5-deoxypentoses the proportion of the pyranose form (which contains the nitrogen atom in the ring) is — 65% for the xylo, 50% for the lyxo, 25% for the arabino, and 10% for the ribo isomer,124 and this is the order found for the parent pentoses. The corresponding 5-(benzyloxycarbonyl)amino-5-deoxypentoses, however, exist in solution almost exclusively in the pyranose form,130 reflecting the diminished extent of deactivation of the amide nitrogen atom but a solution of 5-(benzyloxycarbonyl)amino-5,6-dideoxy-3-0-mesyl-L-idose was found to contain 20% of the furanose forms, because the steric effect of the N-acyl group forces it into the particularly unfavorable 4Cx(l) conformation (26) of the y3-pyranose form.132... 

Hamamelose is 2-C-(hydroxymethyl)-D-ribose. The bulky branch forces the pyranose forms to exist mainly in the (d) conformation (30),... 

It is most straightforward to begin with a-L-sorbopyranose (1) (see Figure 1) because this sugar shows virtually no mutarotational change in water (I, 7) thus, it remains basically in the a-pyranose form. By analogy with the established (8) structure of the crystalline material, this sugar exists in solution in the 1C(l) pyranose chair conformation (1) because all substituents are thereby optimally oriented—i.e., the... 

These spectra have detected only one species of L-sorbose in aqueous solution. However, after prolonged storage in methyl sulfoxide 5-10% of a second component is generated. Although it has not as yet been properly identified, there is no counterpart in spectra of the heptulose, and most likely it is the /f-pyranose—i.e., the existence of furanose forms of 1 and 2 should have about equal probability, whereas the / -pyranose form of 2 in a chair conformation should be much less stable than that of 1. 

Chemical and conformational changes occur on complex formation of D-lyxose. The a-pyranose form lacks the ax-eq-ax sequence, but the / -pyranose form contains it in its 1C(d) conformation. This conformation, which has three axial hydroxyl groups, is not normally observed... 

If it is true that the structural form of D-glucose which reacts with boric acid is the a-D-pyranose form, then that form probably exists in a boat or twist conformation in the complex. This implies that the study of the stability constants of sugar borate ester might give information about the ability of various carbohydrates to form such boat or twist conformations (10, 21). 

Since almost no furanose form of D-glucose is observed in solution, it is expected that the energies of the furanose conformational isomers will be much greater than those of the pyranose form. In fact, the four furanose isomers examined are predicted to be about 4-5 kcal mol higher than the lowest pyranose conformers 6a and 6c at HF/6-31G. Quite remarkable, however, was the B3LYP/6-31G ... 

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. 

On hydrolysis with sulfurous acid, 5-amino-5-deoxy-l,2-0-isopro-pylidene-jS-D-arabinofuranose gives an acyclic bisulfite adduct which, on treatment with barium hydroxide, afiFords a solution of 5-amino-5-deoxy-D-arabinopyranose that behaves similarly to the analogous D-xylose compound. The Amadori rearrangement proceeds with somewhat more difiBculty. The equilibrium between the pyranose form and its dehydration product is recognizable by the presence of a positive Cotton efiFect (300 nm), a result predictable by theory, as 42 should represent the most favored conformation. 

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