Reducing sugars pyranose form

The composition of solutions of the 2-heptuloses has been determined, and discussed, by Angyal and Tran.92 These ketoses are different from other reducing sugars inasmuch as there are two hydroxymethyl side chains attached to the pyranose ring. In the a-pyranose form, they are cis to each other and will therefore both be equatorial in the preponderant chair form. In the / -pyranose, however, one or other of the hydroxymethyl groups has to be axial and, in consequence, the / anomers are disfavored in only one solution (that of the altro isomer) was the yS-pyra-nose detected in the 13C-n.m.r. spectrum.92... 

It is reasonable to assume that reducing sugars which exist in the pyranose or furanose form must revert to their open-chain aldehydo or keto forms before any significant C—H ionization occurs. This reversion is brought about by ionization of the acidic hemiacetal hydroxyl group, followed by ring opening. Subsequent carbanion formation at the carbon... 

Figure 2.19 Conformations of representative pyranose and furanose forms of common reducing sugars.

The cyclic hemiacetal (or hemiketal) can react with an alcohol to form an acetal (or ketal), called a glycoside. If the name pyranose or furanose is used, the acetal is called a pyranoside or a furanoside. The bond between the anomeric carbon and the alkoxy oxygen is called a glycosidic bond. The preference for the axial position by certain substituents bonded to the anomeric carbon is called the anomeric effect. If a sugar has an aldehyde, ketone, hemiacetal, or hemiketal group, it is a reducing sugar. 

Table 11 summarizes methylation analyses carried out on certain GXM listed in Table I. Analysis of the carboxy-reduced C. neoformans serotype A polysaccharide was recently performed at NRRC in cooperation with Dr. Errol Reiss of the Center for Disease Control, Atlanta. The results are in accord with a structure consisting of a main chain of (l->3)-linked g-mannosyl residues, two-thirds of which are also (1- 2)-linked by single residue branches of g-xylose and g-glucuronic acid. (All sugar residues referred to in text and figures are in the pyranose form.) The polysaccharide from the cell walls of T. fusiformis has a related structure. It appears, however, that there are also backbone... 

Lactose, milk sugar a reducing disaccharide. M, 324.3, a-form m.p. 223°C, [a]g-t-89.4°55.5° (water), p-form m.p. 252°C, [a]p-H34.9°- 55.3° (water). L. crystallizes from water as the p-form above 93 °C, and as a-lactose monohydrate below 93 °C. It consists of galactose P-1,4 glycosidically linked to glucose, both monosaccharide residues in the pyranose form. L. is not fermented by ordinary yeasts, but it is fermented by yeasts such as kefir. The souring of milk consists of the eonversion of L. into lactic acid by lactic acid bacteria. 

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