Both derivatives do not react with Tollens' reagent. However, the same reaction with D-galactose, shown in the blue-shaded box, produces a pyranose product in which the C-6 hydroxyl is unprotected. Finally, direct di-O-isopropylidene derivatization of glucose by reaction with excess acetone results in a change to a furanose structure in which the C-3 hydroxyl is again unprotected. Selective oxidation to a ketone is then possible. The formation of an isopropylidene acetal at C-1 and C-2, center structure, leaves the C-3 hydroxyl as the only unprotected function. A methyl glycoside derivative of this compound (see below) leaves the C-2 and C-3 hydroxyl groups exposed to reactions such as the periodic acid cleavage, shown as the last step. The top equation shows the formation and some reactions of the 4,6-O-benzylidene acetal, a commonly employed protective group. As a rule, benzaldehyde forms six-membered cyclic acetals, whereas acetone prefers to form five-membered acetals. Acetal derivatives have been prepared by acid-catalyzed reactions with benzaldehyde and acetone. A pyranose structure for D-glucose is drawn in the rose-shaded box on the left. The formation of acetal derivatives illustrates how subtle changes may alter this selectivity. Aldolhexoses usually form pyranose rings and their pentose homologs tend to prefer the furanose form, but there are many counter examples. The size of the cyclic hemiacetal ring adopted by a given sugar is not constant, but may vary with substituents and other structural features. Examples of four typical pyranose structures are shown below, both as Haworth projections and as the more representative chair conformers. We know that these molecules are actually puckered in a fashion we call a chair conformation. These Haworth formulas are convenient for displaying stereochemical relationships, but do not represent the true shape of the molecules. In the D-family, the alpha and beta bonds have the same orientation defined for the furanose ring (beta is up
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