Pentamethyl glucose

Acetals are easily hydrolyzed by dilute acid, but ethers are stable under these conditions. Treatment of the pentamethyl glucose derivative with dilute acid hydrolyzes only the acetal methyl group. Haworth determined that the free hydroxyl group is on C5 of the hydrolyzed ether, showing that the cyclic form of glucose is a pyranose. 

Glucose is converted to a pentamethyl derivative by treatment with excess methyl iodide and silver oxide (Section 23-12). The five methyl groups are not the same, however. Four are methyl ethers, but one is the glycosidic methyl group of an acetal. 

All OH groups of a cyclic monosaccharide are converted to ethers by treatment with base and an alkyl halide. For example, a-D-glucose reacts with silver(I) oxide (Ag20, a base) and excess CH3I to form a pentamethyl ether. 

The four ether bonds that are not part of the acetal do not react with any reagents except strong acids like HBr and HI (Section 9.14). The acetal ether, on the other hand, is hydrolyzed with aqueous acid (Section 27.7B). Aqueous hydrolysis of a single glycoside (like the pentamethyl ether of a-D-glucose) yields both anomers of the product monosaccharide. 

Amino groups in dithioacetals undergo normal substitution reactions. Conventional reactions have been reported to occur with acetic anhydride,88,90,91 benzoyl chloride,46,92 l-fluoro-2,4-dinitrobenzene,250 p-toluenesulfonyl chloride,247 and benzyloxycarbonyl chloride92 the reported92 application of sodium chloride and methyl iodide to the conversion of 2-amino-2-deoxy-D-glucose diethyl dithioacetal hydrochloride into a pentamethyl derivative is presumed to be an error. Although crude 2-acetamido-2-deoxy-D-glucose diethyl dithioacetal was in one instance converted in low yield into a poorly... 

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