Three stereogenic centers occur in the furanose form, namely, the anomeric carbon C-1 andthe original stereogenic center C-2, as well as a new stereogenic center at C-3.In addition to the two furanose forms just shown, two more are possible. Instead of thereaction of the CH2OH group that was shown to form the cyclic hemiacetal, the other CH2OHgroup may add to the aldehyde carbonyl.25.25The most reasonable conclusion is that all four are methyl glycosides. Two are the methyl glycosidesof the a- and b-pyranose forms of mannose and two are the methyl glycosides of the a- and b-furanose forms.In the case of the methyl glycosides of mannose, comparable amounts of pyranosides and furano-sides are formed. The major products are the aisomers.25.26(a)Disaccharides, by defnition, involve an acetal linkage at the anomeric position; thus all the
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