Pyranose forms of carbohydrates

Sfrucfurally O glycosides are mixed acefals fhaf involve fhe anomeric posifion of furanose and pyranose forms of carbohydrates Recall fhe sequence of mfermediafes m acefal formalion (Secfion 17 8)... 

Haworth formulas (Section 25.6) Planar representations of furanose and pyranose forms of carbohydrates. 

Section 23.9 Pyranose forms of carbohydrates resemble cyclohexane in their conformational preference for chair forms. The anomenc effect causes an electronegative substituent at the anomeric (C-1) carbon to be more stable when it is axial than when it is equatorial. The effect is believed to result from the delocalization of an electron pair of the ring oxygen into an antibonding orbital of the anomeric substituent. 

Entry 6 reminds us that the furanose and pyranose forms of carbohydrates are specialized examples of hemiacetals and capable of interconversion. Entries 7 and 8 connect enolization of the open-chain form to interconversion of C-2 epimers. Entry 9 illustrates another reaction involving the enol of the open-chain form. Here, the enediol intermediate connects aldose and ketose isomers. In entry 10, an enzyme-catalyzed reverse aldol reaction cleaves a 6-carbon chain to two 3-carbon fragments. 

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