Glucose and fructose exist mainly as their cyclic hemiacetals (hemiketal for fructose), the result of hemiacetal formation between the carbonyl group and the hydroxyl group on the C-5 carbon. Through hemiacetal formation, glucose forms a six membered ring, called a pyranose because of the resemblance to pyran. In the chair conformation, the CH2OH substituent occupies an equatorial position. With fructose, a five or six membered ring is formed (the five membered ring is called a furanose because of the resemblance to furan). Because an additional chiral carbon is formed upon cyclization of glucose (the original carbonyl carbon, also called the anomeric carbon), the nomenclature α or β is used to distinguish orientation at that carbon, whether the hydroxyl is below or above the plane of the ring (trans or cis to the methoxyl group on C-6). In general, for pyranoses, the chair form will predominate in which hydroxyl and methoxyl groups are allowed to occupy equatorial positions with the hydrogens occupying axial positions.

You won't need to reproduce this discussion on the MCAT, but you want to feel comfortable with the nomenclature.












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