Reactions of Aldehydes and Ketones
|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.