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![[Figure 2]](ba5257fig2mag.jpg)
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Figure 2
Interconversions between open-chain and cyclic forms of D-fructose. A furanose ring (on the left) is formed after the 5-hydroxyl (O atom in orange) performs a nucleophilic attack on the ketone (carbonyl containing the O atom in blue). This results in two anomeric configurations (α or β, resulting from the blue O atom lying on the lower or upper side of the ring. respectively), as the ketone C atom is sp2-hybridized and thus planar, and the attack can be performed from either side of the plane. The same holds true for pyranose-ring formation, except that now it is the 6-hydroxyl (O atom in green) which attacks the ketone. A similar mechanism occurs in aldoses (e.g. D-glucose or D-galactose), where the 4- and 5-hydroxyls attack the aldehyde group in position 1 to form furanose and pyranose rings, respectively. Numbers in gold denote all of the potential positions that substituents can adopt in a pyranose ring (1, up and axial; 2, up and equatorial; 3, down and axial; 4, down and equatorial). |
![[Figure 2]](ba5257fig2.jpg)
 | STRUCTURAL BIOLOGY |
ISSN: 2059-7983
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