Glucose and Fructose: Open-Chain and Cyclic Forms

Open-chain D-glucose (aldohexose):

SMILES: OC[C@@H](O)[C@H](O)[C@@H](O)[C@@H](O)C=O

Structure: $HOCH_{2}$ — [C2(OH)] — [C3(OH)] — [C4(OH)] — [C5(OH)] — CHO

Glucose is an aldohexose: the carbonyl group is an aldehyde at C1.

Open-chain D-fructose (ketohexose):

SMILES: OC[C@@H](O)[C@@H](O)[C@H](O)C(=O)CO

Fructose is a ketohexose: the carbonyl group is a ketone at C2.

Cyclisation of glucose (pyranose ring formation):

The C5-OH acts as a nucleophile attacking the C1 aldehyde:

C1(CHO) + HO-C5 → C1-C5 ring closure (6-membered pyranose ring)

Two faces of the aldehyde give two anomers:

• Attack from below C1 → alpha-D-glucose (C1-OH axial, down in Haworth)
• Attack from above C1 → beta-D-glucose (C1-OH equatorial, up in Haworth)

The greater stability of the beta anomer is due to the equatorial position of C1-OH, minimising 1,3-diaxial steric interactions in the ^{4}$C_{1}$ chair conformation.