Carbohydrates (Chapter 24)

Cm(H2O)n

An aldehyde/ketone containing at least three carbon atoms and two hydroxyl groups. A monosaccharide can be either an aldose OR a ketose.

Ether Linkage

• Aldose: A monosaccharide containing an aldehyde group. • Ketose: A monosaccharide containing a ketone group.

• Triose: 3 Carbons • Tetraose: 4 Carbons • Pentose: 5 Carbons • Hexose: 6 Carbons

• Aldose: Numbering begins at the aldehydic carbon. • Ketose: Numbering begins at the terminal carbon closest to the ketone.

The highest-numbered hydroxyl group substituent is located on the left side of the projection. The highest-numbered carbon stereocenter has the S-conformation.

The highest-numbered hydroxyl group substituent is located on the right side of the projection. The highest-numbered carbon stereocenter has the R-conformation.

Furanose The five-membered furanose ring is more stable than the six-membered pyranose ring.

Pyranose The six-membered pyranose ring is more stable than the five-membered furanose ring.

The pyranose stereoisomer that places the C1 hydroxyl group at the equitorial position (i.e. the R-isomer) will be the dominant product. The placement of the C1 hydroxyl group at the equitorial position minimizes steric effects.

Hemiacetal cyclization forms a pair of diasteromers (i.e. anomers) due to stereoisomerization at the aldehydic (C1) carbon. The new stereocenter of hemiacetal cyclization is formed at the former aldehydic carbon.

A stereoisomer of a cyclic hemiacetal

The interconversion between the α-anomer and β-anomer of a cyclic hemiacetal. The mutarotation interconversions occur while the aldoses are in the acyclic form. (The α-anomer and β-anomer of the cyclic aldose are in equilibrium in solution).

• α-Anomer: The C1 hydroxyl group and highest-numbered hydroxyl group are placed on the same side of the cyclic projection. • β-Anomer: The C1 hydroxyl group and highest-numbered hydroxyl group are placed on opposite sides of the cyclic projection. The stereochemistry/configuration at the anomeric carbon is the ONLY difference between the α-anomer and β-anomer.

C1 Carbon The anomeric carbon is the hemiacetal carbon of a cyclic monosaccharide.

—OH > —COOH > —CH2OH

Hemiacetal Cyclization The hemiacetal cyclization reaction can occur under basic conditions OR acidic conditions.

Cyclic Hemiacetal Ring-Opening The hemiacetal cyclization reaction can is reversible.

• Acid-Catalyzed: H+, H2O • Base-Catalyzed: OH–, H2O The hemiacetal cyclization reaction CAN occur under neutral conditions (but the cyclization rate is much slower without a catalyst).

All chiral centers within the aldose/ketose are inverted to the opposite stereoisomer configuration.

• Carbon atoms are numbered top-to-bottom. • Horizontal bonds are depicted as wedges.

• α-Anomer: The anomeric/hemiacetal carbon and the highest-numbered carbon have OPPOSITE stereoisomer configuration. • β-Anomer: The anomeric/hemiacetal carbon and the highest-numbered carbon have the SAME stereoisomer configuration.

Interconversion results from the varying plane-of-attack of the alcohol/oxide group to form the cyclic hemiacetal. (Attack of the alcohol/oxide group from above the molecule plane OR below the molecule plane creates differing anomeric forms of the cyclic monosaccharide.) Both cyclic anomeric forms of the monosaccharide are in equilibrium with the acyclic aldose conformation.