Lecture 3 (II) F24 (Carbohydrate chemistry; linear and ring structures)

Page 1: Representing Sugar Structures

  • Fischer Projection Formula

    • Used to represent three-dimensional sugar structures in a two-dimensional format.

    • Vertical bonds project behind the plane.

    • Horizontal bonds project out of the plane.

  • Perspective Formula

    • Another drawing method for sugar structures.

    • Solid wedge-shaped bonds project in front, dashed bonds point away.

Page 2: Monosaccharides and Asymmetric Carbons

  • Chiral Carbon Atoms

    • Monosaccharides contain one or more chiral carbon atoms, except for dihydroxyacetone.

    • Optically active isomeric forms arise from these chiral centers.

  • Examples

    • Dihydroxyacetone

    • D-Glyceraldehyde and L-Glyceraldehyde are enantiomers (mirror images).

Page 3: Enantiomers

  • Characteristics of Enantiomers

    • Differ in configuration at every chiral carbon atom.

    • Referred to as "left-handed" and "right-handed" forms.

    • Identical chemical properties (e.g., melting point, water-solubility).

    • Differ in optical activity: rotate polarized light in opposite directions.

Page 4: Diastereomers

  • Definition

    • Stereoisomers differing in handedness at some chiral carbons but not all.

    • They do not possess identical chemical properties due to different spatial relationships among atoms.

Page 5: D and L Designation of Sugars

  • Naming Convention

    • "D" sugar: Chiral carbon furthest from carbonyl similar to D-glyceraldehyde.

    • "L" sugar: Configuration similar to L-glyceraldehyde.

    • Most naturally occurring sugars are D-sugars.

Page 6: Epimers

  • Definition

    • Sugars identical except for configuration at one carbon atom.

    • A specific type of diastereomer.

Page 7: Identifying Sugar Structures

  • Analyzing Sugar Structures

    • The two structures presented can be identified as:

      • Both D sugars

      • Diastereomers of each other

      • Enantiomers of each other

      • Epimers of each other

Page 8: Further Analysis of Sugar Structures

  • Analyzing Sugar Structures

    • Identifications include:

      • Both D sugars

      • Diastereomers of each other

      • Enantiomers of each other

      • Epimers of each other

      • A, B, and D as all correct options.

Page 9: Stereoisomers Calculation

  • Formula

    • A sugar with n chiral centers has 2^n stereoisomers.

    • Example: Glyceraldehyde (21) has 2 stereoisomers.

    • An aldose with 5 carbons and 3 chiral centers has 2^3 = 8 stereoisomers.

    • Half are D sugars and half are L sugars.

Page 10: Generic Sugar Molecule

  • Understanding Generic Structures

    • Represents possible stereoisomers stemming from the chiral configurations.

    • Example Calculation: X possible stereoisomers, Y of which are D sugars leads to options like 8 and 4, etc.

Page 11: Understanding D-Ketoses

  • Isomer Dynamics

    • Adding a carbon doubles the number of isomers.

    • All sugars shown are D sugars except dihydroxyacetone.

    • The 4 hexoses are diastereomers of each other; some are also epimers.

Page 12: D-Aldoses Structures

  • Relationship of Suagrs

    • Understanding the dynamics of sugars in each group to others without the need to memorize structures or names.

Page 13: Hemiacetals and Hemiketals Formation

  • Definition and Formation

    • Aldehydes and ketones can react with alcohols to produce hemiacetals and hemiketals.

    • The original carbonyl carbon becomes chiral upon formation.

Page 14: Cyclization of Sugars

  • Cyclization Process

    • Sugars have both alcohol and carbonyl groups present, leading to intramolecular reactions.

    • This formation leads to ring structures.

Page 15: Cyclization of Glucose

  • Formation Details

    • OH group at C-5 reacts with carbonyl carbon of the aldehyde.

    • Creates a stable six-membered ring with C-1 becoming asymmetric, resulting in α and β isomer forms.

    • Isomeric forms differing at the hemiacetal or hemiketal carbon are called anomers.

Page 16: Cyclization of Ketoses

  • Similar Cyclization Process

    • The electrophilic carbonyl carbon reacts similarly to aldoses with OH group of nucleophiles forming rings.

Page 17: Mutarotation

  • Process Description

    • Glucose can be found in either α or β form in solid state.

    • Upon dissolving in water, it establishes an equilibrium mixture of the α, β, and linear forms over time.

    • Identical optical properties achieved with a composition ideally approximating ⅓ α-D-Glucose, ⅔ β-D-Glucose.

Page 18: Ring Forms of Sugars

  • Cyclization Types

    • Sugars can form either pyranose (6-membered) or furanose (5-membered) rings.

    • The stability of the formed ring involves the geometry of the molecule.