Chem 336 - Spring 1999 - Organic Chemistry III
Portland State University - Dr. Carl C. Wamser

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Chapter 24 - Carbohydrates

Simple Sugars (Monosaccharides)

• general formula: (CH₂O)_n
• general structure: one carbonyl group, every other carbon has an OH group
  
  
  
• monosaccharide classifications:
  aldose: aldehyde
  ketose: ketone
  triose: 3 carbons
  tetrose: 4 carbons
  pentose: 5 carbons
  hexose: 6 carbons
  ketopentose: ketose with 5 carbons
  etc.

Stereocenters

• D-glucose has 4 chiral carbon atoms (2⁴ = 16 possible stereoisomers)
  the name D-glucose implies just one of those stereoisomers
  one stereoisomer is the enantiomer of D-glucose
  the other 14 stereoisomers are diastereomers of D-glucose

Fischer Projections

• a method for depicting stereochemistry at a series of chiral centers
  arrange the chiral center so that:
  • horizontal groups are forward
  • vertical groups are oriented backward

• Note that there are numerous ways to show a given chiral center
  • 12 different Fischer projections represent (R)
  • 12 different Fischer projections represent (S)

D-Glyceraldehyde

• the simplest chiral sugar
• the reference for D & L designation of stereochemistry

Writing D-Sugars

• orient the carbonyl end up
• write all chiral centers in Fischer projections
• D means the lowest OH group is right
• L means the lowest OH group is left

The D-Aldose Family

• trioses: D&L glyceraldehyde
• tetroses: D&L erythrose and D&L threose
• pentoses: 4 pairs of stereoisomers (including D-ribose)
• hexoses: 8 pairs of stereoisomers (including D-glucose)

Cyclic Sugar Structures

• internal hemiacetal formation
  an alcohol group adds to the carbonyl
  furanose: a 5-membered ring
  pyranose: a 6-membered ring

Haworth Projections

• a method to depict ring structures (flat)
  arrange the ring with O in the back (or back-right)
  • an OH to the right (Fischer) is down (Haworth)
  • an OH to the left (Fischer) is up (Haworth)
  • D-sugars will have the last CH2OH group up
  • the new hemiacetal could have either configuration

Anomers

• the two stereoisomers at the hemiacetal (anomeric) carbon
  • alpha anomer: OH group is down (Haworth)
  • beta anomer: OH group is up (Haworth)
• anomers are diastereomers (different physical properties)

Mutarotation of Glucose

• alpha-D-glucopyranose and beta-D-glucopyranose interconvert
  starting with either one, a mixture results in solution
• actual structure of glucose in solution is about 64% beta, 36% alpha, <1% open-chain

Conformations of Sugars

• 5-membered rings are close to planar
  (Haworth projections are OK)
• 6-membered rings are chair conformations
  (Haworth projections are inaccurate)
• from a Haworth 6-membered ring,
  flex the right down and the left up

• beta-D-glucose has every substituent equatorial (most stable)

Reactions of Monosaccharides

• hemiacetals to acetals with alcohol + acid
  (an acetal, rather than a hemiacetal, is called a glycoside)
  
• OH groups to esters with acetic anhydride
  
• OH groups to ethers with methyl sulfate (+ base such as NaH )
  
• reduction of the carbonyl with NaBH₄ (polyalcohol is called an alditol)
  
• oxidation of aldehydes to carboxylic acids with Tollen's reagent
  (polyhydroxy carboxylic acid is called an aldonic acid)
  
• oxidation of aldehydes and primary alcohol (both ends) to acids with HNO₃ (polyhydroxy dicarboxylic acid is called an aldaric acid)

Glycosides

• acetals do not interconvert with the open-chain form
  (they don't mutarotate like hemiacetals)

• glycoside linkages are used to connect sugars to other biomolecules, including other sugars or nucleic acid bases

Reducing Sugars

• sugars that react positively with Tollen's reagent (Ag⁺) or Cu⁺²
  indicates a free aldehyde (or hemiacetal)
  
  aldoses are reducing sugars
  glycosides are not reducing sugars
  ketoses are not reducing sugars

Periodate Cleavage Reactions

• periodic acid ( HIO4 or H5IO6 ) cleaves the C-C bond between an alcohol and an adjacent alcohol or carbonyl group
  
• excess periodate can cleave apart all carbons of a sugar
  
• 1° alcohols oxidize to formaldehyde after cleavage
  2° alcohols oxidize to aldehydes after cleavage
  aldehydes oxidize to formic acid after clevage
  ketones oxidize to carboxylic acids after cleavage
  carboxylic acids oxidize to CO2 after cleavage
  
• a useful technique for decomposing sugars and deducing structure

Disaccharides

• linkage of two monosaccharides,
  usually by glycoside (acetal) formation

Glucose Disaccharides - Maltose and Cellobiose

• maltose: glycoside link from a 4'-OH to make an acetal at C-1 (alpha)
  a 1,4'-alpha-glycoside

• cellobiose: glycoside link from a 4'-OH to make an acetal at C-1 (beta)
  a 1,4'-beta-glycoside

Sucrose - A Disaccharide of Glucose + Fructose

• glucose C-1 and fructose C-2 join one another at their acetal carbons
  sucrose is not a reducing sugar

Polysaccharides

• cellulose: long chains of glucose, joined 1,4'-beta
  wood is mainly cellulose
  
• starches: storage form of glucose in plants
  long chains of glucose, joined 1,4'-alpha (amylose)
  some 1,6'-alpha linkages also occur (amylopectin)
  we can digest alpha-linkages but not beta-linkages
  
• glycogen: storage form of glucose in animals
  very long chains (1,4'-alpha), with lots of branching (1,6'-alpha)