Stereochemistry

1
Stereochemistry

• L D L
  Rotate 180 D
• 2,3-dihydroxybutanedioic acid meso achiral
  diastereomer, (optically inactive overall)

()-tartaric acid aD 12 m.p. 170
C (-)-tartaric acid aD -12 m.p.
170 C meso-tartaric acid aD 0 m.p.
140 C
2
Aldaric Acids
3
Question 1

• Which of the following aldaric acids are
  optically active?

C and D
A B C D E
meso
No stereocenter
meso
R, R
S, S
4
Question 2

• Draw a hexose that would give the same aldaric
  acid product as D-Glucose

D
5
Question 2

• Draw a hexose that would give the same aldaric
  acid product as D-Glucose

D
6
Question 3

• There are four D-aldopentoses. Draw Fischer
  projections of each of them. Then draw Fischer
  projections of the aldaric acids they would
  yield. Which of those aldaric acids would be
  optically inactive?

7
Question 3

• There are four D-aldopentoses. Draw Fischer
  projections of each of them. Then draw Fischer
  projections of the aldaric acids they would
  yield. Which of those aldaric acids would be
  optically inactive?

Ribaric acid
Arabinic acid
Xylaric acid
Lyxaric acid
(S,S)
meso
meso
(S,S)
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Osazone Formation

• Both C1 and C2 react with phenylhydrazine.

9
Osazone

• Sugars that differ in configuration only at the
  a-carbon
• Give the same product.

10
Question 4

• Select the compounds that would produce the same
  osazone.

A and D, B and C
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Aldoses versus Ketoses

• Reductions provide the same alcohol products.
• Ketoses are not easily oxidized by Tollens'
  reagent but slowly react via enol.
• Tollens is not useful for distinguishing aldoses
  from ketoses
• Oxidation by HOBr reacts only with aldehydes to
  form aldonic acids.
• Oxidation with HNO3 gives the same aldaric acids.
  
• Both form the same osazones. (phenylhydrazones)

12
Ruff Degradation

• Aldose chain is shortened by oxidizing the
  aldehyde to -COOH, then decarboxylation.

13
Question 5
6

• Aldohexose 2 would yield the same aldopentose as
  _____
• Aldohexose 3 would yield the same aldopentose as
  _____
• Aldohexose 4 would yield the same aldopentose as
  _____

7
8
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Question 6

• Draw a Fischer projection of D-mannose, Then
  select the aldose that is formed by Ruff
  degradation of D-mannose.

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Kiliani-Fischer Synthesis

• This process lengthens the aldose chain.
• A mixture of C2 epimers is formed.

2- epimers
16
Chain Lengthening
17
Question 7

• Select the aldoses that would be formed by a
  Kiliani-Fischer synthesis in which D-threose was
  the starting material

18
Determination of Ring Size

• Haworth determined the pyranose structure of
  glucose in 1926.
• The anomeric carbon can be found by methylation
  of the -OHs, then hydrolysis.

19
Periodic Acid Cleavage

• Periodic acid cleaves vicinal diols to give two
  carbonyl compounds.
• Separation and identification of the products
  determine the size of the ring.

20
Fischers Proof

• Emil Fischer determined the configuration around
  each chiral carbon in D-glucose in 1891, using
  Ruff degradation and oxidation reactions.
• He assumed that the -OH is on the right in the
  Fischer projection for D-glyceraldehyde.
• This guess turned out to be correct.

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Reactions for identification
hexane 6 Cs unbranched
(HI) reductively lose oxygen
aldehyde
Cyanohydrin formation (HCN)
Reduction (Ni / H2)
hexa-alcohol sorbitol
aldehyde ? carboxylic acid mono acid, glucuronic
acid
Mild oxidation (Br2/ H2O)
diacid, glucaric acid optically active
Strong oxidation (dil.HNO3)
five OHs ? penta-acetate ester
React with Acetate
aldehyde? 6 OHs ? hexa-acetate ester
Reduce then react with Acetate
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Glucose Reactions
(C6H12O6) monosaccharide an aldohexose a
reducing sugar.
The four chiral centers in glucose tells us there
are sixteen (24) stereoisomers. of
stereoisomers 2n where n is the of
asymmetric Cs. There are eight diastereomeric
pairs of enantiomers
23
The First Elimination
Fischer selected the D- sugars with C5 OH on the
right
24
The eight possible D-Aldohexoses
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Reactions for Configuration

• Oxidation by HNO3
• Kiliani synthesis of aldonic acids
• Osazone formation
• Oxidation of arabinose with nitric acid

26
Oxidation by HNO3
Optically Active
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Question 8

• Which of the following aldohexoses produces
  optically inactive aldaric acids?

28
Question 8

• Which of the following aldohexoses would produce
  optically inactive aldaric acids?

Allaric and Galactaric
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The Second Elimination

• Eliminate the aldohexoses that would produce
  optically inactive aldaric acids.