Discussion Carbohydrates
Nomenclature
(D)-glucose
(2R,3S,4R,5R)-2,3,4,5,6-pentahydroxyhexanal
α-pyranose form:
α-(D)-glucopyranose
Dextrose
Hawthorne structure
α-pyranose form:
α-(D)-glucose
β-pyranose form:
β-(D)-glucopyranose
Another Chiral sugar: Erythrose
How do you convert to a regular zig-zag
structure? Three ways:
1) use models
2) determine R or S configurations, draw a four
carbon chain and draw substituents then recheck configurations
3) Draw intermediate structures and rotate
molecule and bonds to get into zig-zag
Method 2:
Method 3: By drawing & rotating
Takes time, can make mistakes and you still should check the
configurations (R or S) at the end
Draw the enantiomer of erythrose
Draw the enantiomer of erythrose
You are switching two groups –involves breaking and making two bonds
– to interconvert each center (Four bonds total)
Draw the diastereomers of the
erythrose isomers shown
To create diastereomers
1) first figure out how many total possible diastereomers ( = 2n, n = #
chiral carbons). There are 22 = 4 diastereomers possible (two pair of
enantiomers)
2) then switch two groups to just one of the chiral centers to form one
diastereomer.
3) then draw its mirror image as above.
Meso compounds
Found among diastereomers of molecules with plane of symmetry between two (or
four, six, eight… ) chiral centers.
With this plane of symmetry the RS & SR enantiomers are not enantiomers but are
superimposable. These are achiral and will not show optical activity.
Meso is an issue with stereoselective reactions (syn or trans) to symmetrical olefins
To generate 1,2-dihalides, 1,2-diols, epoxides or cyclopropanes
For example:
[1] If the ring closes on a hydroxyl which is on the right in the Fischer projection, the
hydroxymethyl group (tail) points up; if it closes on a hydroxyl which was on the left in
the Fischer projection, the tail points down.
[2] The ring hydroxyls point down if they are on the right in the Fischer projection, and
up if they are on the left in the Fischer projection. (Note that these are also the positions
they are in after the Fischer projection has been rotated 90° clockwise to lie on its side.)
[3] The hydroxyl on the anomeric carbon points down in the D series if it is α and up if β
In the L series, α is up and β is down
Nomenclature
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Nomenclature
(D)-fructose
α-(D)-fructofuranose
(2S,3S,4S,5R)-2,5bis(hydroxymethyl)tetrahydrofuran-2,3,4-triol
(2S,3S,4S,5R)-2,5bis(hydroxymethyl)tetrahydrofuran-2,3,4-triol
S
R
R
S
S
S
R
Nomenclature
(D)-fructose
Sucrose
Fisher Projections
Epimers
Optically active
Optically active
C2 epimers
C2 epimers
C2 epimers
HNO3
Optically active
Mirror image is same compound
One segment of Fischer Proof