discussion carbohydrates
DESCRIPTION
Discussion Carbohydrates. Nomenclature. (D)-glucose. α-pyranose form: α-(D)-glucopyranose. (2 R,3S,4R,5R)-2,3,4,5,6-pentahydroxyhexanal. Dextrose. Hawthorne structure α-pyranose form: α -(D)-glucose. β-pyranose form: β-(D)-glucopyranose. Another Chiral sugar: Erythrose. - PowerPoint PPT PresentationTRANSCRIPT
Discussion Carbohydrates
Nomenclature
(D)-glucose
(2R,3S,4R,5R)-2,3,4,5,6-pentahydroxyhexanal
Dextrose
α-pyranose form:α-(D)-glucopyranose
β-pyranose form:β-(D)-glucopyranose
Hawthorne structureα-pyranose form:α-(D)-glucose
Another Chiral sugar: Erythrose
How do you convert to a regular zig-zag structure? Three ways:1) use models2) determine R or S configurations, draw a four carbon chain and draw substituents then re-check configurations3) 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 diastereomers1) 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 olefinsTo 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)-2,5-bis(hydroxymethyl)tetrahydrofuran-2,3,4-triol
(2S,3S,4S,5R)-2,5-bis(hydroxymethyl)tetrahydrofuran-2,3,4-triol
RR
S
S
R
S
S
Nomenclature
(D)-fructose
Sucrose
Fisher Projections
Epimers
Optically active Optically active
C2 epimers C2 epimersC2 epimers
HNO3
Mirror image is same compound
Optically active
One segment of Fischer Proof