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