Projection
representation Fischer of a 3D molecule as a flat structure where a tetrahedral carbon is represented as two crossed lines. vertical line is going back behind of the plane of the page carbon C substituent
horizonal line is coming out of the plane of the page CHO HOH2C OH H H CHO OH CH2OH H CHO OH CH2OH Fischer projection
(R)-(+)-glyceraldehyde Manipulation of Fischer Projections: 1. Fischer Projection can be rotated by 180 only! CHO H OH CH2OH (R)-(+)-glyceraldehyde Rotation by 90 or -90 (270 ) invert the stereochemistry CHO H OH CH2OH (R)-(+)-glyceraldehyde 90 rotation H HOH2C CHO OH (S)-(-)-glyceraldehyde 180 rotation HO
CH2OH H CHO still (R)-(+)-glyceraldehyde
CHO H OH CH2OH HOH2C
H CHO OH
Because of the convention of the Fischer Projection (i.e., the vertical lines are going back and the horizonal lines are coming forward) the 90 rotated Fisher projection is actually the enantiomer of the original
These two compounds are enantiomers
2. If one group of the Fischer projection is held steady, the other groups can be rotated either clockwise or counter clockwise. CHO H OH CH2OH (R)-(+)-glyceraldehyde Assigning R and S-configurations to Fischer projections 1. Assign priorities to the four substituents according to the Cahn-Ingold-Prelog convention. 2. Perform the two allowed manipulations of the Fischer projection to place the lowest priority group on one of the vertical positions (either top or bottom) 3. If the priorities of the other three groups (1-2-3) proceed clockwise, the stereogenic center is assigned as R. If the priorities of the other three groups (1-2-3) proceed counter clockwise, the stereogenic center is assigned as S. 2 CHO 4 H lowest priority group. Move to a vertical position OH 1 CH2OH 3 hold steady hold one group steady and rotate the other three 1 HO 3 The priorities of the other three groups (1-2-3) is clockwise. The stereogenic center is assign as R 4 H CHO 2 CH2OH hold this group steady HOH2C CHO H
OH still (R)-(+)-glyceraldehyde
For more structures with more than one stereogenic center such as carbohydrates, the tetrahedral carbons are "stacked" on top of one another. OH HO H HO Threose For carbohydrates, the convention is to put the carbonyl group the at top for aldoses and closest to the top for ketoses. The carbons are numbered from top to bottom. 1 CHO H HO H H OH H OH OH CH2OH D-Glucose HO H H 1 CH2OH 2 O H OH OH CH2OH D-Fructose O HO H CHO H OH CH2OH
Carbohydrates and amino acids are designated as D- or L- according to the stereochemistry of the highest numbered carbon in the Fischer projection. If the hydroxyl group (or amino group for amino acids) is pointing to the right in the Fischer Projection, the sugar (or amino acid) is designated as D. If the hydroxyl group (or amino group for amino acids) is pointing to the left in the Fischer projection, the sugar (or amino acid) is designated as L. Most naturally occurring carbohydrates are of the D-configuration while most naturally occurring amino acids are L. 1 CHO H 2 OH 3 HO H 4 H OH highest numbered stereogenic carbon H 5 OH 6 CH2OH D-Glucose 1 CHO H 2 OH HO 3 H H highest numbered stereogenic carbon HO 4
5 CH2OH L-Arabinose
Converting Fischer Projections into Haworth Projections. 1. Identify the hydroxyl group which is cyclizing onto the carbonyl group. This hydroxy will become the ring oxygen in the hemiacetal or hemiketal form of the carbohydrate. For D-glucose, it is the C5 hydroxyl in the pyranose form; for D-ribose, it is the C4-hydroxyl for the furanose form. 2. Manipulate the Fischer projection so this hydroxyl group is on the bottom. 3. Draw the Haworth projection so that the ring oxygen is on the top. For pyranoses, draw the sixmembered ring laying on it side with a oxygen at the upper right. 4. Substituents on the right side of the Fischer projection will be on the bottom face of the Haworth projection. Substituents on the left side of the Fischer projection will be on the top face of the Haworth projection. 5. Remember that in the cyclic form, the C1-hydroxyl group (the anomeric center) can usually adopt either the up or down configuration. T T T B B B Furanose O T B B B B T T O B T T B T T= top B= bottom
Pyranose
CHO H HO H H OH H OH OH CH2OH Ring oxygen
1 CHO H HO H HOH2C OH OH H OH H 4 OH
6 CH2OH 5 OH 3 O 2 OH OH 1
D-glucose
-D-glucopyranose
CHO H H H OH OH OH CH2OH Ring oxygen
1 CHO H H HOH2C OH OH OH H
HO 4
6 3 OH
O 2 OH
OH 1
D-Ribose
-D-ribofuranose