Chem help- Fischer/Haworth projections?

Question

Can someone tell me how to turn a Fischer projection for aldohexoses, aldopentoses, and ketopentoses into a Haworth projection? I know it goes through a hemiacetal sythesis reaction, but I'm not sure of the steps. Thank you!

Answer 1

Fischer projections were invented by the German sugar chemist Emil Fischer to simplify displaying the stereochemical relationships among carbohydrate structures. Why they persist in textbooks, when the dash-and-wedge notation is available, is beyond my understanding.

A research project in which I participated some years ago demonstrated conclusively that individuals are better able to perceive structures written with dash-and -wedge notation as three-dimensional, and make more accurate assignments of absolute configuration.

Furthermore, a Fischer projection cannot simultaneously represent the arrangement of ligands around a chiral center and the most stable conformation of the molecule as can a dash-and-wedge picture.
If that were not enough to doom these projections, one must accept limitations on one's mental manipulation of the structures:


Fischer projections may not be lifted from the plane of the paper, nor rotated by other than 180 degrees.

They also are supposed to be drawn only with the most highly oxidized carbon at the top, and the "main" carbon chain vertically oriented.
When confronted with one of these fiendish contraptions, I simply translate quickly into dash-and-wedge, and work with the new projection. Here is an example translation, of glyceric acid:



To view the main chain as vertical and completely aligned, implies that the sideways projecting bonds of the Fischer structure are actually coming up out of the the page, whereas the vertical bonds are oriented behind it.


Making this change turns the Fischer at left into the dash-wedge at center.

A 90 degree rotation to the right produces a more familiar view of the molecule.
That the Fischer projection formally places the molecule into an all eclipsed conformation is shown in the second example, where we step through the transformation from Fischer to dash-wedge for a molecule having three chiral centers.


Once translated, the structure can be rotated easily into a fully staggered conformation by simply turning the two end chiral centers 180 degrees.

Such a manipulation is not permitted with a Fischer projection because it changes the absolute configuration.
Fischer Projection and Hawoorth Projections:
The relationship between Fischer and Haworth projections and their relationship with the three-dimensional structure of each anomer. Each of the sugars is in its reducing form and shown as both the alpha and beta anomer. The 3-D conformation of the sugar may not be in its lowest energy chair conformation. However, in many cases it is. The translation is directly from Haworth projection to 3-D structure.
Red = Oxygen, Grey = Carbon, White = Hydrogen

to see and manipulate the images you must be sure that the chime plug-in is installed Sugar Fischer
Projection Haworth Projection
alpha anomer 3-D image
alpha anomer Haworth Projection
beta anomer 3-D image
beta anomer
D-Glucose
D-Allose
D-Mannose

Methods of Converting Fischer Projections of Sugars to Haworth Projections
Three methods of generating Haworth Projections from Fischer projections follow. The first is similar to the one presented in the book on p. 533 but makes placement of the –OH’s simpler. Use whichever method you find easiest to remember. Let’s consider two sugars, one an aldohexose, the other an aldopentose. Fischer projections are shown below. One the following two pages, we’ll generate Haworth projections of each. Recall the carbon with the star (*) next to it is the one that determines if the sugar is a D-sugar or
an L-sugar.
CHO
OH
OH
HO
HO
CH2OH
CHO
OH
OH
CH2OH
OH * *
Method 1
1) Draw the basic structure for the sugar.
O
O
2) If the sugar is a D-sugar place a –CH2OH above the ring on the carbon to the left of the
oxygen, for an L-sugar place it below the ring.
O
OH
CH2OH
OH CH2OH O
-sugar -sugar
3) For an -sugar place an –OH below the ring on the carbon to the right of the ring oxygen, for
an -sugar place the –OH above the ring.
O
OH
CH2OH
OH CH2OH O
L--sugar D--sugar
4) Finally, –OH groups on the right go below the ring and those on the left above, using the
–CH2OH group as the reference point for both projections.
O OH
OH
OH OH
CH2OH
OH CH2OH
OH
OH
O
-L-sugar -D-sugar
Method 2
1. Take the Fischer projection and rotate the 3 groups forming the upside down triangle on the
starred to get the –OH group on the bottom.
CHO
OH
OH
HO
CH2OH
OH
CHO
OH
HO
OH
HOCH2 * *
2. Imagine the structure above turned on its side with the aldehyde to the right. Note that the vertical lines now are coming out of the paper and the horizontal one fade into it (the opposite of a Fischer projection).
CHO
OH OH
OH
HOCH2
HO CHO
OH
OH
HO
HOCH2
3. Draw a basic Haworth projection; remembering to place an –OH on the right carbon below the ring on the right carbon for an -sugar, for a -sugar place it above the ring.
O
OH
CH2OH
OH CH2OH O
-sugar -sugar
4. Place the –OH groups on the ring exactly as they appear on the rotated Fischer projections from the previous step.
O OH
OH
OH OH
CH2OH
OH CH2OH
OH
OH
O
-L-sugar -D-sugar
Method 3
This method involves memorizing both the Fischer and Haworth projections of glucose, then determining all other structures relative to it. First produce the two structures of -Dglucose.
CHO
OH
HO
OH
OH
CH2OH
O
OH OH
OH
OH
CH2OH
-D-glucose
If you remember that the –CH2OH group is up, then all you have to do is recall that the –OH groups follow an alternating pattern from it. You then compare the position of the –OH groups on the Fischer projection of the sugar you are interested in with that of glucose to place the –OH
groups. Those on the same side go in the same position, those on the opposite side go in the opposite position.

Answer 2

Consult a standard textbook on Organic Chemistry.