Wize University Organic Chemistry Textbook > Carbohydrates
Converting Fischer Structures
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Converting Fischer Structures
Carbohydrates are interesting molecules because they can rapidly interconvert between their closed and open forms to form either a pyranose (6-membered) or furanose (5-membered) ring:
You will need to be able to draw the cyclized version of monosaccharides from Fischer projections. The steps are as follows:
- rotate the bottom chiral carbon so that the -OH group is on the bottom
- rotate the -CH2OH and the -H group appropriately (this will depend on the D vs. L forms)
- groups on the left of the Fischer projection point up on the chair conformation
- groups on the right of the Fischer projection point down on the chair conformation
Example: draw the alpha and beta versions of the monosaccharide below in chair conformations
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Example: Converting Fischer Structures
The most common cyclic structure we see for carbohydrates is the pyranose form, but the furanose form also persists. Draw D-glucose as a β-furanose.
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Example: Converting Fischer Structures
Draw L-Galactose (shown below) as an α-pyranose. Hint: The trick with L-carbohydrates is we rotate the Fischer projections to the left in the first step.
Mark Yourself Question
- Grab a piece of paper and try this problem yourself.
- When you're done, check the "I have answered this question" box below.
- View the solution and report whether you got it right or wrong.
Draw the molecule below as a chair conformation.
