2. Diastereomers: differ at one or more, but not all stereocenters
- Epimers: diastereomers differ at only one stereocenter (and have multiple stereocenters)
- Anomers: differ at new stereocenters created during ring closing
- Some diastereomers are neither epimers nor anomers
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Anomers
Finally, we will draw examples of anomers. Anomers are specifically diastereomers of ring structures, so for these next drawings we will be drawing ring form aldohexoses.
First, let's draw the sugar alpha-D-glucose.
- Draw a hexagon with a pair of parallel sides going horizontally, and insert an oxygen atom at the apex of the two lines on the top right of the hexagon.
- Recall from organic chemistry that in stick drawings of organic compounds, carbon atoms are at the end of the lines or at the points where two lines meet.
Let's label the carbon atoms in this molecule.
- Moving clockwise from our oxygen atom, label the carbons 1-5.
- Notice that our sixth carbon atom is missing. Let's add it as an attachment to carbon 5 as follows:
Now let's add the other hydroxyl groups.
- To carbons 1, 2, and 4, add hydroxyl groups going vertically down.
- To carbon 3, draw a hydroxyl group going vertically up.
Next, let's draw its anomer, beta-D-glucose.
- Redraw the hexagon, insert the oxygen, label the carbon atoms, add the sixth carbon and its attachments.
Now let's add the hydroxyl groups, which determine the difference between these two molecules.
- To carbons 1 and 3, add a hydroxyl group going vertically up.
- To carbons 2 and 4, add the hydroxyl groups going vertically down.
- Dash a box around carbon 1 (and its attachments) on each molecule to show that the chirality at carbon 1 only exists in the ring form of glucose, and that these molecules only differ in the position of the hydroxyl group at carbon 1.
- In the straight chain form of glucose, carbon 1 has a double-bond to an oxygen atom, making it achiral.
- However, with the formation of the ring in alpha- and beta- D-glucose, carbon 1 is now a fifth chiral carbon in the glucose molecule.