![]() The only problem is, it is drawn with the carbon backbone in a different orientation from what we have seen. The sugar below is one of the stereoisomers that we have been discussing. In addition, the specific rotation values of diastereomers are unrelated – they could be the same sign or opposite signs, similar in magnitude or very dissimilar. Diastereomers, in theory at least, have different physical properties – we stipulate ‘in theory’ because sometimes the physical properties of two or more diastereomers are so similar that it is very difficult to distinguish between them. We know that enantiomers have identical physical properties and equal but opposite magnitude specific rotation. Two examples of epimerase-catalyzed reactions are below. The epimer term is useful because in biochemical pathways, compounds with multiple chiral centers are isomerized at one specific center by enzymes known as epimerases. The R R R and S S R stereoisomers shown earlier are diastereomers but not epimers because they differ at two of the three chiral centers.Ī) Draw the structure of the enantiomer of the S R S stereoisomer of the sugar used in the previous example.ī) List (using the X X X format, not drawing the structures) all of the epimers of S R S.Ĭ) List all of the stereoisomers that are diastereomers, but not epimers, of S R S. ![]() For example, R R R and S R R are epimers : ![]() One more definition at this point: diastereomers which differ at only a single chiral center are called epimers. Let's invert the configuration at chiral center 1 and 2, but leave chiral center 3 unchanged. To draw one of them, we just invert the configuration of at least one, but not all three, of the chiral centers. Try making models of R R R and S S S and confirm that they are in fact nonsuperimposable mirror images of each other. If we want to draw the enantiomer of R R R, we don't need to try to visualize the mirror image, we just start with the R R R structure and invert the configuration at every chiral center to get S S S. Now, using the above drawing as our model, drawing any other stereoisomer is easy. Being careful to draw the wedge bonds correctly so that they match the R R R configurations, we get: Going through all the possible combinations, we come up with eight total stereoisomers - four pairs of enantiomers. Now, let's extend our analysis to a sugar molecule with three chiral centers. We also know that R S and S R are diastereomers of R R, because in each case one - but not both - chiral centers are different. We know, using the shortcut above, that the enantiomer of R R must be S S - both chiral centers are different. Pairs of enantiomers are stacked together. Here's another way of looking at the four stereoisomers, where one chiral center is associated with red and the other blue. ( Note: these shortcuts to not take into account the possibility of additional stereoisomers due to alkene groups: we will come to that later) If at least one, but not all of the chiral centers are opposite between two stereoisomers, they are diastereomers. If all of the chiral centers are of opposite R/S configuration between two stereoisomers, they are enantiomers. This can also seem very confusing at first, but there some simple shortcuts to analyzing stereoisomers: Compound D is also a diastereomer of compounds A and B. Therefore, C and D are a pair of enantiomers. Does compound C have its own enantiomer? Compound D is the mirror image of compound C, and the two are not superimposable. So, compounds A and B are a pair of enantiomers, and compound C is a diastereomer of both of them. Notice that compounds C and B also have a diastereomeric relationship, by the same definition. By definition, they are diastereomers of each other. However, they are not mirror images of each other (confirm this with your models!), and so they are not enantiomers. Compounds A and C are stereoisomers: they have the same molecular formula and the same bond connectivity, but a different arrangement of atoms in space (recall that this is the definition of the term 'stereoisomer). Now, look at compound C, in which the configuration is S at chiral center 1 and R at chiral center 2. A and B are nonsuperimposable mirror images: in other words, enantiomers. If we were to pick up compound A, flip it over and put it next to compound B, we would see that they are not superimposable (again, confirm this for yourself with your models!). The mirror image of Compound A is compound B, which has the S configuration at both chiral centers. Both chiral centers in have the R configuration (you should confirm this for yourself!). To avoid confusion, we will simply refer to the different stereoisomers by capital letters. We'll start with some stereoisomeric four-carbon sugars with two chiral centers. ![]() Next, we turn our attention to those which have multiple chiral centers. So far, we have been analyzing compounds with a single chiral center.
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