Let’s analyse our set of four stereoisomers a little more closely. You may have already noticed that these structures all contain stereogenic centres—two in each case. Go back to the diagram of the four structures at the bottom of p. 312 and, without looking at the structures overleaf, assign an R or S label to each of the stereogenic centres. You should have made assignments of R and S like this.

● Converting enantiomers and diastereoisomers R R O cis epoxide

• To go from one enantiomer to another, both stereogenic centres are inverted.

• To go from one diastereoisomer to another, only one of the two is inverted.

All the compounds that we have talked about so far have been cyclic because the diastereo isomers are easy to visualize: two diastereoisomers can be identifi ed because the substituents are either on the same side or on opposite sides of the ring (cis or trans). But acyclic compounds can exist as diastereoisomers too. Take these two, for example. Both ephedrine and pseudo ephedrine are members of the amphetamine class of stimulants, which act by imitating the action of the hormone adrenaline.

Ephedrine and pseudoephedrine are stereoisomers that are clearly not mirror images of each other—only one of the two stereogenic centres in ephedrine is inverted in pseudoephedrine—so they must be diastereo isomers. Thinking in terms of stereogenic centres is useful because, just as this compound has two stereogenic centres and can exist as two diastereo isomers, any compound with more than one stereogenic centre can exist in more than one diastereo isomeric form. Both ephedrine and pseudoephedrine are produced in enantiomerically pure form by plants, so, unlike the anti-asthma intermediates above, in this case we are talking about single enantiomers of single diastereo isomers. Adrenaline (also known as epinephrine) is also chiral. In nature it is a single enantiomer but it cannot exist as other diastereo isomers as it has only one stereogenic centre.

Ephedrine and pseudoephedrine

Ephedrine is a component of the traditional Chinese remedy ‘Ma Huang’, extracted from Ephedra species. It is also used in nasal sprays as a decongestant. Pseudoephedrine is the active component of the decongestant Sudafed.

The ‘natural’ enantiomers of the two diastereomers are (–) ephedrine and (+)-pseudoephedrine, which does not tell you which is which, or (1R,2S) -(–)-ephedrine and (1S,2S)- (+ (pseudoephedrine, which does. From that you should be able to deduce the corresponding structures. Here are some data on (1R,2S) -(–)-ephedrine and (1S,2S) -(+)-pseudoephedrine and their ‘unnatural’ enantiomers (which have to be made in the laboratory), (1S,2R) -(+)-ephedrine and (1R,2R)- (–)-pseudoephedrine.

● The two diastereoisomers are different compounds with different names and different properties, while the pairs of enantiomers are the same compound with the same properties, differing only in the direction in which they rotate polarized light.

We can illustrate the combination of two stereogenic centres in a compound by consider ing what happens when you shake hands with someone. Hand-shaking is successful only if you each use the same hand! By convention, this is your right hand, but it’s equally possible to shake left hands. The overall pattern of interaction between two right hands and two left hands is the same: a right-handshake and a left-handshake are enantiomers of one another; they differ only in being mirror images. If, however, you misguidedly try to shake your right hand with someone else’s left hand you end up holding hands. Held hands con sist of one left and one right hand; a pair of held hands have totally different interactions from a pair of shaking hands; we can say that holding hands is a diastereoisomer of shaking hands. We can summarize the situation when we have two hands, or two chiral centres, each one R or S.

What about compounds with more than two stereogenic centres? The family of sugars pro vides lots of examples. Ribose is a fi ve-carbon sugar that contains three stereogenic centres. The enantiomer shown here is the one used in the metabolism of all living things and, by convention, is known as D-ribose. The three stereogenic centres of D-ribose have the R con figuration. For convenience, we will consider ribose in its open-chain form, but more usually it would be cyclic, as shown underneath.

In theory we can work out how many ‘stereoisomers’ there are of a compound with three stereogenic centres simply by noting that there are 8 (= 23) ways of arranging Rs and Ss.

But this method blurs the all-important distinction between diastereoisomers and enantiomers. In each case, the combination in the top row and the combination directly below it are enantiomers (all three centres are inverted); the four columns are diastereoisomers. Three stereogenic centres therefore give four diastereoisomers, each a pair of two enantiomers. Going back to the example of the C5 aldoses, each of these diastereoisomers is a different sugar. In these diagrams each diastereo isomer is in a frame but the top line shows one enantiomer (D) and the bottom line the other (L).

مواضيع ذات صلة

Significance of the SN2 rate equation

Mechanisms for nucleophilic substitution

Resolutions using diastereoisomeric salts

Separating enantiomers is called resolution

Chiral compounds with no stereogenic centres

Absolute and relative stereochemistry

Diastereoisomers can be chiral or achiral

Dia stereoisomers are stereoisomers that are not enantiomers

The rotation of plane-polarized light is known as optical activity

R and S can be used to describe the configuration of a chiral centre

Many chiral molecules are present in nature as single enantiomers

Stereogenic centres