This pair of epoxides was produced by chemists in Pennsylvania in the course of research on drugs intended to alleviate the symptoms of asthma. Clearly, they are again diastereoisomers, and again they have different properties. Although the reaction they were using to make these compounds gave some of each diastereoisomer, the chemists working on these compounds only wanted to use the first (trans) epoxide. They were able to separate it from its cis diastereo isomer by chromatography because the diastereoisomers differ in polarity.

This time, the diastereoisomers are a little more complex than the examples above. The first two pairs of diastereoisomers we looked at were achiral—they each had a plane of symmetry through the molecule.

The epoxide diastereoisomers, on the other hand, are chiral. We know this because they do not have a plane of symmetry and we can check that by drawing the mirror image of each one: it is not superimposable on the first structure.

If a compound is chiral, it can exist as two enantiomers. We’ve just drawn the two enantiomers of each of the diastereoisomers of our epoxide. This set of four structures contains two diastereoisomers (stereoisomers that are not mirror images). These are the two different chemical compounds, the cis and trans epoxides, that have different properties. Each can exist as two enantiomers (stereoisomers that are mirror images) indistinguishable except for rotation. We have two pairs of diastereoisomers, each being a pair of enantiomers. When you are considering the stereochemistry of a compound, always distinguish the diastereoisomers first and then split these into enantiomers if they are chiral.

In fact, the chemists working on these compounds wanted only one enantiomer of the trans epoxide—the top left stereoisomer. They were able to separate the trans epoxide from the cis epoxide by chromatography because they are diastereoisomers. However, because they had made both diastereoisomers in the laboratory from achiral starting materials, both diastereoisomers were racemic mixtures of the two enantiomers. Separating the top enantiomer of the trans epoxide from the bottom one was much harder because enantiomers have identical physical and chemical properties. To get just the enantiomer they wanted the chemists had to develop some completely different chemistry, using enantiomerically pure compounds derived from nature.

مواضيع ذات صلة

Absolute and relative stereochemistry

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

Chiral molecules have no plane of symmetry

Some compounds can exist as a pair of mirror-image forms

Coupling in the proton NMR spectrum Nearby hydrogen nuclei interact and give multiple peaks

Exchange of acidic protons is revealed in proton NMR spectra

Protons on heteroatoms have more variable shifts than protons on carbon

Non-aldehyde protons in the aldehyde region: pyridines