Resolution of EnantiomersSeparation of a racemic mixture into its two

respective enantiomers

Properties of Diastereomers

Diastereomers have different physical properties, so they can be easily separated.

Enantiomers differ only in reaction with other chiral molecules and the direction in which polarized light is rotated.

Enantiomers are difficult to separate.

Convert enantiomers into diastereomers to be able to separate them.

Resolution of Enantiomers

React the racemic mixture with a pure chiral compound, such as tartaric acid, to form diastereomers, then separate them.

ChromatographicResolution of Enantiomers

Prochirality

a molecule is prochiral if it can become chiral in a single chemical step

sp2 carbons are designated: Re (similar to R) or Si (similar to S)

sp3 hybridized carbon atoms

Prochiral center

Becomes chiral by changing one attached group

Consider the following:

pro-R: replaced atom leads to R configuration

pro-S: replaced atom leads to S configuration

Prochiral distinctions, paired atoms or groups

An sp3 carbon with two groups that are the same is a prochirality center

The two identical groups are distinguished by considering either and seeing if it was increased in priority in comparison with the other

If the center becomes R the group is pro-R and pro-S if the center becomes S

4Prochiral distinctions, paired atoms or groups

An sp3 carbon with two groups that are the same is a prochirality center

The two identical groups are distinguished by considering either and seeing if it was increased in priority in comparison with the other

If the center becomes R the group is pro-R and pro-S if the center becomes S

5Prochiral Distinctions in Nature Biological reactions often involve making distinctions

between prochiral faces or or groups

Chiral entities (such as enzymes) can always make such a distinction

Example: addition of water to fumarate

Questions: Identify the indicated Hs as Pro R or Pro S

Questions: Identify the indicated faces as Re or Si

Questions: Lactic acid in tired muscles results from reduction (addition of H) of pyruvate from the Re face. What is the stereochemistry of the product?

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Prochiralallene