Chiral Configurations Designating the Configuration of Chiral Centers

Chiral Configurations We have referred to the mirror-image configurations of enantiomers as "right- handed" and "left-handed" An early procedure assigned a D prefix to enantiomers chemically related to a right- handed reference compound and a L prefix to a similarly related left-handed group of enantiomers.

Chiral Configurations Although this notation is still applied to carbohydrates and amino acids, it required chemical transformations to establish group relationships, and proved to be ambiguous in its general application. A final solution to the vexing problem of configuration assignment was devised by three European chemists: R.S.Cahn, C. K. Ingold and V. Prelog C. K. IngoldV. PrelogC. K. IngoldV. Prelog. The resulting nomenclature system is sometimes called the CIP system or the R-S system.

The Sequence Rule for Assignment of Configurations to Chiral Centers Assign sequence priorities to the four substituents by looking at the atoms attached directly to the chiral center. Assign sequence priorities to the four substituents by looking at the atoms attached directly to the chiral center. The higher the atomic number of the immediate substituent atom, the higher the priority. For example, H– < C– < N– < O– < Cl–. (Different isotopes of the same element are assigned a priority according to their atomic mass.) The higher the atomic number of the immediate substituent atom, the higher the priority. For example, H– < C– < N– < O– < Cl–. (Different isotopes of the same element are assigned a priority according to their atomic mass.)

If two substituents have the same immediate substituent atom, evaluate atoms progressively further away from the chiral center until a difference is found for example:. CH 3 –< C 2 H 5 –< ClCH 2 –< BrCH 2 –< CH 3 O–. If double or triple bonded groups are encountered as substituents, they are treated as an equivalent set of single-bonded atoms. For example, C 2 H 5 – < CH 2 =CH– < HC≡C–

Once the relative priorities of the four substituents have been determined, the chiral center must be viewed from the side opposite the lowest priority group

It is important to remember that there is no simple or obvious relationship between the R or S designation of a molecular configuration and the experimentally measured specific rotation of the compound it represents

Two or More Chiral Centers The Chinese shrub Ma Huang Ephedra vulgaris)) contains two physiologically active compounds ephedrine and pseudoephedrine Both compounds are stereoisomers of 2- methylamino-1-phenyl-1-propanol, and both are optically active, one being levorotatory and the other dextrorotatory.

Compounds Having Two or More Chiral Centers Ephedrine: this isomer may be referred to as (–)-ephedrine, m.p º C 41º, moderate water solubility. – = D [α] Pseudoephedrine this isomer may be referred to as (+)-pseudoephedrine m.p. 119 º C, [α] D = +52º, relatively insoluble in water

As a general rule, a structure having n chiral centers will have 2n possible combinations of these centers

Using the Fischer projection notation, the stereoisomers of 2-methylamino-1- phenylpropanol are drawn in the following manner Using the Fischer projection notation, the stereoisomers of 2-methylamino-1- phenylpropanol are drawn in the following manner

Diastereomers The usefulness of this notation to Fischer, in his carbohydrate studies, is evident in the following diagram.

Epimers The aldopentose structures are all diastereomers. A more selective term, epimer, is used to designate diastereomers that differ in configuration at only one chiral center.

Epimers ribose and arabinose are epimers at C-2 arabinose and lyxose are epimers at C-3 However, arabinose and xylose are not epimers, since their configurations differ at both C-2 and C-3.

Meso Compounds Meso compounds are achiral (optically inactive) diastereomers of chiral stereoisomers

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The syn-anti nomenclature may be applied to acyclic compounds having more than two chiral centers, as illustrated by the example in the colored box. The stereogenic center nearest carbon #1 serves as a reference. At sites having two substituents, such as carbon #5, the terms refer to the relative orientation of the highest order substituent, as determined C.I.P. sequence rules. by the C.I.P. sequence rules