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  Compounds with Several Stereogenic Centers Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Compounds with Several Stereogenic Centers

The Chinese shrub Ma Huang (Ephedra vulgaris) consists of two physiologically active compounds that are pseudoephedrine and ephedrine. Both compounds are stereoisomers of 2-methylamino-1-phenyl-1-propanol and both are optically active, one being levorotatory and the other dextrorotatory. Because the properties of these compounds are considerably distinct, they cannot be enantiomers.

Ephedrine from Ma Huang:               m.p. 35 - 40 º C,   [α]D = -41º,   moderate water solubility [this isomer may be referred to as (-)-ephedrine]

Pseudoephedrine from Ma Huang:   m.p. 119 º C,   [α]D = +52º,       relatively insoluble in water [this isomer may be referred to as (+)-pseudoephedrine]

Each must have an enantiomer because these two compounds are optically active. Even though these missing stereoisomers were not exist in the natural source, they have been prepared synthetically and have the expected similar physical properties and opposite-sign specific rotations with those listed above. The structural formula of 2-methylamino-1-phenylpropanol has two stereogenic carbons (#1 & #2). Each may suppose an R or S configuration; therefore, there are four stereoisomeric combinations possible. These are displayed in the examples, together with the assignments that have been made on the basis of chemical interconversions.

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Like a general rule, a structure having n chiral centers will have 2n possible combinations of these centers. Depending on the entire symmetry of the molecular structure, some of these combinations may be similar but in the absence of this type of identity, we would suppose to find 2n stereoisomers. Few of these stereoisomers will have enantiomeric relationships but enantiomers come in pairs, and so non-enantiomeric stereoisomers will be common. We consider such type of stereoisomers as diastereomers. In the instance above, either of the ephedrine enantiomers has a diastereomeric relationship with either of the pseudoephedrine enantiomers.

Stereochemical Terminology

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In the development of organic chemistry Concept of the asymmetric carbon has played an essential role. Certainly, it provided the focus for the revolutionary structural theory that is introduced by Joseph A. Le Bel and Jacobus van't Hoff. Though, in recent years the nuance of differentiate among the purely structural definition of an asymmetric carbon and the operational definition of a stereogenic element (for example a chiral center) has become increasingly essential. 

For an instance, consider the tetrasubstituted adamantane molecule displayed in the diagram on the left. Even though four asymmetric carbon atoms (that is blue in color) are exist in this compound, it present in the form of only two stereoisomers (enantiomers) rather than the sixteen expected by the 2n rule. There is, actually, only one stereogenic center, displayed by the red dot at the center of the molecule and the configuration displayed in the diagram is (R). The asymmetric carbon atoms are not stereogenic centers, since any two substituents on any of these bridgehead carbons cannot be exchanged without destroying the constitutional integrity of this rigid, highly bridged molecule.
The configurations of the asymmetric bridgehead carbon units in this structure are (R), (S), (R) and (R) correspondingly, for the Br, Cl, CO2H and CH3 substituents. Its enantiomer would. Certainly, have the opposite configurations.

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