Molecules with More than One Chirality Center

Selective Binding of Drug Enantiomers to Left- and Right-Handed Coiled DNA 

Would you like left- or right-handed DNA with your drug  

Enantiomeric forms of daunorubicin bind with DNA and cause it to coil with opposite handedness. [Graphic courtesy John O. Trent, Brown Cancer Center, Department of Medicine, University of Louisville, KY. Based on work from Qu, X. Trent, J.O., Fokt, I., Priebe, W., and Chaires, J.B., Allosteric, Chiral-Selective Drug Building to DNA, Proc. Natl. Acad. Sci. U.S.A., 2000 (Oct. 24) 97(22), 12032-12037.] 

12 Molecules with More than One Chirality Center 

So far we have mainly considered chiral molecules that contain only one chirality center. Many organic molecules, especially those important in biology, contain more than one chirality center. Cholesterol (Section 23.4B), for example, contains eight chirality centers. (Can you locate them ) We can begin, however, with simpler molecules. Let us consider 2,3-dibromopentane, shown here in a two-dimensional bond-line formula. 2,3-Dibromopentane has two chirality centers  

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One final very important point Everything we have said in this section concerns molecules that have one and only one chirality center molecules with more than one chirality center may or may not be chiral Molecules that have more than one chirality center will be discussed m Sections 7 10 through 7 13... 

MOLECULES WITH MORE THAN ONE CHIRAL CENTER... 

Epimers differ in configuration about a single chiral center in molecules with more than one chiral center. Anomers are epimers in which the chiral site was formerly a carbonyl C. 

Another system of specifying configuration around a chiral center is the RS system, which is used in the systematic nomenclature of organic chemistry and describes more precisely the configuration of molecules with more than one chiral center (see p. 18). 

Stereoisomers that are not enantiomers are called diastereoisomers. Three classes may be distinguished configurational, geometrical, and conformational isomers. Configurational diastereomers include molecules with more than one chiral center. Thus 2,3-dichlorobutane can exist in three configurationally... 

Molecules with More Than One Chiral Center. Diastereomers... 

Molecules with more than one chirality center can also exist as enantiomers, but only if the molecule is not superposable on its mirror image. (We shall discuss that situation later in Section 5.12.) For now we will focus on molecules having a single chirality center. 

An ability to find chirality centers in structural formulas will help us in recognizing molecules that are chiral, and that can exist as enantiomers. The presence of a single chirality center in a molecule guarantees that the molecule is chiral and that enantiomeric forms are possible. However, as we shall see in Section 5.12, there are molecules with more than one chirality center that are not chiral, and there are molecules that do not contain a chirality center that are chiral. 

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