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Stereochemistry 93

called “heterotopic” (from Greek “heteros,” different, and THEORY • ORGANIC CHEMISTRY,SYNTHESIS • PHYSICAL
“topos,” place). In contrast, the carbon atom in CH 3 Cl is ORGANIC CHEMISTRY • PROTEIN STRUCTURE • PROTEIN
not prochiral since replacement of H by, say, D would SYNTHESIS • RHEOLOGY OF POLYMERIC LIQUIDS
produce CH 2 DCl, which remains achiral.
In the former case (CH 2 ClBr) replacement of one or
other of the two hydrogen atoms gives rise to enantiomeric BIBLIOGRAPHY
products. The hydrogen atoms are therefore called “enan-
tiotopic.” In the latter case, replacement gives the same
Addadi, L., Berkovitch-Yellin, Z., Weissbuch, I., Lahav, M., and
compound and the hydrogens in CH 3 Cl are called “ho- Leiserowitz, L. (1986). “A link between macroscopic phenomena and
motopic.” In a molecule such as CH 2 BrCHOHCO 2 H, molecular chirality: Crystals as probes for the direct assignment of
replacement of one of the terminal hydrogens by, say, absolute configuration of chiral molecules. In “Topics in Stereochem-
chlorine would give one or other of the diastereomers of istry,” Vol. 16, pp. 1–85, Wiley, New York.
Cahn, R. S., Ingold, C., and Prelog, V. (1966). “Specification of molec-
CHBrClCHOHCO 2 H; in this case the terminal hydrogens
ular chirality,” Angew. Chem. Int. Ed. Engl. 5, 385–415 .
are said to be diastereotopic. These definitions of homo- Eliel, E. L. (1982). “Prostereoisomerism (prochirality).”In “Top-
topic, enantiotopic, and diastereotopic ligands also pro- ics in Current Chemistry,” Vol. 105, pp. 1–76, Springer-Verlag,
vide a means for their recognition: Replacement of one Heidelberg.
of two or more homotopic ligands by a different ligand Eliel, E. L., and Wilen, S. H. (1994). “Stereochemistry of Organic Com-
pounds,” Wiley, New York.
gives identical products, analogous replacement of enan-
Eliel, E. L., Allinger, N. L., Angyal, S. J., and Morrison, G. A. (1965).
tiotopic ligands gives enantiomeric products, and such re- “Conformational Analysis,” Wiley, New York [reprinted (1981),
placement of diastereotopic ligands gives diastereomeric American Chemical Society, Washington, DC.]
products. There is also a symmetry criterion which may Fasman, G. D. (ed.). (1996). “Circular Dichroism and the Conforma-
tional Analysis of Biomolecules,” Plenum Press, New York.
be applied to the appropriate molecules above: Homotopic
Gawley, R. E., and Aub´e, J. (1996). “Principles of Asymmetric Synthe-
ligandsinamoleculeareinterchangedbyoperationofboth
sis,” Pergamon Press, Oxford.
simple symmetry axes and symmetry planes; enantiotopic Hegstrom, R. A., and Kondepudi, D. K. (1990). “The handedness of the
ligands are interchanged by operation of a symmetry plane universe,” Sci. Am. 262(January), 108–115.
but not by operation of a simple symmetry axis, and di- Jacques, J., Collet, A., and Wilen, S. H. (1981). “Enantiomers, Race-
astereotopic ligands are interchanged neither by symmetry mates and Resolutions,” Wiley, New York.
Juaristi, E. (ed.). (1995). “Conformational Behavior of Six-Membered
axes nor by symmetry planes.
Rings,” VCH, New York.
Diastereotopic ligands (e.g., protons or C-13 atoms) Kagan, H. B., and Fiaud, J. C. (1988). “Kinetic resolution.” In “Topics
generally display distinct signals in NMR spectra, but ho- in Stereochemistry,” Vol. 18, pp. 249–330, Wiley, New York.
motopic and enantiotopic ligands have coincident (iden- Klyne, W., and Prelog, V. (1960). “Description of stereochemical rela-
tical) signals (except possibly in the case of enantiotopic tionships across single bonds,” Experientia 16, 521–523.
Kondru, R. K., Wipf, P., and Beratan, D. N. (1998). “Atomic contribu-
ligands, in a chiral solvent, or in the presence of a chi-
tions to the optical rotation angle as a quantitative probe of molecular
ral complexing agent) since NMR is an achiral technique. chirality,” Science 282, 2247–2250; id. (1998). Theory-assisted de-
Both enantiotopic and diastereotopic ligands may be dis- termination of absolute stereochemistry for complex natural products
tinguished by enzymes (which are chiral). Thus in citric vid computation of molecular rotation angle, J. Am. Chem. Soc. 120,
acid,HO 2 CCH 2 C(OH)(CO 2 H)CH 2 CO 2 H,allfourmethy- 2204–2205.
Mislow, K., and Raban, M. (1967). “Stereoisomeric relationships of
lene hydrogen atoms are distinguished by enzymes in the
groups in molecules.” In “Topics in Stereochemistry,” Vol. 1, pp. 1–
citric acid cycle (to demonstrate this distinction, they must 38, Wiley, New York.
be individually labeled as deuterium atoms). On the other Nakanishi, K., Berova, N., and Woody, R. W. (eds.). (2000). “Cir-
hand, the CH 2 groups are pairwise identical in NMR (e.g., cular Dichroism: Principles and applications,” Wiley-VCH, New
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C-13) but the geminal hydrogen atoms in each are di-
Pauling, L., Corey, R. B., and Branson, H. R. (1951). “The structure of
astereotopic and provide an (AB) 2 system in the proton
proteins: Two hydrogen bonded helical configurations of the polypep-
NMR spectrum. Further details may be found in Eliel tide chain,” Proc. Natl. Acad. Sci. U.S.A. 37, 205–211.
(1982) and Eliel and Wilen (1994). Prelog, V., and Helmchen, G. (1982). “Basic principles of the CIP system
and proposals for a revision,” Angew. Chem. Int. Ed. Engl. 21, 567–
SEE ALSO THE FOLLOWING ARTICLES 583.
Ramsay, O. B. (1981). “Stereochemistry,” Heyden & Son, Philadelphia.
Sih, C. J., and Wu, S.-H. (1989). “Resolution of enantiomers via bio-
BIOPOLYMERS • ENZYME MECHANISMS • NUCLEAR catalysis.” In “Topics in Stereochemistry,” Vol. 19, pp. 63–125, Wiley,
MAGNETIC RESONANCE • ORGANIC CHEMICAL SYSTEMS, New York.