Enantiotopic atoms

Compound (I)—The protons are enantiotopic because respective replacement of enantiotopic atoms by another ligand affords enantiomeric molecules. 

Asymmetric synthesis starts with a prochiral compound. This is a compound which is not chiral, but can be converted into a chiral compound by a chiral (bio) catalyst. Subsequently, two types of prochiral compounds can be distinguished The first one has a stereoheterotopic face (which usually is a double bond) to which an addition reaction takes place. An example is the conversion of the prochiral compound propene into 1,2-epoxypropane (which has two enantiomers, of which one may be preferentially formed using an enantioselective catalyst). The second type of prochiral compound has two so-called enantiotopic atoms or groups. If one of these is converted, the compound becomes chiral. Meso-compounds belong to this class. Figure 10.5 and 10.6 show some examples of the different types of asymmetric catalysis with prochiral compounds. 

The term prochiralm is used for a compound or group that has two enantiotopic atoms or groups, e.g., CX2WY. That atom or group X that would lead to an R compound if preferred to the other is called pro-R. The other is pro-S e.g.,... 

Enantiotopic atoms or groups 479 Endergonic reactions 286 Endo ring conformation 212 Endocytosis 13,425-427 Endoderm 23 Endoglucanases 602 Endoglycanases 593, 602 Endonexin SeeAnnexin Endoplasmic reticulum (ER) 10,13,14, 521 cisternae of 14 definition of 14 in micrograph 13 rough 14 smooth 14 Endosomes 13, 426 Endosperm 30... 

Chirotopic The property of any atom, and, by extension, any point or segment of the molecular model, whether occupied by an atomic nucleus or not, that resides in a chiral environment [83]. Achirotopic is the property of any atom or point that does not reside in a chiral environment (see also [84]). Chirotopic atoms located in chiral molecules are enantiotopic by external comparison between enantiomers. Chirotopic atoms located in achiral molecules are enantiotopic by internal and therefore also by external comparison. All enantiotopic atoms are chirotopic [83]. 

The different chemical shifts that arise from enantiotopic atoms and groups in chiral solvents can be explained by assuming that the chiral solvent or additive must be part of the solvation shell of the solute molecule. Thus, the presence of chiral molecules nearby will result in different intermolecular interactions depending, on the right- or left-handedness of the prochiral atoms or groups. 

Figure 14.8. Four classes of reactions that form chiral products. A illustrates prochiral faces and B enantiotopic atoms. C and D are examples of a dynamic kinetic resolution and kinetic resolution, respectively.

Enantiotopic atoms or groups are equivalent in all chemical respects except toward a chiral reagent. An important enzymic reaction that discriminates between enantiotopic ligands is the oxidation of ethanol catalyzed by liver alcohol... 

Draw a three-dimensional representation of 4-methylheptane. Identify a pair of enantiotopic atoms. Identify a pair of diastereotopic atoms. 

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