Enantiotopic atoms or groups

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... 

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... 

Replacing one of them by some different atom or group gives the enantiomer of the structure obtained by replacing the other therefore the methylene hydrogens at C 2 of butane are enantiotopic The same is true for the hydrogens at C 3... 

Enantiotopic (Section 7.9) Describing two atoms or groups in a molecule whose environments are nonsuperimposable mirror images of each other. The two protons shown in bold in CH3CH2CI, for example, are enantiotopic. Replacement of first one, then the other, by some arbitrary test group yields compounds that are enantiomers of each other. 

The term enantiotopic was invented because the replacement of one proton of the pair by another atom or group, such as deuterium, produces the enantiomer (nonsuperimposable mirror image, 4-4c) of the molecule that results when the other proton is replaced by the... 

Some of the compounds shown below contain enantiotopic or diastereotopic atoms or groups. Which possess this characteristic For those that do, indicate the atoms or groups that are diastereotopic and assign the groups as pro-R and pro-S. 

Enantiotopic. Two atoms or functional groups in a molecule are said to be enantiotopic if marking one of the atoms or groups renders a molecule that is the mirror image of the molecule obtained when the other atom or group is marked. The resonances of NMR-active enantiotopic groups or atoms will be isochronous in the absence of a chiral compound. 

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. 

This concept of the enantiotopic faces of a prochiral molecule is very important as it allows us to show that the introduction of an atom or group of atoms in the third dimension leads to a chiral molecule. This is the case for all such complexes of benchrotrene, ferrocene, cymanthrene, and so on (Figure 2.18). 

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