Enantiotopic atoms, groups

The concept of heterotopic atoms, groups, and faces can be extended from enantiotopic to diastereotopic types. If each of two nominally equivalent ligands in a molecule is replaced by a test group and the molecules that are generated are diaster-eomeric, then the ligands are diastereotopic. Similarly, if reaction at one face of a trigonal atom generates a molecule diastereomeric with that produced at the alternate face, the faces are diastereotopic. 

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... 

PLE) transforms the meso substrate into chiral compound 5 with >98% te. This en/yme is capable of differendating between the two enantiotopic ester groups on the prochiral carbon atom and hydrolyzing only one of them to a carboxylic acid. Maximum enan-tioselectivity is achieved by carrying out the reaction in 25% aqueous DMSO solution at 35 C. 

Stereoheterotopic A term that includes both enantiotopic and diastereotopic atoms, groups and faces. Equivalent atoms, groups and faces would be homotopic. 

We are concerned with the absolute stereochemistry of the product does epoxidation give 5a or 5b Does the Diels-Alder reaction give 7a or 7b There is also a small group of prochiral tetrahedral carbon atoms with enantiotopic functional groups such as the diester 8 or the diene 9. We shall meet examples of all these (and more ) in this chapter. 

An asymmetric synthesis at silicon was developed starting from tetragonal silicon compounds. Prochiral compounds of type R1R2SiX2 having two enantiotopic functional groups X are of particular interest. Owing to the many stereospecific substitution reactions that can be performed at a functional silicon atom, the preparation of optically... 

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 method is not only restricted to dimethylphosphine boranes and sulfides. It has been also applied to cyclic diphosphine derivatives by deprotonation of one of the enantiotopic methylene groups a to the phosphorus atom (Scheme 5.3). In this case, a second stereogenic centre is created and therefore it is necessary to control also the diastereoselectivity of the reactions. 

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