Halogens, chiral centers

The situation is the same when the two chirality centers are present m a ring There are four stereoisomeric 1 bromo 2 chlorocyclopropanes a pair of enantiomers m which the halogens are trans and a pair m which they are cis The cis compounds are diaste reomers of the trans... 

Some of the newer compounds may contain both saturated and unsaturated rings, heteroatoms such as oxygen, nitrogen, or sulfur, and halogen substituents. Others, such as synthetic pyrethroids, may have one or more chiral centers, often needing stereospecific methods of synthesis or resolution of isomers (42). Table 4 Hsts examples of some more complex compounds. Stmctures are shown ia Eigure 1 (25). 

Highly enantioselective atom transfer radical cydization reactions catalyzed by chiral Lewis acids have been reported by Yang et al. [80]. Two main advantages of these enantioselective cyclizations include installing multiple chiral centers and retaining a halogen atom in the product, which allows for further functionalization. 

Priority is given to the substituent atoms that have the highest atomic number. This means that four different atoms arranged tetrahedrally about the chiral center have a priority sequence that decreases with decreasing atomic number. For example, the sequence among the halogens is I > Br >... 

An unusual feature of dactomelyns is the presence of two halogen-substituted chiral centers. Since few methods are so far known for selective introduction of halogen atoms ai sp -hvbridi/ed carbon, the dactomelyns pose an extraordinary synthetic challenge. Another interesting feature of these natural products is the appearance of both chlorine and bromine atoms in the same molecule. One is struck, upon closer examination of the bicyclic pyran skeleton 1. by the fact that the chlorine atom is oriented toward the steri-cally less lavorable side of the molecule (the concave side), whereas the bromine atom is directed toward the convex side. 

Analytical Properties Substrate has 38 chiral centers and 7 aromatic rings surrounding 4 cavities (A, B, C, D), making this the most structurally complex of the macrocyclic glycopeptides substrate has a relative molecular mass of 2066 this phase can be used in normal, reverse, and polar organic phase separations selective for anionic chiral species with polar organic mobile phases, it can be used for a-hydroxy acids, profens, and N-blocked amino acids in normal phase mode, it can be used for imides, hydantoins, and N-blocked amino acids in reverse phase, it can be used for a-hydroxy and halogenated acids, substituted aliphatic acids, profens, N-blocked amino acids, hydantoins, and peptides Reference 47, 48... 

To differentiate between these alternative mechanisms, H. C. Brown, M. S. Kharasch, and T. H. Chao, working at the University of Chicago, carried out the photochemical halogenation of optically active S-(H-)-l-chloro-2-methylbutanc. A number of isomeric products were, of course, formed, corresponding to attack at various positions in the molecule. Problem What were these products ) They focused their attention on just one of these products i,2--dichloro-2-mcthyl-butane, resulting from substitution at the chiral center (C-2). 

A Generation of a Second Chirality Center in a Radical Halogenation... 

Any reaction that creates a new bond to an achiral carbon has the potential to create a new chiral center if that carbon has a tetrahedral geometry in the product. Examples in which the starting carbon was also tetrahedral include free-radical halogenation (replace H- with X-) and a-alkylation of an enolate... 

In the case with 4-nitrochalcone, two chiral centers are generated in the bromonium ion and we might, therefore, expect that two diastereomeric pairs would be formed. However, due to the stereoselectivity of the reaction, only a single diastereomer is generated, as a racemic pair of enantiomers (refer to Experiment [D2] for a further discussion of the stereochemistry of this halogenation). 

When radical halogenation produces a chiral center or takes place at a hydrogen on an existing chiral center, the product is an equal mixture of R and S enantiomers. Consider, for example, radical bromination of butane, which produces 2-bromobutane. 

The stereospecificity of halogenation reactions can be explored in a case where two new chirality centers are formed. For example, consider the products that are formed when cyclopen-tene is treated with molecular bromine (Br2) ... 

In some case, halogenation will occur at an existing chirality center. Consider the following example ... 

When a new chirality center is created during a radical halogenation process, a racemic mixture is obtained. 

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