Chiral compounds carbons

When the asymmetric carbon atoms in a chiral compound are part of a ring, the isomerism is more complex than in acyclic compounds. A cyclic compound which has two different asymmetric carbons with different sets of substituent groups attached has a total of 2 = 4 optical isomers an enantiometric pair of cis isomers and an enantiometric pair of trans isomers. However, when the two asymmetric centers have the same set of substituent groups attached, the cis isomer is a meso compound and only the trans isomer is chiral. (See Fig. 1.15.)... 

Chiral separations are concerned with separating molecules that can exist as nonsupetimposable mirror images. Examples of these types of molecules, called enantiomers or optical isomers are illustrated in Figure 1. Although chirahty is often associated with compounds containing a tetrahedral carbon with four different substituents, other atoms, such as phosphoms or sulfur, may also be chiral. In addition, molecules containing a center of asymmetry, such as hexahehcene, tetrasubstituted adamantanes, and substituted aHenes or molecules with hindered rotation, such as some 2,2 disubstituted binaphthyls, may also be chiral. Compounds exhibiting a center of asymmetry are called atropisomers. An extensive review of stereochemistry may be found under Pharmaceuticals, Chiral. 

Compounds in which one or more carbon atoms have four nonidentical substituents are the largest class of chiral molecules. Carbon atoms with four nonidentical ligands are referred to as asymmetric carbon atoms because the molecular environment at such a carbon atom possesses no element of symmetry. Asymmetric carbons are a specific example of a stereogenic center. A stereogenic center is any structural feature that gives rise to chirality in a molecule. 2-Butanol is an example of a chiral molecule and exists as two nonsuperimposable mirror images. Carbon-2 is a stereogenic center. 

The chiral information of stereogenic centers in the allyl moiety of the precursor is destroyed on deprotonation. While an i/3-bound ion pair with a planar carbon frame is a chiral compound, usually rapid racemization takes place by intra- or intermolecular migration of the cation from one face to the opposite one. The sole exceptions known at present are secondary 2-alkenyl carbamates with X = dialkylaminocarbonyloxy21, in which the cation is tied by the chelating ligand, see Section 1.3.3.3.1.2. 

Note. In these compounds carbon atom number 1 has become chiral. When known, the stereochemistry at this new chiral centre is indicated using the R,S system ([13], Section E). 

The system that has replaced the dl system is the Cahn-Ingold Prelog system, in which the four groups on an asymmetric carbon are ranked according to a set of sequence rules. For our purposes, we confine ourselves to only a few of these rules, which are sufficient to deal with the vast majority of chiral compounds. 

Although four is the maximum possible number of isomers when the compound has two chiral centers (chiral compounds without a chiral carbon, or with one chiral carbon and another type of chiral center, also follow the rules described here), some compounds have fewer. When the three groups on one chiral atom are the same as those on the other, one of the isomers (called a meso form) has a plane of symmetry, and hence is optically inactive, even though it has two chiral carbons. Tartaric acid is a typical case. There are only three isomers of tartaric acid a pair of enantiomers and an inactive meso form. For compounds that have two chiral atoms, meso forms are found only where the four groups on one of the chiral atoms are the same as those on the other chiral atom. 

The great majority of known chiral compounds are naturally occurring organic substances, their molecules having one or more asymmetrically substituted carbon atoms (stereogenic atoms). Chirality is present when a tetrahedrally coordinated atom has... 

In a-methyl-benzylcalcium and a-Me3Si-benzylcalcium complexes, energy barriers for inversion of the chiral benzylic carbon (17-19 kcal mol-1) are concentration independent, suggesting that a dissociative mechanism is involved that involves Ca-Ca bond breakage. The a-methyl-benzylcalcium compounds are less stable and show a... 

Enantiomers of a chiral compound show many different physiological responses, including those of odor and taste2 and it has long been known that enantiomers of some sulfur-containing compounds may have different odors. The examples discussed here are for sulfur-containing compounds where the chirality is based on carbon. While certain compounds can show sulfur-based chirality, there are apparently no known cases where enantiomers dependent on sulfur chirality exhibit different odors. 

As mentioned in Section 1.2, the presence of an asymmetric carbon is neither a necessary nor a sufficient condition for optical activity. Each enantiomer of a chiral molecule rotates the plane of polarized light to an equal degree but in opposite directions. A chiral compound is optically active only if the amount of one enantiomer is in excess of the other. Measuring the enantiomer composition is very important in asymmetric synthesis, as chemists working in this area need the information to evaluate the asymmetric induction efficiency of asymmetric reactions. 

The previous section discussed chelation enforced intra-annular chirality transfer in the asymmetric synthesis of substituted carbonyl compounds. These compounds can be used as building blocks in the asymmetric synthesis of important chiral ligands or biologically active natural compounds. Asymmetric synthesis of chiral quaternary carbon centers has been of significant interest because several types of natural products with bioactivity possess a quaternary stereocenter, so the synthesis of such compounds raises the challenge of enantiomer construction. This applies especially to the asymmetric synthesis of amino group-substituted carboxylic acids with quaternary chiral centers. 

Enzyme reductions of carbonyl groups have important applications in the synthesis of chiral compounds (as described in Chapter 10). Dehydrogenases are enzymes that catalyse, for example, the reduction of carbonyl groups they require co-factors as their co-substrates. Dehydrogenase-catalysed transformations on a practical scale can be performed with purified enzymes or with whole cells, which avoid the use of added expensive co-factors. Bakers yeast is the whole cell system most often used for the reduction of aldehydes and ketones. Biocatalytic activity can also be used to reduce carbon carbon double bonds. Since the enzymes for this reduction are not commercially available, the majority of these experiments were performed with bakers yeast1 41. 

The prochirality concept is useful if it is applied to factored structures within a molecule rather than to the whole, because chiral compounds may also contain centers of prostereoisomerism that would become chiral if their homomorphic ligands were made distinct. The methylene carbons of cholesterol or C(3) of chiral trihydroxyglutaric acid (20b) are appropriate examples. 

Sum = 7.41 10i2 3 chiral centers (carbons where Rj, R3 and R5 are attached to the backbone) in this molecule X 8 = 5.93 lOH or more than 59 trillion compounds included in the patent. 

In a typical application, C-labeling is used to distinguish between the (R)- and (S)-forms of a chiral compound. Essentially, any carbon atom in the compound of interest can be labeled (Fig. 11), but methyl groups in which the H signals are not split by coupling are preferred because the relevant peaks to be integrated are... 

A stereogenic center may be located at carbon, as in numerous ordinary chiral compounds and in spiranes12, or at other elements such as sulfur13-14 (see the chiral sulfoxides pp 401 and 411), phosphorus15-17, nitrogen18, silicon19-20, or a metal21. 

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