Other Chiral Compounds

However, compounds containing other pyramidal atoms, with larger barriers to inversion, can be resolved. Examples include the following phosphorus compound (inversion barrier of about 30 kcal/mol [126 kJ/mol]) and the sulfur compound (inversion barrier of about 35 kcal/mol [146 kJ/mol])  

Allene has two adjacent carbon-carbon double bonds. Its geometry is not planar like a normal alkene. The central carbon is sp hybridized and has linear geometry. The two p orbitals that it uses for the two double bonds are perpendicular, so the planes of the two double bonds are perpendicular. The two hydrogens on one end of allene lie in a plane perpendicular to the two hydrogens on the other end. 

As early as 1875, van t Hoff pointed out that properly substituted allenes would be chiral. When the two groups on one end of the allene are different and the two groups on the other end of the allene are also different, the compound is chiral and exists as a pair of enantiomers, rather than as cis-trans isomers as is the case with simple alkenes. 

A number of chiral allenes, such as the following dicarboxylic acid, have been prepared and resolved. 

Another interesting group of chiral compounds results when molecules are forced to adopt a helical geometry. Like the turn of a screw, the turn of the helix can be either right-handed or left-handed. One example is the compound known as hexahelicene, which has six aromatic rings fused together. The molecule is forced to adopt a helical 

There are also classes of organic compounds that are chiral and cannot be superimposed on their mirror images, but which do not contain any classical asymmetric atoms. The stereogenic unit in these molecules is essentially a twisted structure, with either a right- or left-handed twist. The first category comprises the allenes. If you look back to Problem 3.8(b), you should remember that allenes are not planar. If one double bond is formed from p atomic orbitals, then the other must be formed from p orbitals (7.84). So the shape of 2,3-pentadiene, 7.85, is twisted, and the two mirror images cannot be superimposed. 

Thanks to Prof. Ken Seddon, Queens University Belfast for this image. 

Assign an absolute configuration to each of the following molecules. 

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Enantiomers have identical chemical properties, except when they react with other chiral compounds. Because many biochemical substances are chiral, one consequence of this difference in reactivity is that enantiomers may have different odors and pharmacological activities. The molecule has to fit into a cavity, or slot, of a certain shape, either in an odor receptor in the nose or in an enzyme. Only one member of the enantiomeric pair may be able to fit. 

They react at different rates with other chiral compounds. These rates may be so close together that the distinction is practically useless, or they may be so far apart that one enantiomer undergoes the reaction at a conveni t rate while the other does not react at all. This is the reason that many compounds are biologically active while their enantiomers are not. Enantiomers react at the same rate with achiral compounds. ... 

As in the case of other chiral compounds, the optical and enantiomeric purity of chiral organosulfur compounds can be determined by various methods (241). The simplest and most common method for the determination of optical purity of a mixture of enantiomers is based on polarimetric measurements. However, this method requires a knowledge of the specific rotation of the pure enantiomer. In the... 

User s Guide Separation of Chiral Drugs and Other Chiral Compounds on Chiral-AGP, Chiral-CBH and Chiral-HSA, Chrom Tech, Hagersten, Sweden (1994). 

Except for reactions with plane-polarized light and with other chiral compounds, enantiomers have the same physical and chemical properties. 

For enantiomers, you must know that they have the same chemical and physical characteristics except for two cases 1. reactions with other chiral compounds 2. reactions with polarized light... 

With few exceptions, enantiomers cannot be separated through physical means. When in racemic mixtures, they have the same physical properties. Enantiomers have similar cliemi cal properties as well. The only chemical difference between a pair of enantiomers occurs in reactions with other chiral compounds. Thus resolution of a racemic mixture typically takes place through a reaction with another optically active reagent. Since living organisms usually produce only one of two possible enantiomers, many optically active reagents can be obtained from natural sources. For instance muscle tissue and (S)-<-)-2-methyl-l-butanol, from yeast fermentation. 

The aminotransferase class of enzymes (E.C. 2.6.1.x), also known as transaminases, are ubiquitous, PLP-requiring enzymes that have been used extensively to prepare natural L-amino acids and other chiral compounds.30 123 124 The L-aminotransferases catalyze the general reaction shown in Scheme 19.19 where an amino group from one L-amino acid is transferred to an a-keto acid to produce a new L-amino acid and the respective a-keto acid (see also Chapter 3). Those enzymes most commonly used as industrial biocatalysts have been cloned, overexpressed, and generally used as whole-cell or immobilized preparations. These include branched chain aminotransferase (BCAT) (E.C. 2.6.1.42), aspartate aminotransferase (AAT) (E.C. 2.6.1.1), and tyrosine aminotransferase (TAT) (E.C. 2.6.1.5). 

The classical method to resolve a racemate is to react the mixture of enantiomers with one enantiomer of some other chiral compound. The products are diastereomers and can be separated by using the usual methods, such as recrystallization or chromatography. Then the separated diastereomers are individually converted back to the enantiomers of the original compound. Figure 7.5 shows how a racemic carboxylic acid can be resolved. 

Chiral Gompounds. As CD s are composed of D-glucose they are chiral. CD complexation represents therefore a potential tool for separation of other chiral compounds into enantiomers. In fact various interesting separations of chiral compounds into enantiomers have been achieved using p -CD silica stationary phases (3,9, 12-16). ... 

The trivial names of the aldoses can be used as prefixes to determine the stereochemistry for other chiral compounds. However, in this case, the -ose suffix is omitted and the prefix is written in italics (Figure 2.8). In this respect, it should be mentioned that the chiral centers can be separated by one or more nonchiral centers. If the number of chiral centers exceeds four (e.g., in heptose, octose, nonose, etc. derivatives), a multiple configurational prefix is added to the stem name (Figure 2.9). In this case, the first four chiral centers are selected, followed by the remaining ones. However, the name of the compound starts with the prefix of the chiral center with the highest location. In the case of ketoses, if the carbonyl group is not at C-2, then its location should also be given in the name. 

Asymmetric allyation of carbonyl compounds to prepare optically active secondary homoallyhc alcohols is a useful synthetic method since the products are easily transformed into optically active 3-hydroxy carbonyl compounds and various other chiral compounds (Scheme 1). Numerous successful means of the reaction using a stoichiometric amount of chiral Lewis acids or chiral allylmetal reagents have been developed and applied to organic synthesis however, there are few methods available for a catalytic process. Several reviews of asymmetric allylation have been pubHshed [ 1,2,3,4,5] and the most recent [5] describes the work up to 1995. This chapter is focussed on enantioselective allylation of carbonyl compounds with allylmetals under the influence of a catalytic amount of chiral Lewis acids or chiral Lewis bases. Compounds 1 to 19 [6,7,8,9,10,11,12,... 

Each of the isomers will react at the same rate with simple monodentate ligands such as water. They differ only in their reactivity toward other chiral compounds and toward polarized light. 

Since tartaric acid had been converted chemically into other chiral compounds and these in turn into still others, it became possible as a result of Bijvoet s work to assign absolute configurations (that is, the correct R or S configuration for each stereocenter) to many pairs of enantiomers. 

In addition to the chiral pollutants discussed above, certain other chiral compounds are found in the environment, and sometimes become harmful to the biota. For example, toxicological information on individual chlorobomanes is scarce, but some reports have recently appeared. The neurotoxic effects of toxaphene exposure on behaviour and learning have... 

Throughout this chapter we have written three-dimensional structures for particular stereoisomers and it is important to know how these have been determined. How, for example, do we know that (+)-alanine has the absolute configuration (1) and not (2) The answer is that until 1951 this was not known and the three-dimensional structure of stereoisomers was shown according to a convention introduced in the last century by Emil Fischer. According to this convention it was assumed that (+)-glyceraldehyde had the three-dimensional structure (67). Once this assumption had been made a self-consistent system of conventional representations could be applied to many other chiral compounds by chemical correlation with (67). In 1951 a group led by Bijvoet in Utrecht determined the absolute configuration of the... 

Chiral compounds are frequently foimd among the flavor volatiles of fruits and, like many naturally occurring chiral compounds, one enantiomer usually exists with a greater preponderance when compared with its antipode. Chiral odor compounds may show qualitative and quantitative differences in their odor properties (7). For example, (/ )-(+)-limonene has an orange-like aroma while (5)-(-)-limonene is turpentine-like (5)-(+)-carvone is characteristic of caraway while its enantiomer has a spearmint odor (2). However, other chiral compounds, such as y 6-lactones, show very little enantioselectivity of odor perception (7). The occurrence of chiral flavor compounds in enantiomeric excess provides the analyst with a means of authenticating natural flavorings, essential oils, and other plant extracts. The advent of cyclodextrin-based gas chromatography stationary phases has resulted in considerable activity in the analysis of chiral compounds in flavor extracts of fruits, spices and other plants (i-7). 

When most of the accessible hydroxyl groups in CF6 are derivatized with aromatic groups, however, the general enantioselectivity for primary amines is almost completely lost (Fig. 9c), whereas enhanced separations of a great variety of other chiral compounds can be obtained (Fig. 9f). These observations are readily... 

While polarimetric detectors usually use fight sources in the visible-near IR region, OR and CD detectors use ultraviolet (UV) lamps. The sensitivity of the former is appropi-ate for the detection of sugars, but it is very low for many other chiral compounds (dmgs and synthetic intermediates, for instance). As a result, polarimetric detection is less and less used. As refractive index (Rl) detectors, OR detectors are broadly applicable. They do not require a chromophore... 

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