Enantiomers chemical differences

An article entitled "When Drug Molecules Look in the Mirror" in the June 1996 issue of the Journal of Chemical Education (pp. 481 -484) describes numerous examples of common drugs in which the two enantiomers have different biological properties. 

J. Manz Prof. M. Quack has just presented to us a fascinating strategy for laser control of chemical enantiomers with different parities [1]. 

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. 

A is correct. Strychnine is the only chiral molecule and thus the only possibility. The passage states that the only chemical difference between enantiomers is their reactions with chiral compounds. Strychnine is often employed as a resolving agent for racemic acids. 

Several prominent types of host molecule, such as the steroidal bile acids and the cyclodextrins, are chiral natural products that are available as pure enantiomers. Chemical modification of these parent compounds provides an easy route to the preparation of large numbers of further homochiral substances. Since all these materials are present as one pure enantiomer, it automatically follows that their crystalline inclusion compounds must have chiral lattice structures. It is not currently possible to investigate racemic versions of these compounds, but the examples discussed previously in this chapter indicate that very different behaviour could result. 

It was Pasteur, in the middle of the 19th century, who first recognized the breaking of chiral symmetry in life. By crystallizing optically inactive sodium anmonium racemates, he separated two enantiomers of sodium ammonium tartrates, with opposite optical activities, by means of their asymmetric crystalline shapes [2], Since the activity was observed even in solution, it was concluded that optical activity is due to the molecular asymmetry or chirality, not due to the crystalline symmetry. Because two enantiomers with different chiralities are identical in every chemical and physical property except for optical activity, in 1860 Pasteur stated that artificial products have no molecular asymmetry and continued that the molecular asymmetry of natural organic products establishes the only well-marked line of demarcation that can at present be drawn between the chemistry of dead matter and the chemistry... 

The complexes 9c and 11c with 2-methyl-l,3-butadiene as ligands form three diasteromers, according the coordinations modes of the diene ligands, ARR,ASS, ASS,ARR, and ARS,ASR. The enantiomers ARR,ASS and ASS,ARR have C2 symmetry and contain two chemically equivalent diene ligands. The ARS = A SR and A SR = ARS forms are asymmetric, containing diene ligands of opposite chirality which are chemically different. 

The single stereogenic or chiral center in the chemical structure of verapamil results in two stereoisomers of verapamil S(-)-verapamil and R(+)-verapamil (Fig. 1). These enantiomers have different pharmacokinetic and pharmacodynamic properties (3,6-9). Although both enantiomers have similar types of pharmacologic activity, the S enantiomer has been shown to be the more potent with respect to several of the effects (3,6-8). 

Diastereoisomers (unlike enantiomers) have different physical properties such as boiling point, density, etc. These differences between diastereoisomers can be exploited to resolve (or separate) mixtures of enantiomers. The principle behind this technique is to resolve the mixture of enantiomers by chemically converting them into a pair of diastereoisomers. This is achieved by reacting the racemic mixture with an optically pure reagent. These reagents are usually natural products for example, if the racemic mixture contains acidic compounds, reaction is with an optically pure alkaloid such as strychnine or brucine. 

Interestingly, this enzyme also accepted cis- and tra s-2,3-disubstituted olefins. Moreover, it appeared that racemic czs-2,3-epoxyheptane led to almost total deracemisation, thus affording the corresponding R,R) diol with 79% chemical yield and 91 % ee [187] (Fig. 15). This is due to the fact that the regioselectivity of the enzymatic attack on each of the two enantiomers was different. Recent work described that this enzyme could be immobilized on DEAE-cellulose, thus... 

Enantiomers of a chiral molecule have identical melting and boiling points, densities, and other physical and chemical properties. However, enantiomers show different behaviour towards plane-polarized light. When a beam of plane polarized light passes through an enantiomer, the plane of polarization rotates. For this reason chiral molecules are known as optical isomers and are said to be optically active. 

The selectivity of a chemical reaction is a very important criterion. Besides the chemo- and regioselectivity, the stereoselectivity, i.e. the favored or excluded formation of one or several stereoisomers in the course of a chemical reaction, plays an important role. If there is a formation of (S)- and (K)-enantiomers from a prochiral compound, an enantioselective reaction takes place. What are the reasons for the growing interest in enantioselective reactions and preparation of homochiral compounds Firstly, it is certainly the wish of the chemist to imitate the ability of nature by stereospecific synthesis in the laboratory. Secondly, there are some practical and economic reasons many natural products and a great number of synthetic drugs have a chiral structure and the enantiomers can differ markedly in their biological activity. Sometimes only one of the enantiomers exhibits the wanted optimal activity, while the other is less active or totally inactive, or even toxic. 

A molecule that is asymmetric or dissymmetric (and therefore not superimposable on its mirror image) is called a chiral compound this means that all enantiomers are chiral. Such a compound will display optical activity as an individual enantiomer, which is the ability to rotate the plane of plane-polarized light (measured using a polarimeter), which is one way that we can detect the presence of an enantiomer and define its optical purity. Whereas diastereomers usually differ appreciably in their chemical and physical properties, enantiomers differ only in their ability to rotate polarized light and related optical properties. Normally, when a diastereomer that has an enantiomer is synthesized, a 50 50 mixture of the two enantiomeric forms of the compound is produced, and thus no optical activity is observed. However, if the compound is separated into its two enantiomers (or resolved), each enantiomer will show optical activity in a polarimeter the responses of the two enantiomers will differ only in the sign of rotation of plane-polarized light. 

The two nonsuperimposable mirror image molecules are called an enantiomeric pair and each is the enantiomer of the other. The separated enantiomers have identical properties with respect to achiral environments. They have the same solubility, physical, and spectroscopic properties and the same chemical reactivity toward achiral reagents. However, they have different properties in chiral environments. The enantiomers react at different rates toward chiral reagents and respond differently to chiral catalysts. Usually enantiomers cause differing physiological responses, since biological receptors are chiral. For example, the odor of the R- (spearmint oil) and S- (caraway seed oil) enantiomers of carvone are quite different. 

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