Isomer enantiomers

This chapter has reported the only extensive and coordinated investigation of the effects of chirality on the properties of monolayer films spread at the air-water interface. Twenty compounds of varied headgroup and chain length have been examined carrying one and two chiral centers. In every case, all of the optical isomers—enantiomers and diastereomers—were made and their properties measured both as pure compounds and as mixed monolayers in order to compare phase changes in the films with mixed melting points of the crystals. 

Stereoisomers are chemical componnds having the same elemental composition bnt differing in strnctnre. We have seen three snbtypes of stereoisomers constitutional isomers, enantiomers, and diastereomers. 

Optical isomerism is the result of a dissymmetry in molecular suhstitution. The basic aspects of optical isomerism are discussed in various textbooks of organic chemistry. Optical isomers (enantiomers) may have different physiological activities from each other provided that their interaction with a receptor or some other effector structure involves the asymmetric carbon atom of the enantiomeric molecule and that the three different substituents on this carbon atom interact with the receptor. The Easson-Stedman hypothesis assumes that a three-point interaction ensures stereospecificity, since only one of the enantiomers will fit the other one is capable of a two-point attachment only, as shown in figure 1.13 for the reaction with a hypothetical planar receptor. However, it is reasonable to assume that receptor stereospecificity can also undergo a change when the receptor conformation is altered by a receptor-drug interaction. 

There are two major types of stereoisomer conformational isomers and configurational isomers. Configurational isomers include optical isomers, geometrical isomers, enantiomers and diastereomers. 

This is a problem similar to 5-29, except that the structures are written mostly as projection formulas of the Fischer or Newman type. Determine the relationship between the pairs of compounds as one of the following identical, position isomers, enantiomers, diastereomers, conformational isomers, or cis-trans isomers. (D stands for deuterium, the hydrogen isotope of mass 2.)... 

One of the most interesting developments in the stereochemistry of organic compounds in recent years has been the demonstration that trans-cyclooctene (but not the cis isomer) can be resolved into stable chiral isomers (enantiomers, Section 5-IB). In general, a Wa/w-cycloalkene would not be expected to be resolvable because of the possibility for formation of achiral conformations with a plane of symmetry. Any conformation with all of the carbons in a plane is such an achiral conformation (Figure 12-20a). However, when the chain connecting the ends of the double bond is short, as in trans-cyclooctene, steric hindrance and steric strain prevent easy formation of planar conformations, and both mirror-image forms (Figure 12-20b) are stable and thus resolvable. 

To this point, various physicochemical properties of drugs such as lipophilicity, ionization, and partition coefficient have been discussed. While these are certainly major factors, there is an additional factor that can influence drug distribution, namely chirality. Chirality is a relatively unique structural characteristic of certain molecules that can exist in two asymmetric, nonsuperimposable isomers (enantiomers) due to the presence of a chiral center (a carbon atom that is attached to four different functional groups (see Chapters 5 and 13). 

Identify these pairs of compounds as identical, structural isomers, enantiomers, or diastereomers ... 

We can draw two nonsuperimposable mirror images of the most stable chair conformation of trans-1,2-dibromocyclohexane with both bromines equatorial. These structures cannot interconvert by ring-flips or other rotations about bonds, however. They are mirror-image isomers enantiomers. 

Give the stereochemical relationships between each pair of structures. Examples are same compound, structural isomers, enantiomers, diastereomers. Which pairs could you (theoretically) separate by distillation or recrystallization ... 

X-ray crystal structures were used for the production of computer projected images of inclusion complexes of structural isomers, enantiomers and dlastereomers with a- or B-cyclodextrin. These projections allow for a visual evaluation of the interaction that occurs between various molecules and cyclodextrin, and an understanding of the mechanism for chromatographic resolution of these agents with bonded phase chromatography. 

Green (crystal structure Table 40) Orange-red, tram (P,P) and trans (P,N) isomers, enantiomers resolved 13C NMR, visible-UV Yellow optical isomers separated... 

In either case a pheromone identification should be followed by an investigation of the response of other species to synthetic compounds, blends, isomers, enantiomers, etc., and comparison of the results with the natural interspecific responses or behavior. Additionally, odors from host plants or pheromones of other species may affect the response of members of a species to their pheromones. 

The final aspect of selectivity is related to forming optically active mirror image isomers (enantiomers). The problem of enantioselectivity is extremely important in organic synthesis, but will not be addressed in this text since this is an independent topic in its own right (see, however, discussion of some aspects of this problem in Chapter 4). 

The first step in method development is selecting an adequate HPLC mode for the particular sample. This choice depends on the character of the sample compounds, which can be either neutral (hydrophilic or lipophilic) or ionic, low-molecular (up to 2000 Da) or macromolecular (biopolymers or synthetic polymers). Many neutral compounds can be separated either by reversed-phase or by normal-phase chromatography, but a reversed-phase system without ionic additives to the aqueous-organic mobile phase is usually the best first choice. Strongly lipophilic samples often can be separated either by non-aqueous reversed-pha.se chromatography or by normal-phase chromatography. Positional isomers are usually better separated by normal-phase than by reversed-phase chromatography and the separation of optical isomers (enantiomers) requires either special chiral columns or addition of a chiral selector to the mobile phase. 

Ion association or ion-pairing reactions are most commonly studied for clathrochelate complexes exhibiting unique inertness. These reactions attract particular interest due to their marked effect on the kinetics and direction of the redox and photochemical reactions and on the characteristics of electrochemical processes. In certain cases, ion association reactions govern the catalytic activity of compounds. The ion-pairing ability of clathrochelates is utilized to resolve racemates into optical isomers (enantiomers) and to separate optically active anions using clathrochelates as chiral eluents. 

Pasteur devised three methods to resolve paratartaric acid the first was manual, the second was chemical, and the third could be considered biological or physiological. Because paratartaric acid (also called racemic acid) was the first inactive compound to be resolved into optical isomers (enantiomers, an equimolar mixture of two enantiomers is now called a racemate. 

There is additional, positive evidence for the tetrahedral carbon atom the finding of just the kind of isomers—enantiomers—ih2ii are predicted for compounds of formula CWXYZ. It was the existence of enantiomers that convinced van t Hoff that the carbon atom is tetrahedral. But to understand what enantiomers are, we must first learn about the property called optical activity. 

There remained only for van t Hoff to point out that a tetrahedral carbon atom would account not only for the absence of isomers of formula CH3Y and CH2YZ, but also for the existence of mirror-image isomers—enantiomers—like Pasteur s tartaric acids. ... 

GEOMETRICAL ISOMERS, OPTICAL ISOMERS, ENANTIOMERS, and DIASTEREOMERS. 

See optical isomer enantiomer asymmetry polymer, stereospecific Ziegler catalyst geometric isomer. 

Pasteur, Louis (1822-1895). (1) First to recognize infective bacteria as disease-causing agents (2) developed concept of immunochemistry (3) initiated heat sterilization of wine and milk (pasteurization) (4) observed optical isomers (enantiomers) in tartaric acid. 

Diastereoisomers When a molecule contains several asymmetrical atoms of carbon (see chiral), two types of isomers, enantiomers and diastereoisomers, exist. The former are mirror images. The latter contain some carbon atoms which have an identical structure, others which are enantiomeric. For example, if we have a molecule with two asymmetrical carbons, the first carbon atom can have... 

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