Enantiomers/structural isomers

Identical molecules (Both are the R enantiomers) Structural isomers (one is optically active, the other is not) Enantiomers (R and S optical isomers)... 

For each set of examples, make a model of the first stmcture, and indicate the relationship of each of the other stmctures to the first structure. Examples of relationships same compound, enantiomer, structural isomer. 

Enantiomers/Structural Isomers. The particular optical isomer of a drug being used in a formulation is quite important. For example, quinine is used to treat malaria quinidine, its optical isomer, is used for heart arrhythmia. In 1985, Ciurczak observed that although pure d- and L-amino adds gave identical spectra, the racemic mixtures (dl-) produced an entirely different spectra. Some work was presented by Ciurczak in 1986, which was later expanded and published by Buchanan et al. in 1988, where the enantiomer ratio was determined via NIR. 

Propylene glycol, dipropylene glycol, and tripropylene glycol all have several isomeric forms. Propylene glycol has one asymmetric carbon and thus there are two enantiomers (R)-l,2-propanediol and (3 -1,2-propanediol. 1,3-Propanediol is a structural isomer. Dipropylene glycol exists in three structural forms and since each structural isomer has two asymmetric carbons there are four possible stereochemical isomers per structure or a total of twelve isomers. These twelve consist of four enantiomer pairs and two meso- compounds. Tripropylene glycol has four structural isomers and each structural isomer has... 

Structural isomers of pentapeptide 1A and tripeptide 8 are represented in Scheme 1. For pentapeptide 1A, its retro-peptide 6A, enantiomer (all-D-peptide) IB, retro-inverso-peptide (all-D-retro-peptide) 6B, partially modified, retro-inverso-peptide 7, end-group-modified, partially modified, retro-inverro-peptides 2-4, and end-group-modified, retro-inverso-pep-tide 5 are shown. For end-group-blocked tripeptide 8, its retro-peptide 13, end-group-modified, partially modified, retro-inverso-peptides 9-11, and end-group modified retro-inverso-peptide 12 appear. 

Stereoisomers Structural isomers having an identical chemical constitution but exhibiting differences in the spatial arrangement of their atoms are called stereoisomers [7], One case of stereoisomerism, denoted asymmetric chirality, comprises molecules that are mirror images of each other. Such pairs of molecules are called enantiomers. Figure 1.2.3 illustrates the two chiral molecules of 1-bromo-1-chloroethane. The line in the middle represents a symmetry plane. Note that it is... 

Amphetamine and methamphetamine occur as structural isomers and stereoisomers. Structural isomers are compounds with the same empirical formula but a different atomic arrangement, e.g., methamphetamine and phentermine. Stereoisomers differ in the three-dimensional arrangement of the atoms attached to at least one asymmetric carbon and are nonsuperimposable mirror images. Therefore, amphetamine and methamphetamine occur as both d- and L-isomeric forms. The two isomers together form a racemic mixture. The D-amphetamine form has significant stimulant activity, and possesses approximately three to four times the central activity of the L-form. It is also important to note that the d- and L-enantiomers may have not only different pharmacological activity but also varying pharmacokinetic characteristics. 

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

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. 

The wide interest in the use of cyclodextrins as a separation medium has led to a number of useful applications. The ability of these molecules to bind other molecules to form an inclusion complex, has provided for their use in typically difficult separations of enantiomers, diasastereomers, and structural isomers. Through the coupling of cyclodextrin to a solid support, such as silica gel, a chromatographic resin can be made, and has been developed as a useful chromatographic procedure. 

This review will illustrate examples of computer projected models of inclusion complexes of structural isomers (ortho, meta, para nitrophenol), enantiomers (d- and 1- propranolol) and diastereomers [cis and trans. l(p-B-dimethylaminoethoxy-phenyl-butene), tamoxifen] in either a- or B-cyclodextrin. The use of these computer projections of the crystal structures of these complexes allows for the demonstration and prediction of the chromatographic behavior of these agents on immobilized cyclodextrin. 

Data which establishes the compound s molecular formula is required. Traditionally an accurate combustion analysis (within 0.3 - 0.5%) has been used to determine the empirical formula of a compound, and also to justify that the compound is of high chemical purity. However, combustion analysis data will be identical for structural isomers of any type (geometrical isomers, diastereoisomers, enantiomers etc.) and other spectroscopic or chromatographic methods will therefore be required in order to determine levels of isomeric impurities. 

A chiral molecule and its non-superimposable image are called enantiomers. So isomers that are mirror images of each other are enantiomers. For molecules to have enantiomers, their structure must be asymmetrical. The simplest asymmetrical molecule is the tetrahedrally bonded carbon atom with four different types of atoms or groups attached to it. These kinds of carbon atoms are called asymmetric or chiral carbon atoms and the molecules are called chiral molecules. Some compounds may occur in nature with both enantiomers present. For example, lactic acid can be found in nature this way. 

High pressure catalytic hydrogenation of dodecahydrotriphenylene (Pd/C, 150-250 atm, 300°C, 48 hr) affords a mixture of perhydrotriphenylene stereoisomers which theoretically should comprise four meso forms plus three pairs of enantiomers. An isomer melting at 124°C isolated by Farina (177) was shown to be the thermodynamically controlled product. 11s trans-transoid-trans-transoid-trans structure (215) with D3 symmetry was established by NMR study and con-... 

Indicate whether the following pairs of compounds are identical, enantiomers, diastereomers, or structural isomers. 

Therefore, there are three stereoisomers. Isomers I and II are enantiomers, while isomers I and III are diastereomers. Note that isomer III has a plane of symmetry, therefore it has no enantiomers. Structure III is called a meso compound. 

Aqueous and methanolic solutions of cyclodextrins have been employed as mobile phases in high performance liquid chromatography (HPLC), and as stationary phases by bonding the cyclodextrin to silica packing by several workers (1-8). They have been shown to be especially suitable for separation of structural isomers, cis-trans geometric isomers, and enantiomers. Cyclodextrins (CD) are toroidal-shaped, cyclic, oligosaccharides made up of o-l,4 linked,... 

CD-containing mobile phases in HPLC have been successfully used for the separation of various isomers such as structural isomers, diastereomers and enantiomers. For example, ortho, meta, para isomers of cresol, xylene and aU six isomers of nitrocinnamic acid were separated on the Lichrosorb RP-C18 column with jS-CD solution as mobile phase [41]. Similar results were also obtained for ortho, meta and para isomers of nitrophenol, nitroaniline, fluoronitrobenzene. 

In capillary electrophoresis (CE), CDs and their ionic and neutral derivatives have been successfully used as additives in the carrier system for the separation of structural isomers and structurally related compounds [53]. The commonly used neutral CDs are the native a-, /3- and y-CDs and the dimethyl, trimethyl, hydroxyethyl and hydroxypropyl forms [54]. The charged CDs are carboxymethyl, sulfobutyl ether, sulfated and amino CDs. The methyl derivatives of the CD are effective in separating chiral compounds, enantiomers of terbutaline, ephedrine and carnitine. The neutral derivatives of hydroxyalkylated /3-CD and the mixture... 

Enantiomers (optical isomers) A chiral molecule and its nonsuperimposable, mirror-image molecule. Enantiomers are one type of stereoisomer Stereoisomers Molecules with the same molecular formula but different three-dimensional structures... 

The chiral resolution of environmental pollutants by CE is a very interesting feature, since one of the chiral isomers may be more toxic than the other. In addition, biological transformation of the enantiomers is many times stereoselective, and, therefore, their uptake, metabolism, and excretion can be different. Besides, CE has also been utilized to separate the structural isomers of various toxic pollutants such as phenols, polyaromatic hydrocarbons, etc. 

CE has been used for the analysis of chiral pollutants, e.g., pesticides, polynuclear aromatic hydrocarbons, amines, carbonyl compounds, surfactants, dyes, and other toxic compounds. Moreover, CE has also been utilized to separate the structural isomers of various toxic pollutants such as phenols, polyaromatic hydrocarbons, and so on. Sarac, Chankvetadze, and Blaschke " resolved the enantiomers of 2-hydrazino-2-methyl-3-(3,4-dihydroxyphenyl)propanoic acid using CD as the BGE additive. The CDs used were native, neutral, and ionic in nature with phosphate buffer as BGE. Welseloh, Wolf, and Konig investigated the CE method for the separation of biphenyls, using a phosphate buffer as BGE with CD as the chiral additive. Miura et al., used CE for the chiral resolution of seven phenoxy acid herbicides using methylated CDs as the BGE additives. Furthermore, the same group resolved 2-(4-chlorophenoxy) propionic acid (MCPP), 2-(2,4-dichlorophenoxy) propionic acid (DCPP), (2,4-dichlorophenoxy) acetic acid (2,4-D), 2-(4-chlorophenoxy) propionic acid (2,4-CPPA), [(2,4,5-... 

More recently, several reports have appeared which describe the preparation of HPLC columns which contain CD chemically bonded to silica gel [15-18]. Of these, there are presently two types. The first consists of CD bonded to the silica via amide or amine bonds [15,16] while the second contains no nitrogen linkages [17,18]. This review article summarizes our chromatographic work to date with the latter type of CD bonded phases. In particular, we demonstrate the successful HPLC separation of enantiomers, epimers, cis-trans and other structural isomers as well as important classes of routine compounds by use of a P- or y-CD bonded phase. The obtained chromatographic separations are compared to those obtained on the more conventional normal or reversed phase packings. Additionally, the effect of changes of the pertinent chromatographic variables (such as flow rate, temperature, and solvent composition of the mobile phase) upon the separations are described. Lastly, a brief prospectus on the future of CD bonded phases in HPLC is presented. 

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