Stereoisomers enantiomers and

In a series of papers, the methodologies applied for amino acids were extended to peptide stereoisomer (enantiomer and diastereomer) separations [59,116,121-123],... 

Figure 13. Rapid simultaneous separation of eight stereoisomers (enantiomers and diastereomers) of 2-mcthyl-3-(l -methylpropyl)oxiranc on 0.125 M nickel(ll) bis[3-(heptafluorobutanoyl)-(l / )-camphorate] in SB-30 [20 m x 0.25 mm (i.d.) glass capillary column. 90 C. 1 bar nitrogen].

One of the problems attaching to the HPLC separation of peptides is the analysis of stereoisomers (enantiomers and diastereoisomers), that is, of peptides that differ only in the configuration of their amino acid residues. 

Two compounds are stereoisomers of one another if the bonding arrangement in one is identical to that in the other except in the way the atoms are oriented in space. Thus, 1 and 2, 3 and 4, 5 and 6, 7 and 8 are four pairs of stereoisomers. There are two types of stereoisomers enantiomers and diasteriomers. 

Two molecules with the same molecular formula and the same bond-to-bond connectivity that are not the same compound are called stereoisomers. Unless they are geometric isomers, stereoisomers must each contain at least one chiral center in the same location. There are two types of stereoisomers enantiomers and diastereomers. 

Stereoisomers Enantiomers and Diastereomers Figure 9. conformational changes. 

There are only two kinds of stereoisomers enantiomers and diastereomers. 

For convenience, we laid down (Sec. 4.20) the following ground rule for discussions and problems in this book unless specifically indicated otherwise, the terms stereoisomers," ""enantiomers," and ""diastereomers" will refer only to configurational isomers, including geometric isomers, and will exclude conformational isomers. The latter will be referred to as conformational isomers," con-formers, conformational enantiomers, and conformational diastereomers. ... 

Stereoisomers have the same number of atoms connected in the same order, but differ in their arrangement in space. There are two major types of stereoisomers enantiomers and diastereomers. 

Generally, in liquid crystals, it is only phases that contain optically active materials that exhibit chiral liquid crystal modifications. Thus at least one substance in the liquid crystal system must be a stereoisomer that contains at least one asymmetrically substituted atom, and which is present in a greater concentration than its opposite enantiomer. It is the configurational isomers in the system that give rise to chiral properties. Included in configurational isomers are two distinct classes of stereoisomers enantiomers and diastereoisomers [2]. Enantiomers are two molecules that are related to one another as object and nonsuperimposable mirror image, as shown in the upper part of Fig. 1. Diastereoisomers usually contain more than one asymmetric atom, and pairs of diastereoisomers do not share a superim-posable mirror image, as shown in the lower part of Fig. 1 [1, 2]. 

Some Physical Properties of Selected Stereoisomers (Enantiomers and Diastereoisomers)... 

Figure 7 8 presents structural formulas for these four stereoisomers Stereoisomers I and II are enantiomers of each other the enantiomer of (R R) is (S S) Likewise stereoiso mers III and IV are enantiomers of each other the enantiomer of (R S) being (S R)... 

FIGURE 7 8 Stereo isomeric 2 3 dihydroxybu tanoic acids Stereoisomers I and II are enantiomers Stereoisomers III and IV are enantiomers All other rela tionships are diastereomeric (see text)... 

FIGURE 7 10 Stereo isomeric 2 3 butanediols shown in their eclipsed con formations for convenience Stereoisomers (a) and (b) are enantiomers of each other Structure (c) is a diastereo mer of (a) and (b) and is achiral It is called meso 2 3 butanediol... 

Steroids are another class of natural products with multiple chirality centers One such compound is cholic acid which can be obtained from bile Its structural formula IS given m Figure 7 12 Cholic acid has 11 chirality centers and so a total (including cholic acid) of 2" or 2048 stereoisomers have this constitution Of these 2048 stereoiso mers how many are diastereomers of cholic acid s Remember Diastereomers are stereoisomers that are not enantiomers and any object can have only one mirror image Therefore of the 2048 stereoisomers one is cholic acid one is its enantiomer and the other 2046 are diastereomers of cholic acid Only a small fraction of these compounds are known and (+) cholic acid is the only one ever isolated from natural sources... 

Three stereoisomers are possible a parr of enantiomers and a meso form... 

Consider the stereochemical relationships between these flexible stereoisomers. A flexible molecule is chiral only if each of its conformers is chiral and if no two conformers are mirror images. Which, if any, of the stereoisomers are chiral Rexible chiral molecules are enantiomers only if each of their conformers are mirror-images. Which, if any, of the stereoisomers are enantiomers and which are diastereomers ... 

Now, consider the physical properties of these stereoisomers. Enantiomers should have many of the same physical properties, such as energy and dipole moment, but diastereomers should not. Obtain the energy of each conformer and use equation (1) to calculate the composition of a large sample of each stereoisomer at 298 K. Then, obtain the dipole moment of each conformer and use equatiori (2) to calculate the dipole moment of a large sample of each stereoisomer at 298 K. Do enantiomers have the same dipole moment Do diastereomers have different dipole moments ... 

Note carefully the difference between enantiomers and diastereomers. Enantiomers have opposite configurations at all chirality centers, whereas diastereomers have opposite configurations at some (one or more) chirality centers but the same configuration at others. A full description of the four stereoisomers of threonine is given in Table 9.2. Of the four, only the 2S,3R isomer, [o]D= -28.3, occurs naturally in plants and animals and is an essential human nutrient. This result is typical most biological molecules are chiral, and usually only one stereoisomer is found in nature. 

Stereoisomers (Section 4.2) Isomers that have their atoms connected in the same order but have different three-dimensional arrangements. The term stereoisomer includes both enantiomers and diastereomers. 

The analogy goes like this when yon have a lot of stereocenters in a com-ponnd, there will be many stereoisomers (brothers and sisters). Bnt, they will be paired up into sets of enantiomers (twins). Any one molecule will have many, many diastereomers (brothers and sisters), bnt it will have only one enantiomer (its mirror image twin). For example, consider the following compound ... 

Answer HBr indicates that we will be adding H and Br across the double bond. The presence of peroxides indicates that the regiochemistry will be anti-Markovnikov. To determine whether stereochemistry is relevant in this particular case, we need to look at whether we are creating two new stereocenters. When we place the Br on the less substituted carbon (and the H on the more substituted carbon), we will only be creating one new stereocenter. With only one stereocenter, there are not four possible stereoisomers but just two possible products (a pair of enantiomers). And we will get this pair of enantiomers regardless of whether the reaction was syn or anti ... 

The importance of the three stereogenic centers became evident as two of the stereoisomers [(2S,3S,7S) and (2S,3R,7R)] were recognized early as sex pheromones and that other enantiomers and diastereoisomers were often found to be inhibitory to the attractive response. Recently, the sawfly pheromone field has undergone a major advance with the recognition that several sawfly species synthesize and utilize sex pheromones of different structural types than the 3,7-dimethylpentadecan-2-ols. Shorter and longer chain lengths (undecan-2-ols 2 and tridecan-2-ols 3) and an additional methyl group in position 9 or 11 (4 and 5) characterize these new pheromone discoveries (Fig. 1). With an ad-... 

Technical 1,2, 5, 6, 9,10-HBCD is produced industrially by addition of bromine to cis-trans-trans-1,5,9-cyclododecatriene. This process leads theoretically to a mixture of 16 stereoisomers (six pairs of enantiomers and four mesoforms) and the product usually is a mixture of the three diastereoisomers a-, p- and y-isomer [14]. Normally, the y-isomer is the most dominant in the commercial mixtures (ranging between 75 and 89%), followed by a- and then p-isomer (10-13% and 1-12%, respectively) [15]. The dissimilarities in the structure of a-, p- and y-isomer might raise differences in polarity, dipole moment and in solubility in water. For example, the solubility of a-, p- and y-HBCD in water was 48.8,14.7, and 2.1 pg/L, respectively. Therefore, these different properties may explain the differences observed in their environmental behavior [16]. Covaci et al. [17] and Morris et al. [18] found that in sediments, the distribution of HBCD isomers was the same of... 

Similar schemes can be developed easily for analogous reactions of acceptor-substituted polyenes. For example, a triene with an acceptor group in 1-position can form six regioi-someric products of Michael addition and electrophilic capture, and each of these exists as E/Z stereoisomers, diastereomers and/or enantiomers. Thus, reactions of this type are only useful if both the regio- and stereoselectivity can be controlled fortunately, only one isomeric Michael adduct is formed in many cases. This is true in particular for polyunsaturated Michael acceptors which bear at least one triple bond besides one or more double bonds. An additional feature of the latter substrate type is that nucleophilic additions can... 

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