Geometrical isomers, optical activity

When molecules composed of the same constituents have the same structural formulas but differ only with respect to the spatial arrangement of certain atoms or groups of atoms, they are defined as stereoisomers. Stereoisomers can be optical isomers or geometrical isomers. Optical isomers are members of a set of stereoisomers, at least two of which are optically active or chiral geometrical isomers are members of a set of Stereoisomers that contains no optically active members. If the relationship between optical isomers is one of nonsuperimposable mirror images, the isomers are defined as enantiomers. Molecules having at least one pair of enantiomers are considered chiral. Optical isomers not related to each other as enantiomers are diastereomers. 

There is no reported separation by Johnson and coworkers of geometric or optical isomers from their reaction mixtures. Connick, Pepperman, and coworkers (10,24) and Cook et al. (22) separated geometric isomers into their two diastereomeric racemates and noted differences in activity for the geometric isomers. Many of the reported activities of strigol-related compounds have been obtained with mixtures of isomers. 

A good example of applying the hydrazone method is the preparation of the optically active pheromone 34 (Scheme 2-22).38 Further study of the crude product prepared from SAMP-hydrazone and 3-pentanone 33 shows that, among the four possible stereoisomers, (Z,S, S )-isomer 35 predominates along with the minor (E,S,S)-isomer, the geometric isomer of 35. The final product 34 was obtained with over 97% enantiomeric excess (ee). 

They may or may not be optically active. The geometric isomers are not optically active, but diastereomers other than geometric isomers are optically active. 

The geometrical isomers are xxii and xxiii. The cis and trans both will exist in two optically active forms along with their one racemic modification. Therefore, the optically active forms of cis and trans will all be different and we will have two pairs of enantiomers. This is also according to the rule of 2 optically active forms where n represents the number of different chiral centres. 

Now, let us examine the case where all three carbon atoms of cyclopropane become asymmetric and satisfied by different groups. In such a case because the molecule contains three different chiral centres, therefore, there will be 23 = 8 optically active forms (four pairs of enantiomers) and there will be four geometrical isomers. The different forms are (xxv) a-d). 

A number of examples involving the stereochemistry of five membered rings are met in furanose sugars. An interesting example is that of 2, 5 dimethylcyclopentane 1, 1 dicarboxylic acid. This acid can exist in two geometrically isomeric forms which can be distinguished by decarboxylation. The cis xxvii isomer forms two monocarboxylic acids which are meso because they possess a vertical plane of symmetry. The trans isomer xxviii forms only one monocarboxylic acid and since it possesses no elements of symmetry, therefore, exists in optically active forms and a meso variety. 

Since all the physical properties of two given enantiomers are the same in the absence of a chiral, or optically active, medium, their chromatographic resolution needs a different approach from the relatively simple separation of geometrical isomers, stereoisomers or positional isomers. Two methods are used. The older technique of indirect resolution, requires conversion of the enantiomers to diastereoisomers using a suitable chiral reagent, followed by separation of the diastereoisomers on a non-chiral GC or LC stationary phase. This technique has now been largely superseded by direct resolution, using either a chiral mobile phase (in LC) or a chiral stationary phase. A variety of types of chiral stationary phase have been developed for use in GC, LC and SFC(21 23). 

Diasteroisomers, also known as geometric isomers, have different relative orientations of their metal-ligand bonds. Enantiomers are stereoisomers whose molecules are nonsuperposable mirror images of each other. Enantiomers have identical chemical and physical properties except for their ability to rotate the plane of polarized light by equal amounts but in opposite directions. A solution of equal parts of an optically active isomer and its enantiomer is known as a racemic solution and has a net rotation of zero. 

Disubstituted alkenyl sulfoxides can also be isomerized to the more stable geometric isomer of the lithiated species through chelation effects, and then reacted with electrophiles such as carbon dioxide. This is illustrated by, for example, the synthesis of optically active 2(5//)-fura-nones64. 

Pentaethylenehexamine (l,14-diamino-3,6,9,12-tetra-azatetradecane) reacts with [Co(NH3)5Br]Br2 in aqueous solution in the presence of active charcoal to form [CoL]C13, nH20 (in the presence of NaCl). Geometrical and optical isomers were separated by column chromatography, and structures assigned on the basis of n.m.r., absorption, i.r., and c.d. spectra.301 The structure of ( —)589-[CoL][Co(CN)6], 3H20 [L = AWIV. /V -tetrakis-(2 -aminoethyl)-l,2-diaminopropane] has been reported, and has the absolute configuration AAA.302... 

Sex pheromonal activity of geometric and optical isomers of synthetic contact pheromone to males of the yellow-spotted longicom beetles, Psacothea hilaris (Pascoe) (Coleoptera Cerambycidae). Appl. Entomol. Zool., 32, 654-656. 

The effect of substitution of a methyl group for hydrogen at Cx of 81 on the product distribution is illustrated by cases 2a-c of Table 6 the yield of cyclized products generally increases. The stereochemistry of the reactions shown under 2a and 2b of Table 6 was also investigated. Optically active tosylate 83 was shown to yield the cyclopropyl derivative 84 (two geometric isomers) with virtually complete inversion of configuration at the reaction centre (Bertrand and Santelli, 1968). 

N-methyl derivative resulted in oxidation of the ligand with concomitant reduction of Co (III) to Co (II). The preparation of tris (benzohydroxa-mato) chromium (III), Cr(benz)3, was successful and resulted in the separation and characterization of its two geometric isomers (2). The half-lives for isomerization of these complexes near physiological conditions is on the order of hours. To facilitate the separation of all four optical isomers of a simple model tris (hydroxamate) chromium (III) complex, we prepared (using Z-menthol as a substituent) the optically active hydroxamic acid, N-methyl-Z-menthoxyacethydroxamic acid (men). This resulted in the separation of the two cis diastereoisomers of tris(N-methyl-Z-menthoxyacethydroxamato) chromium (III) from the trans diastereoisomers and their characterization by electronic absorption and circular dichroism spectra. 

We have prepared the optical isomers of the former (49) and the geometric and optical isomers of the latter (48) and biological evaluations have demonstrated that one isomer of each component is significantly more active than the others (50). Thus the natural sex pheromone probably consists of a mixture of (3j3,6R)-XV and (3Z,6R)-XVI (Figure 5). Field tests have also shown that the compounds XV and XVI are independently attractive to males, and that there is no synergistic effect when XV and XVI are combined. In addition, the presence of the inactive stereoisomers does not inhibit the trap catch of males. Thus synthetic compound for use in monitoring traps in the field can be either XV or XVI and need not be stereochemically pure. 

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