Geometric isomerism stereoisomers

We should compare this system with a 1,4-disubstituted cyclohexane such as 4-methylcyclo-hexanecarboxylic acid (see Section 3.4.4). There is a plane of symmetry in this molecule, so there are no chiral centres but geometric isomers exist, allowing cis and trans stereoisomers. The restrictions imposed by bridging have now destroyed any possibility of geometric isomerism. 

Whenever a chemical structure has one or more asymmetric centers or double bonds, either diastereoisomerism or geometric isomerism is possible. Usually only one of the specific stereoisomers is responsible for all or most of the biological activity. An example is... 

Isomers that differ by their chemical constitution are constitutionally isomeric. As traditionally defined, stereoisomers are molecules with the same chemical constitution that differ with respect to the relative spatial arrangement of their constituent atoms. Since many types of flexible molecules exist whose shape rapidly change with time and that are not adequately representable by any geometric model, stereoisomers must be defined as follows, without reference to molecular geometry ... 

Chemical compounds that have the same molecular formula but different structural formulas are said to be isomers of each other. These constitutional isomers (or structural isomers) differ in their bonding sequence, i.e. their atoms are connected to each other in different ways. Stereoisomers have the same bonding sequence, but they differ in the orientation of their atoms in space. Stereoisomerism can be further divided into optical isomerism (enantiomerism) and geometrical isomerism (cis—trans isomerism). The relationships between the different types of isomerism are shown in Figure 4.1. 

According to lUPAC the term geometric isomerism" is an obsolete synonym of cis-trans isomerism and its use is strongly discouraged. Sometimes the term geometric isomerism has been used as a synonym of stereoisomerism, i.e. optical isomers being considered to be geometric isomers. This, however, is not consistent with current standard chemical nomenclature. The exact term for stereoisomers that are not optical isomers is diastereomers. 

Cis-trans isomerism (Often called geometric isomerism although this term refers to all stereoisomers) is a form of stereoisomerism and describes the orientation of functional groups at the ends of a bond around which no rotation is possible. Both alkenes and cycloalkanes have restricted rotation around certain bonds. In alkenes, the double bond restricts movement and rotation, as does the looped structure of cycloalkanes. 

The reaction is typically performed under elevated temperature to afford the final products in yields ranging from 17 to 11% depending on the particular substituents. However, using DBU (ca. 0.6 equiv.) allows the desired compounds to be obtained in yields close to the quantitative ones, even at room temperature (rt) [13, 14], Despite the possibility of geometric isomerism for A,5-thiaphosphiranes, the formation of two stereoisomers was mentioned in only one publication [14],... 

For polymers, two types of configurational isomers are of importance (1) geometric isomerism, e.g., cis and trans and (2) stereoisomers. 

Cycloalkylidene acetates The geometrically isomeric E- and Z- disubstituted cycloalkylidene acetates (e.g. E-8 and Z-8) exhibit a highly specific El-induced loss of one of the two substituents R and R at the two homoallylic positions, as shown in Scheme 7. The specific H-transfer from the allylic position adjacent to the carbonyl toward the carbon-ylic oxygen atom has been proposed as explanation for the stereospecificity of this process. These results enable easy distinction and configurational assignment of stereoisomers in analogous systems... 

Geometrical isomeric mixtures of steroidal oxime esters have been evaluated for the enzymatic transformations. In this respect, the work by Adamczyk and co-workers [27] describes the use of lipase from CCL in the diastereoselective hydrolysis of 3-(0-carbox-ymethyl)oxime methyl esters, 17a-hydroxyprogesterone 122, progesterone 124, testosterone 126, and cortisol 128. CCL proved to be effective in carrying out hydrolysis of methyl esters of steroidal 3-carboxymethyl oximes in a mild manner affording the E and Z dia-stereoisomers 123/122, 125/124, 127/126, and 129/128 in different ratios. The enzyme exhibited a preference for the anti isomer (Table 11). The faster rate and greater selectivity observed for 128 is probably due to the cortisol derivative s better solubility in the reaction media. 

In general, the same sense of chiral induction is obtained with either geometrical stereoisomer, which facilitates the use of (E/Z)-isomeric mixtures. An exception to this was recently reported by Heller and Bomer [56d]. Remarkably, hydrogenation of methyl (Z)-/3-acetylamino pentenoate with [(S,S)-Et-DuPhosRh (COD)]BF4 at 1 bar gave the (R)-enantiomer of product in 31% ee, whereas the same reaction at 30 bar resulted in an inversion of configuration and the (S)-product in 77% ee. 

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