Stereoisomerism enantiomorphs

Since all the molecules are asymmetric and have no plane of symmetry, all are optically active. Further structures I and II are enantiomorphs and so are structures III and IV. But structures III and I or IV and I are although stereoisomers but are not enantiomorphs. Such pairs of steroisomers which possess chirality but are not the mirror images of each other are called diastereomers. Thus III and IV are diastereomers of 1. So diastereomers will always be formed when the compound contains two dissimilar asymmetric carbon atoms and will exist in four stereoisomeric forms. 

A second problem that has repeatedly concerned us is the inability of the Sequence Rule to provide descriptors for some elements of stereoisomerism. When Cahn et al. (16) first encountered this problem with the all-cis and all-trans isomers of inositol, they attributed it to the fact that the symmetry has become so high that they have no asymmetric, nor even a pseudo-asymmetric atom. This interpretation, we believe, is incorrect. If the two ring ligands of any carbon atom of m-inositol were not heteromorphic, their exchange could not yield an isomer, as it clearly does. Each atom is a center of stereoisomerism with a pair of enantiomorphic ligands (Cg+g hi) and indistinguishable from the traditional pseudoasymmetric atom. The description of cu-inositol as all-5 could be accomplished by the same device that would allow one to specify the configurations of C(l) and C(4) of 4-methylcyclohexanol. 

Stereoisomeric specificity Only the l(-) enantiomorph of morphine and certain other opioids can interact with (enter) the receptor site. For example, levorphanol, a synthetic narcotic, is 5 to 10 times more potent than morphine, but its l(+) enantiomorph dextror-phan is devoid of analgesic activity. 

Figure 5.1 Stereoisomerism of cis-, A polymers of 1,4-disubstituted butadienes, poly(3,4-dialkyl-cw-l-butenylene) CH=CH CH(R) CH(R) n- or [-CH(R>-CH=CH—CH(R)—] —. Diisotactic and disyndiotactic erythro and threo polymers only one of the enantiomorphic forms of the polymers is shown...

The simplest type of stereoisomerism for hexacovalent systems occurs in cases where four of the six substituents on a complex are the same. The remaining two, which may or may not be different from each other, may occupy either cis or irans positions (Fig. 22-14). Both of these complexes are superimposable on their mirror images and cannot be separated into enantiomorphic forms. This can be checked by imagining the complexes to be reflected in a mirror (as was done in Fig. 22-7), or it may be noted that both complexes have at least one plane of symmetry (that is, that there is at least one way in which a plane can bisect the complex into two mirror-image halves). The presence of a plane of symmetry is a convenient indication of the nonresolvability of a complex. The apparent absence of such a plane, however, should be regarded with reserve, for without considerable experience or a good sense of spatial... 

A pair of consecutive, but not necessarily contiguous, tetrahedral stereoisomeric centers defines a diad [16]. Stereosequences terminating in tetrahedral stereoisomeric centers at both ends, and which comprise two, three, four, five, and so on, consecutive centers of that type, may be called diads, triads, tetrads, pentads, and so on, respectively. If the two units belonging to a diad have the same configuration, the diad is defined meso (m) and is characterized by a mirror plane of symmetry if the two units are enantiomorphous, the diad is defined racemo (r) and is characterized by a twofold rotation axis of symmetry (Fig. 2.2b). 

---