Left/right stereoisomerism

The ending caine stems from cocaine, the first clinically employed local anaesthetic. Procaine and tetracaine are ester-linked substances, the others are amides. Amide bonded local anaesthetics usually contain two i s in their name, ester-bonded only one. In the structure drawings, the lipophilic portion of the molecule is depicted at the left, the amine at the right. The asterisk marks the chiral centre of the stereoisomeric drugs. Lipid solubility is given as the logarithm of the water octanol partition coefficient, log(P). 

Enantiomers are characterized as nonsuperimposable mirror images. Enantiomers are said to be chiral (note that some diastereomers may be chiral as well). In the context of the same bonding pattern or connectivity, which atoms are bonded to which, enantiomers have handedness and are related to each other as the right hand is related to the left hand. In the specific example we saw earlier, the carbon atom is linked to four different atoms. Such molecules have non-superimposable mirror images. Stereoisomerism occurs in some molecules that do not have such a carbon atom but these cases are more exotic than we need to worry about here. Stereoisomers frequently have different, and sometimes strikingly different, biological properties, exemplified by the thalidomide case. 

Many drugs are racemates, including 13-blockers, nonsteroidal anti-inflammatory agents, and anticholinergics (e.g benzetimide A). A racemate consists of a molecule and its corresponding mirror image which, like the left and right hand, cannot be superimposed. Such chiral ( handed ) pairs of molecules are referred to as enantiomers. Usually, chirality is due to a carbon atom (C) linked to four different substituents ( asymmetric center ). Enantiomerism is a special case of stereoisomerism. Non-chiral stereoisomers are called diaster-eomers (e.g., quinidine/quinine). 

The absence from a formula of any one of the horizontal or vertical lines at a chiral or prochiral carbon atom (as in examples on pages 27 and 32), or of or Z designations at double bonds, indicates that the configuration of that stereoisomeric centre is not known. Also, as in our previous document [2], the convention of orienting polymer structures (and the corresponding constitutional and configurational units) from left to right is used. Thus, the two bracketted constitutional units in... 

Further model-building experiments have shown that an a helix can form in polypeptides consisting of either L- or D-amino acids. However, all residues must be of one stereoisomeric series a D-amino acid will disrupt a regular structure consisting of L-amino acids, and vice versa. Naturally occurring L-amino acids can form either right- or left-handed a helices, but extended left-handed helices have not been observed in proteins. 

In stereoisomerism, three dimensions must be considered. In stereoisomerism (also termed optical isomerism), there is no plane of symmetry in the molecule, so that the two forms are mirror-images, and thus cannot be turned into a position of coincidence. Thus, compounds containing a carbon atom (or other tetravalent atom) to which four different atoms or radicals are bonded are optical isomers. They receive this name from the fact that one isomer rotates the plane of polarized light to the right (d extra form) the other rotates it to the left (leva form). Lactic acid is an example. See also Lactic Acid, and formulas below ... 

Polyisocyanides have a rigid helical structure with four monomeric units per helix turn [254,255]. The polymers of isocyanides do not display any stereoisomerism, since the carbon atoms in the polymer backbone are not chirotropic. However, it is worth noting that isocyanide polymerisation is stereoselective as regards the screw sense of the polymeric helix (atropisomerism). Therefore, right- and left-handed helices are formed from achiral isocyanides [256], When the monomer is achiral, a racemic mixture of right- and left-handed helices is obtained, e.g. in the case of the polymerisation of t-butylisocyanide [257,258]. When the alkyl... 

The molecular basis for the left- and right-handedness of distinct crystals of the same chemical substance and the associated differences in optical rotation was developed from the hypothesis of Paterno (1869) and Kekule that the geometry about a carbon atom bound to four ligands is tetrahedral. Based on the concept of tetrahedral geometry, Van t Hoff and LeBel concluded that when four different groups or atoms are bound to a carbon atom, two distinct tetrahedral molecular forms are possible, and these bear a non-superimposable mirror-image relationship to one another (Fig. 3). This hypothesis provided the link between three-dimensional molecular structure and optical activity, and as such represents the foundation of stereoisomerism and stereochemistry. 

In a similar set of reactions a 86.14 mixture of the stereoisomeric alcohols 18 and 16 was obtained from diol 17182. For a right to left" synthesis of the vitamin E side chain see p 3371. 

Tropinone is a tertiary amine. Alkylation of nitrogen can occur from either the left or right" (arrows, below), giving stereoisomeric products. 

Fig. 10 Towards new mechanophores for bond creation the bicyclo[3.2.0]-heptanes. The alpha, beta-unsaturated ester formed from mechanically induced [2+2] cycloreversion can undergo subsequently react with nucleophiles (a), such as thiols to form functionalised copolymers left) and cross-linked gels (right). Analysis of the stereoisomerism of the products (b) supported a stepwise mechanism for the ring-opening via a 1,4-diradical intermediate...

The second type of stereoisomerism listed in Figure 23.19 is optical isomerism. Optical isomers, called enantiomers, are mirror images that cannot be superimposed on each other. They bear the same resemblance to each other that your left hand bears to your right hand. If you look at your left hand in a mirror, the image is identical to your right hand ( Figure 23.22). No matter how hard you try, however, you cannot... 

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