Asymmetric carbon atoms, relative

As the absolute configuration of the asymmetric carbon atom is known to be S, it can be concluded from the relative configuration 56) of both chiral centres that (82)B is the S,S-isomer. As far as we know, this is the first absolute configuration... 

R-S convention is based upon the relative positions of the four different groups attached to the asymmetric carbon atom. 

Pyrrolizidine derivatives with at least one substituent, and particularly the pyrrolizidine alkaloid components, have one or more asymmetric carbon atoms. The stereochemistry of pyrrolizidine was clarified for the most part in the course of investigation of the naturally occurring pyrrolizidine alcohols. Here, the problems of relative and absolute configuration and of stereoisomeric transformations will be considered. 

The coordination chemistry of macrocyclic ligands has been extensively studied and aspects of isomerism have been considered in numerous systems.241 Methods whereby two diastereomers of complexes of tetra- N-methylcyclam may be isolated have been discussed previously.184 This, however, is a relatively simple system and it is usually necessary to consider isomerism due to the presence of asymmetric atoms in the chelate arms, as well as that due to asymmetric donor atoms that may be rendered stable to inversion by coordination. An example of a system exhibiting this level of complexity is afforded by the nickel(II) complexes of the macrocyclic ligands generated by reduction of the readily prepared macrocycle (46). These ligands contain two asymmetric carbon atoms and four asymmetric nitrogen atoms but, because AT-inversion is rapid, it is conventional to consider that only three separable stereoisomers exist. There is an enantiomeric pair, (47a) and (47b), which constitutes the racemic isomer (R, R ), and an achiral (R, S ) diastereomer (47c), the meso isomer. 

The alkaloids of this group are derived from a combination of a piperidine and a pyrrolidine ring, designated as tropane (Figure 14.2). The 3-hydroxy derivative of tropane is known as tropine and is the basic component of atropine. When atropine is hydrolyzed, it forms tropine and tropic acid (a-phenyl-p-hydroxy-propionic acid). Atropine is the tropic acid ester of tropine. It has been prepared synthetically. Tropic acid contains an asymmetric carbon atom. The racemic compound (atropine) as obtained naturally or as synthesized may be resolved into its optically active components, d- and /-hyoscyamine. Atropine is racemic hyoscyamine that is, it consists of equal parts of /-hyscyamine and plant cells and also in the process of extraction, so that the relative proportion of the isomers in the plants and in the preparations varies. However, atropine itself does exist in small amounts in the plants, although most of it is formed from the /-hyoscyamine in the process of extraction. 

An isotactic stereospecific polymerization arises essentially from the favored complexation of one prochiral face of the a-olefin, followed by a stereospecific process. The stereospecific insertion process and the stereospecific polymerization of racemic a-olefins giving isotactic polymers may be expected to be stereoselective whenever the asymmetric carbon atom is in an a- or /3-position relative to the double bond, and when the interaction between the chirality center of the olefin and the chiral catalytic site is negligible. 

The chiral center most frequently encountered is the asymmetric carbon atom, a tetrahedral C atom, bonded to four different substituents. Chiral centers of this type are known for many other elements (4). However, chiral centers are also found in other polyhedra, e.g., the metal atoms in octahedral compounds containing three bidendate chelate ligands. Chirality axes, present in the atrop isomers of ortho-substituted biaryls, occur in coordination chemistry in appropriately substituted aryl, pyridyl, and carbene metal complexes. Well known examples of planar chirality in organometallic chemistry are ferrocenes, cymantrenes, and benchrotrenes containing two different substituents in 1,2- or 1,3-positions relative to each other (5-5). 

Chloramphenicol is a relatively simple compound containing two asymmetric carbon atoms (4 isomers exist). Biological activity of all of them was investigated. The results obtained can be summarized as follows ... 

All of the aromatic alkaloids (except for erythratine) and -erythroi-dine possess two asymmetric carbon atoms—the spiro carbon (C-5) and the carbon substituted by methoxyl (C-3). a-Erythroidine has an additional center of asymmetry at C-12. Chemical, spectroscopic, and crystallographic methods have been used to assign relative and absolute configurations at the asymmetric centers. 

The amino acid isoleucine was investigated at a time when63 the relative configuration at the two asymmetric carbon atoms was not known. Phase determination was facilitated by a parallel study of the isotypic hydrobromide. The u-axis projection of the first trial structure is shown in Figure 2a. The visual data used to calculate this projection refined to an R of 23 percent. A difference map calculated at this point revealed that one of the methyl groups must be displaced, and least squares re-... 

Simple alcohols with only one hydroxy function and one asymmetric carbon atom are classical chiral chemicals. While they are often commercially available, they are relatively expensive. Until recently, they were obtained mainly by resolution of the racemates using a reliable but not very convenient technique. Reaction of the racemic alcohol with phthalic acid anhydride gave the monoester of phthalic acid, which was resolved by salt formation with a chiral base, usually brucine, or occasionally also strychnine or cinchonidine. The methyl carbinols from 2-butanol 1 to 2-tridecanol were first obtained by this method1,2 and this was later extended to 3,3-dimethyl-2-butanol3. When crystallization of the diastereomeric salts was performed in the presence of triethylamine, some other methyl carbinols could also be resolved, such as... 

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