Asymmetric atoms

The reason that the third stereoisomer is achiral is that the substituents on the two asymmetric carbons are located with respect to each other in such a way that a molecular plane of symmetry exists. Compounds that incorporate asymmetric atoms but are nevertheless achiral are called meso forms. This situation occurs whenever pairs of stereogenic centers are disposed in the molecule in such a way as to create a plane of symmetry. A... 

Isotactic Type of polymeric molecular structure that contains sequences of regularly spaced asymmetric atoms that are arranged in similar configuration in the primary polymer chain. Materials having isotactic molecules are generally in a highly crystalline form. 

A system of nomenclature has been devised to describe optical isomers conveniently. These isomers differ in configuration and we have to be able to specify the configuration at the asymmetric atom unambiguously. 

Restricted Rotation Giving Rise to Perpendicular Disymmetric Planes. Certain compounds that do not contain asymmetric atoms are nevertheless chiral because they contain a structure that can be schematically represented as in Figure 4.2. For these compounds, we can draw two perpendicular planes neither of which can be bisected by a plane of symmetry. If either plane could be so bisected, the molecule would be superimposable on its mirror image, since such a plane would be a plane of symmetry. These points will be illustrated by examples. 

For reviews of compounds with asymmetric atoms other than carbon, see Aylett, B.J. Prog. Stereochem., 1969.4,213 Belloli, R. J. Chem. Educ., 1969,46,640 Sokolov, V.I. Reutov, O.A. Russ. Chem. Rev., 1965, 34, 1. 

If two asymmetric atoms are present in the molecule (see 4.1), NMR can still be used to determine the configurational stability but intermolecularly diastereo-topic groups are then used instead of intramolecularly diastereotopic ones. 

A triorganotin halide, bis(2-butyl)methyltin chloride14>, with more than one asymmetric atom has been used to show the configurational instability of such compounds in the presence of nucleophiles. It can exist as four different isomers. 

A diorganotin dihalide with two asymmetric atoms, 2-[l-(S)-dimethylaminoethyl-... 

The NMR spectrum of the threo + erythro mixture of methylphenyl(2-phenyl-propyl)tin hydride (51), another compound with two asymmetric atoms, shows two CH3Sn signals (Av = 6.6 Hz in benzene at 270 MHz) which remain aniso-chronous even in the presence of HMPA 22>. The analogous erythro + threo tri-organotin deuteride (52) shows two CH3Sn signals too (Av = 1.5 Hz at 60 MHz... 

If organotin molecules can be made which contain, besides the chiral tin atom, another asymmetric atom which is optically stable, then they can exist as two diastereomeric racemic mixtures if the other atom is not resolved, or as two diastereomers if the other atom is. If these two diastereomeric mixtures (or diastereomers) can be separated (or at least if two mixtures of different compositions, i.e. diastereomeric ratios, can be obtained), then the rate at which these two mixtures (or compounds) are transformed into the equilibrium mixture is a measure for the rate of inversion at the tin atom. 

The bromide ion attacks from the rear and it involves the breaking of the bonds of the asymmetric atom and so a Walden inversion is possible. Further since the brominium ion is symmetrical, there are chances for each carbon atom to be attacked equally well. This is why equal amounts of active isomers is obtained and the product is a racemic mixture. So starting from the cis acid, the trans addition gives a mixture of racemate. 

The two meso forms, although optically inactive differ in chemical properties. For example, on heating the meso form A, readily forms a lactone, whereas the meso form B does not. In such an example the central carbon atom is said to be pseudoasymmetric. But if one of the carboxyl groups is esterified so that the top and bottom parts of the molecule become structually different, then the central carbon atom becomes truly asymmetric and the molecule would have three true asymmetric atoms and it will exist in eight stereoisomeric forms. 

In a Fischer projection of representing an optically active compound, an asymmetric atom is shown at the intersection point of two lines of a cross. The horizontal lines going to left and right of this point are the bonds extending forward from the plane of paper. The two vertical lines going to top and bottom are those going back away from the plane of paper. For example 2-iodobutane can be represented as ... 

Therefore, as we have seen, we can not prepare an optically active compound from an inactive one by the ordinary laboratory methods. Such a synthesis however, can be accomplished by attaching an optically active molecule or groups to the original compound and then removing it after the new asymmetric atom has been produced. 

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. 

Asymmetry. In the most common case, an organic compound having a carbon atom attached to four different atoms or groups is an asymmetrical atom. 

CHR—CO—), polyoxy acids (—O—CHR—CO—), poly-l-alkylbuta-dienes (—CH=CH—CHR—CH2—). To the same class iKlong the polymers with two asymmetric atoms for every monomer unit, such as polysoibates (—CH=CH—CHA—CHB—, 32 and 33) where both eiythio- and thieo-diisotactic forms are chiral, or polyhexadiene (—CH=CH—CHA—CHA—) and poly-2,3-epoxybutane (—O— CHA-CHA-), 39, where only the thieo-diisotactic structures are chiral, and the polymers of some bicyclic monomers such as those shown in 41 and 42. Other examples are the polymers obtained by hydrogen transfer fk m substituted benzalacetone (79, Scheme 17) (266, 267). 

This phenomenon of chirality degradation is carried to the extreme in the polymerization of (-F)-rrinactive polymer One of the two equivalent asymmetric atoms inverts its configuration during polymerization giving rise to a monomer unit with eiythro or meso strac-ture. The isotactic polymer, 40, so formed is clearly achiral (280). 

Ephedrine is an alkaloid that is present in various forms of the ephedrine family, and which is still extracted from Ephedra sinica and Ephedra equisetina. Because of the presence of two asymmetric atoms, there are four isomeric forms. Pseudoepinephrine (d-isoephrine) is a stereoisomer with pharmacological action that differs slightly from ephedrine. The pharmacological action of ephedrine is typical of noncatecholamine sympathomimetics of mixed action. It stimulates both a- and 8-adrenoreceptors, and simultaneously causes a release of norepinephrine from synaptic neurons. Its vasoconstrictive ability is approximately 100 times weaker than that of epinephrine however, the duration of action is approximately 10 times longer. It is much less toxic than epinephrine, which allows it to be used widely in medicine. 

If 1-serine has the configuration represented, d-alanine and 1-cystine will have the groups attached to the asymmetric atom arranged in the same order as in 1-serine —... 

Obviously, permutation and reflection do not, in general, give identical results. Unfortunately, the important classical terms asymmetric atom (center) and pseudoasymmetric atom (center)... 

Asymmetric Atom (Center) - This is an outdated term that usually should be replaced by stereogenic center (unit). 

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