Optically active enantiomers

Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image it has an internal mirror plane. This is an example of a diaster-eomer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound. 

Chemical Properties. The notation used by Chemical Abstracts to reflect the configuration of tartaric acid is as follows (R-R, R )-tartaric acid [S7-69A-] (4) (S-R, R )-tartaric acid [147-71-7] (5) and y j O-tartaric acid [147-73-9] (6). Racemic acid is an equimolar mixture of the two optically active enantiomers and, hence, like the meso acid, is optically inactive. 

Racemic /ru -2-(l-naphthylsulfonyl)-7- [4-amino-2-quinaxolinyl)amino-methyl]perhydropyrido[l,2-u]pyrazine was resolved into the optically active enantiomers by means of a Chiralpack column (01MIP20). 

Today, however, GC-GC coupling is seldom used to determine pesticides in environmental samples (2), although comprehensive MDGC has been applied to determine pesticides in more complex samples, such as human serum (19). On the other-hand, new trends in the pesticide market, which is now moving towards the production of optically active enantiomers and away from racemic mixtures, may make this area suitable for GC-GC application. The coupling of non-chiral columns to chiral columns appears to be a suitable solution to the separation problems that such a trend might cause. 

These cases are completely different from the cis-trans isomerism of compounds with one double bond (p. 157). In the latter cases, the four groups are all in one plane, the isomers are not enantiomers, and neither isomer is chiral, while in allenes, the groups are in two perpendicular planes and the isomers are a pair of optically active enantiomers. 

The optically active (2,5-dimethyl-6-fluorotetrahydroquinolin-l-yl)-methylenemalonates (131) were prepared in the reactions of optically active enantiomers of 2,5-dimethyl-6-fluorotetrahydroquinoline and EMME at 135-140°C for 1 hr (87JMC839). In a similar way, the 2-desmethyl derivative of 131 was prepared. 

Trigonal ML3 metal complexes exist as optically active pairs. The complexes can show enantiomeric selective binding to DNA and in excited state quenching.<34) One of the optically active enantiomers of RuLj complexes binds more strongly to chiral DNA than does the other enantiomer. In luminescence quenching of racemic mixtures of rare earth complexes, resolved ML3 complexes stereoselectively quench one of the rare earth species over the other. 35-39 Such chiral recognition promises to be a useful fundamental and practical tool in spectroscopy and biochemistry. 

Table 4.6 shows some experimental data on a-a dialkyl succinic acids. The most remarkable finding is that some of these molecules have a very large negative cooperativity, far beyond what could be explained by electrostatic theories. These molecules exist in two isomers—the meso and racemic forms. The latter exists in two optically active enantiomers that are mirror images of each other—only one of these is shown in Fig. 4.29. 

Figure 4.29. The meso (a) and racemic (b) forms of a-a dialkyl (R) succinic acid. The racemic form exists in two optically active enantiomers one is shown on the riis, the other is the mirror image of this form.

The enzyme is chiral and causes oxidation of only one enantiomer. The unreacted, optically active enantiomer is isolated. 

In the pseudo-sugar family there are 32 theoretically possible stereoisomers, including anomer-like compounds, and up to the present time, all the predicted sixteen racemic pseudo-sugars have been prepared, as well as ten optically active enantiomers. In the present review, preparations of pseudo-sugars, pseudo-p-fructopyranoses and biological effects of pseudo-sugars will be described. 

Cyclization of the optically active enantiomers of methyl a-(2-carba-moylpiperidinyl)-a-phenylacetate 332 with 1.0 equiv. of NaOH in EtOH provided almost pure optically active l-phenylperhydropyrido[l,2-a]pyr-azine-2,4-diones 333 (09TA1759). 

No. Pasteur separated an optically inactive racemic mixture into two optically active enantiomers. A meso form is achiral, is identical to its mirror image, and is incapable of being separated into optically active forms. 

Optical Activity One cool property of chiral molecules is optical activity. Enantiomers of a chiral molecule interact with polarized light, but one enantiomer tilts the polarization in one direction, while the other enantiomer tilts it in the opposite direction. This phenomenon is called optical activity. 

The cyclic dimer 43 is a me so compound consisting of a pair of different enantiomeric units of 41. [45] All four substituents attached to the two tetra-hydropyran rings are located in the axial positions. This compound has a center of symmetry and is readily crystallized. The cyclic tetramer 44 is a racemic mixture of optically active enantiomers, that is, 44R and its enantiomer 44S. [46] Similarly, the cyclic pentamer 45 is a racemic mixture of 45R and its enantiomer 45S. [47] In these cyclic oligomers, every exo-cyclic acetal oxygen occupies the axial position and every carbonyl carbon occupies the equatorial position of the tetrahydropyran ring. The molecules of these oligomers are chiral but not asymmetric (gyrochiral... 

J0rgenson[189] Another situation which cannot be an eigenfunction of a time-independent Hamiltonian is an optically active enantiomer. 

The idea of lone pairs was originated by W. J. Pope of Cambridge in 1900 who extended the concept of the three-dimensionality of carbon and nitrogen compounds to those of sulfur. His resolution of sulfonium cations RR R"S+ with three different substituents into optically active enantiomers suggested that these species were tetrahedral with an invisible substituent. The influence of these lone pairs can hardly be detected in transition metal compounds, but the situation is different for post-transition group central atoms such as Ge(II) As(III), Se(IV), and Br(V) with 30 electrons, In(I), Sn(II), Sb(III), Te(IV), I(V), and Xe(VI) with 48 electrons, and Au( —I), T1(I), Pb(II), and Bi(III) with 80 electrons (90). 

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