Enantiomeric structures

The H4 system is the prototype for many four-elecbon reactions [34]. The basic tetrahedral sfructure of the conical intersection is preserved in all four-electron systems. It arises from the fact that the four electrons are contributed by four different atoms. Obviously, the tefrahedron is in general not a perfect one. This result was found computationally for many systems (see, e.g., [37]). Robb and co-workers [38] showed that the structure shown (a tetraradicaloid conical intersection) was found for many different photochemical transformations. Having the form of a tetrahedron, the conical intersection can exist in two enantiomeric structures. However, this feature is important only when chiral reactions are discussed. 

The discoveries of optical activity and enantiomeric structures (see the box, page 97) made it important to develop suitable nomenclature for chiral molecules. Two systems are in common use today the so-called d,l system and the (R,S) system. 

It was speculated that 7 obtained in the reaction with (S)-3 is actually the enantiomeric structure (ent-1), and that it derives from (7 )-2-methylbutanal that may have been produced by racemization of the 5-isomer in situ11. It is also possible, however, that 7 derives from the reaction of (5)-2-methylbutanal and (R)-3, since the reagent that was used is not enantiomer-ically pure (ca. 95% ee). In addition, in situ racemization of 3 via reaction with adventitious chloride ions (or trace amounts of hydrogen chloride) is also possible36. 

Most of the time, enantiomers are found equally mixed together. Equally mixed enantiomers are not optically active because the rotation in one direction by one structure is canceled by the rotation in the opposite direction by the other structure. Hence, a sample of 2-butanol, for example, as normally obtained from a chemical vendor, is not optically active. An equimolar mixture of two enantiomers is called a racemic mixture and is optically inactive. Separation of a racemic mixture is not possible by conventional methods because the enantiomers are identical with respect to properties that are used to effect the separation. However, it may be possible to separate them by chemical methods, meaning that one may undergo a chemical reaction that the other does not. Some biological reactions are such reactions, and hence a single enantiomeric structure is sometimes found in nature. 

The X-ray intensity diffraction data of the given crystal do not allow one to specify which of the two sets describes the actual crystal structure and thus the absolute configuration of the molecule when there is no effect of anomalous X-ray dispersion. Under such conditions Friedel s law holds, which states that the X-ray intensity diffraction pattern of a crystal is centrosymmetric whether the crystal structure is centrosymmetric or not. This does not mean that a false crystal structure containing a center of symmetry is obtained as the solution of the structural problem, but rather that the X-ray analysis cannot differentiate between the two enantiomeric structures. A simple mathematical analogy is provided by the two possible square roots of a number Vj = x. 

The essence of the problem, as pointed out in Section II, is that conventional X-ray diffraction does not provide information as to whether the molecule W-Y points in the +h or in the —b direction. In other words, it does not allow one to distinguish between the real crystal (Scheme 9a) and the hypothetical enantiomeric structure in which the orientation of W-Y with respect to the b axis is opposite (Scheme 9b). -... 

Scheme 3 shows the mechanism for the thietane formation, in which the six-membered 1,4-biradical BR is appropriate. There are two ways of cyclization to thietane 2, and each pathway gives an enantiomeric structure of thietanes, (1S,4R)- or (lR,4S)-2, respectively. The absolute structure of (-)-(M)-la and the major isomer (- -)-(lS,4R)-2a was determined by X-ray structural analysis using... 

If one of the ligands is chiral and its absolute configuration is known, one cao readily determine the absolute configuration at the metal atom by X-ray (or neutron) diffraction. Of the two enantiomeric structures consistent with the X-ray data, the one having the correct configuration about the known chiral center is chosen. For example, consider tris(R-propylenediaaune)cobalt(III). This was synthesized ... 

Attempts to produce descriptors similar to cis and trims for stereochemicidly more complicated coordination entities have tailed to achieve generality, and labels such as foe and mer are no longer recommended. Nevertheless, a diastereoisomeric structure may be indicated for any polyhedron using a configuration index as an affix to the name or formula. Finally, the chiralities of enantiomeric structures can be indicated using chirality symbols. 

As the time scale of the Raman scattering event ( 3.3 x 10 14 s for a vibration with a Stokes wave number shift of 1000 cm 1 excited in the visible) is much shorter than that of the fastest conformational fluctuations, an ROA spectrum is a superposition of snapshot spectra from all the distinct conformations present in a sample at equilibrium. Since ROA observables depend on absolute chirality, there is a cancellation of contributions from enantiomeric structures arising as a mobile structure explores the range of accessible conformations. Therefore, ROA exhibits an enhanced sensitivity to the dynamic aspects of biomolecular structure. In contrast, conventional Raman band intensities are blind to chirality and so are generally additive and therefore less sensitive to conformational mobility. Ultraviolet circular dichroism (UVCD) also demonstrates an enhanced sensitivity to the dynamics of chiral structures ... 

In 2, all three substituents may lie on the same side of the reference plane alternatively, A, B, or C may be on the opposite side from the other two. Isomerism resulting from these four arrangements is analogous to the more familiar cis-trans isomerism. Each of these arrangements is chiral, even in the absence of helicity (e.g., even when all rings are perpendicular to the reference plane), and may exist in two enantiomeric forms which differ in configuration with respect to the reference plane. Thus this plane may be treated as a plane of chirality. This is illustrated in the top portion of Fig. 1 where the two enantiomeric structures each have all three substituents on the same side of the reference plane. There are consequently 4x2=8 stereoisomeric structures, i.e., four dl pairs. 

The absolute configurations depicted here for (25)—(28) are based on the absolute stereochemistry deduced for nortryptoquivaline (25) by Springer 26 it should be noted that in ref. 25 the enantiomeric structures are illustrated. 

FIGURE 7.11. Schematic representation of the tree types of crystals formed by the two-dimensional assembly of the enantiomeric structures shown above at the air water interface. Each hand represents an enantiomer within the crystal, e.e. stands for enantiomeric excess. 

In the study of mixtures, differentiation between enantiomers is a two level problem which is somewhat independent of whether the LC system is chiral or conventional. The problems common to both systems are the effects of overlapping bands on the performance of the detectorfs). Overlap can be between chiral-achiral species on the one hand and co-eluted chiral-chiral with achiral on the other. On first thought the chiral-achiral distinction should be relatively easy if a chiroptical detector is used because the achiral compounds will not interfere with the detection measurement. In addition the ability of the chiroptical detector to measure both positive and negative signals makes the confirmation of the enantiomeric structure elementary [3,4], As pointed out earlier, enantiomers co-elute from conventional columns and two detectors in sequence will provide the information to measure the enantiomeric ratio provided the mixture is not racemic. Partial or total overlap of the band for a non-chiral species with the chiral eluate band increases significantly the difficulty in measuring an enantiomeric ratio. In this instance the total absorbance that is measured may include a contribution from the non-chiral species which without correction will lead to an overestimation of the amount of chiral material and an erroneous value for the enantiomeric ratio. Under these circumstances there is no other LC option but to develop a separation that is based upon a chiral system. 

Chiral separation Enantiomeric structure Gassman etal.1... 

Since the two products or complexes are diastereoisomers they have different physical properties and Cushny attributed the different pharmacological properties to the different physical properties. However, he seemed to ignore the overall three-dimensional structure, and Parascandola suggests that Cushny had difficulty in imagining that a receptor could actually differentiate between enantiomeric structures [24]. 

The compound must therefore be one of the stereoisomers of inositol (1,2,3,4,5,6-hexahydroxycyclohexane). There are eight possible isomers of inositol if enantiomeric structures are excluded. All eight isomers are shown opposite, the number inside the ring giving the number of carbon signals expected on the basis of symmetry. 

Since the crystal of pip-1 is chiral, it should be either of the two enantiomer crystals D and L. The absolute structures of 20 crystals obtained from a soluti containing racemic compounds indicated that 12 crystals are D and 8 are L. Wh seed crystals with one of the enantiomeric structures, D or L, were added to racemic solution, all the crystals showed the same enantiomeric structures as of the seed crystals. The enantiomeric D L ratio of 20 crystals became 20 0. 

The powdered sample with the same enantiomeric structures, D or L, w irradiated with a xenon lamp for 20 h and was dissolved in a chloroform solutio The specific rotation [a]D of the chloroform solution was 4 30°. It is clear th the racemic-to-chiral transformation can be observed only by photoirradiatio Using the seed crystals, one of the enantiomeric crystals was selected in ea experiment. This means that the A molecules have R configurations in the pi 1 to pip-5 crystals. 

L-selective) or red color (D-selective), while beads with low selectivity assumed a brown coloration. Among the selected beads, two enantiomeric structures 9.148a,b (Fig. 9.59) showed a significant enantioselectivity that was confirmed upon resynthesis. Their enantioselectivity was also confirmed with the separation ofiV-acyl proline racemates lacking the dye moieties, thus highlighting the possible usefulness of 9.148a,b as chiral selectors. 

The evidence on which Fischer assigned a configuration to (+)-glucose leads to either of the enantiomeric structures I and II. Fischer, we have seen, arbitrarily selected I, in which the lowest chiral center carries —OH on the right. 

Stereochemical descriptors are then introduced as a means of identifying or distinguishing between the diastereoisomeric or enantiomeric structures that may exist for a compound of any particular composition. 

Firm evidence for the absolute configuration of (— )-deoxynupharidine (26) has been obtained. The enantiomeric structure had been previously proposed on the basis of formation of optically active methylsuccinic acid from the ozono-lysis of a Hofmann degradation product. The ambiguity in this assignment was noted and the absolute configuration was established by correlation with R-( — )-a-methyladipic acid which was prepared in two steps from R-(+)-3-methylcyclo-hexanone. Thus the earlier assignment" is incorrect. Dendrine (27 R = CH2C02Me) has been synthesised by treatment of dendrobine (27, R = H) with iV-bromosuccinimide followed by a Reformatsky-type reaction. ... 

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