Enantiomeric configurations

To specify enantiomeric configuration, the pre-numbered perspective formula is so oriented in three dimensions that its numbering will proceed from right to left around the front of the ring, as customary for... 

Figure 8.23 Illustration of chiral layer structure proposed for NOBOW high-temperature smectic banana phase is given. On left layer with (+) configuration is shown, while on right enantiomeric (—) configuration is illustrated. These mirror image fluid smectic layer configurations are not superposable.

There are cases where racemisation takes place under the influence of heat. Under such conditions there is homolytic fission of the bond between the asymmetric carbon atom and one of the substituents. The radical formed may assume either of the two enantiomeric configurations with equal possibility of recombination, giving a racemate. For example a chloroethyl benzene during distillation undergoes thermal racemisation, but in presence of lewis acids it undergoes racemisation with the intermediate formation of a carbocation. 

Allene is a versatile functionality because it is useful as either a nucleophile or an electrophile and also as a substrate for cycloaddition reactions. This multi-reactivity makes an allene an excellent candidate for a synthetic manipulations. In addition to these abilities, the orthogonality of 1,3-substitution on the cumulated double bonds of allenes enables the molecule to exist in two enantiomeric configurations and reactions using either antipode can result in the transfer of chirality to the respective products. Therefore, the development of synthetic methodology for chiral allenes is one of the most valuable subjects for the synthetic organic chemist. This chapter serves as an introduction to recent progress in the enantioselective syntheses of allenes. Several of the earlier examples are presented in excellent previous reviews [ ] ... 

Another mechanism of chiral amplification that extends over an even larger scale has been reported by Huck et al. [119] The molecule 12-(9 H-thioxantbene-9 -yli-dene-12H-benzo[a]xanthene (Fig. 11.6), which has no chiral center, nevertheless exists, like the helicenes, in two chiral forms defined by their enantiomeric configurations. Consistent with the discussion in Section 11.2.3, a small net handedness (ca. 0.7 %) could be induced in racemic solutions of this molecule by use of ultraviolet CPL. However, introducing 20 wt% of this molecule, which contained a 1.5% chiral excess of one roto-enantiomer, into a nematic phase of liquid crystals produced macroscopic (100 pm) regions of a chiral cholesteric liquid crystal phase. The... 

Tactic macromolecule, essentially comprising alternating enantiomeric configurational base units, which have chiral or prochiral atoms in the main chain in a unique arrangement with respect to their adjacent constitutional units. 

Scheme 41 shows a striking stereospecificity in coupling of a metal-complexed carbene species (67). Crossover experiments using the enantiomerically configured Re-carbene complexes revealed that the reaction proceeds through strict self-recognition of the enantiomers to form only homochiral coupling products. 

A consequence of the pyramidal configuration at nitrogen is that, when the attached groups R, R2 and R3 are nonidentical, the nitrogen becomes a chiral atom. Under these circumstances, we would expect two enantiomeric configurations ... 

Stereochemistry in Biological Systems. Amino acids occur naturally in both D and L (R and S) enantiomeric configurations. Amino acids that occur in proteins almost always have the L configuration although amino acids that occur in bacterial peptides may have the enantiomeric D configuration. The two configurations are shown in Figure 1.21 for alanine. 

The GPL from serovar 17 has an inner tetrasaccharide segment identical to that of serovar 12. The enantiomeric configuration of the 3-(3-hydroxy-2-methyl)-bu-tanamido-3,6-dideoxy-/3-glucopyranose residue and the absolute configuration of the N-acyl substituent remain to be determined.11... 

The seroreactivity of a synthetic monoglycosyl NGC related to the major phenolic glycolipid of M. bovis BCG has provided strong confirmatory evidence for revision of the enantiomeric configuration of the glycosyl residue in the so-called mycoside B 78 to that of 2-O-methyl-a-L-rhamnopyranose. The NGC with the glycosyloxynonanoyl linker-arm was suitable for the serodiagnosis of bovine tuberculosis. 

The syndiotactic polypropylene has a melting point up to 133°C (2 74). (Me2C(Flu)(Cp))ZrCl2 and similar metallocenes, in combination with MAO or other perfluorinated borates, can produce chiral metallocenium ions in which chirahty is centered at the transition metal. Because of the flipping of the polymer chain, the metallocene alternates between the two enantiomeric configurations and produces a syndiotactic polymer (272, 175). It had not previously been possible to produce such pure syndiotactic polymer. 

The reactions can also be promoted by SnCl4, the diastereomers with the opposite enantiomeric configuration being obtained by using imines 18 derived from D-ara-binosylamine 10 [32] (see Scheme 26). 

The reaction can be carried out with glycosyl imines derived from a broad range of aliphatic, aromatic, and heteroaromatic aldehydes. Piperidinones of opposite enantiomeric configuration are available by applying the corresponding D-arabinosyl imines 18 [32]. The major diastereomers are isolated by flash chromatography or recrystallization in high yields. The 2-propyl dehydropiperidine 46a can be readily converted into enantiomerically pure (/ )-coniine hydrochloride 47 (Scheme 28). 

Tris-chelate complexes exist in enantiomeric configuration A and D about the metal atom, and when the chelating ligand is unsymmetrical, there are also geometrical isomers, cis and trans. Each geometrical isomer exists in enantiomeric forms thus there are four different molecules. In the case of tris complexes with symmetrical ligands, the process of inversion (interconversion of enantiomers) is important. When the metal ions are of the inert type, it is often possible to resolve the complex then the process of racemisation can be followed by measurement of optical rotation as a function of time. Possible pathways for racemisation fall into two broad classes those without bond rupture and those with bond rupture. 

Scheme 3, bottom). If the energy is transferred to the enantiomeric pyrazoline moiety at different rates, appreciable CD signal may evolve in the course of the photolysis. However, the energy transfer was found to be too fast and/or too efficient to discriminate the enantiomeric configuration of the substrate moiety. 

In Section 4.3 we mentioned two important stereochemical terms, enantiomers and enantiotopic nuclei. Enantiomers are structures related as the left hand is related to the right nonsuperimposable mirror images. Any chiral (dissymmetric) molecule can exist in two (and only two) enantiomeric forms. For example, chiral alcohol 10-11 has two enantiomeric configurations, labeled R and S.10 Enantiotopic nuclei are those related by a plane of symmetry. The methylene hydrogens of benzyl alcohol (10-12) are enantiotopic and are labeled pro-R and pro-S10 ... 

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