Racemic structures

Since we used a non-chiral HPLC system, resolution of enantiomers is not possible. Consequently the racemate (structures 8 and 9, Fig. 27) must appear in peak 3 as well as the two meso forms (structures 6/7 and 10/11) which each are present in double relative molar amounts. The two pairs of racemates (structures 2-5 and 12-15), each present with a relative molar abundance of 4, belong to peaks 3 and 4, since peaks 1 and 5 must, by necessity of a relative abundance of one for each form, occupy peak 1 and 5. 

Chiral resolution on polysaccharide-based CSPs is due to the different types of bonding between racemates and CSP, as discussed later in this chapter. Therefore, different racemate structures provide bondings of different types, which in turn means that different patterns of chiral recognition will be observed. The effects of... 

Figure 5.7 presents a guide to choosing the CSP best fitted to the racemate structure. Optimization strategies for various CSPs, including the effect of organic modifier and eluent pH on enantioselectivity, have been reported [289-2921. 

H-bonding network, whereby the initial unhinging process from the lateral mesh of next neighbors has a higher activation barrier than for the racemic structure. 

In the following, the stereofacial differentiation will be considered in a classical way (Figure 1). Thus, if the direct precursor of the prochiral enolate and/or the reacting electrophile is covalently linked to the source of asymmetry, we will classify the transformation in the category of the diastereospecific reactions. In contrast, if achiral (or racemic) structures are employed and the asymmetry is introduced via the use of non-covalently bonded chiral ligands, we will refer to enantiospecific reactions. 

In a syndiotactic region, the methylene group is in a racemic structure... 

A broad variety of racemic structures have been resolved on CTA I on a preparative scale (1, 15), but this phase suffers from the fact that it is readly soluble in most organic solvents which are usually applied for chromatographic separations. 

In a few cases, the option of preparing tailor-made CSPs for a particular racemic structure has been applied. For example, we prepared on an empirical basis a particular polysaccharide-based CSP for the separation of the enantiomers of the enantiomers of the LTD4 antagonist iralukast and of the antimalaria agent benflumethol [87]. These two racemic drugs were only poorly resolved on the commercially available polysacharide-based phases whereas an excellent separation was obtained on the carbamate derivative of cellulose obtained from cellulose and 3-chloro-4-methylphe-nylisocyanate. The prepared CSP was also used to perform pharmacokinetic studies. 

The molecular conformation of the tartaric acid is not significantly different in the tartrate and racemate structures, but the different modes of molecular packing lead to the existence of the two very different crystal structures (see Fig. 4 for a comparison). For the tartaric acid anhydrate crystal, the molecular planes are held directly together by a complicated network of intermolecular hydrogen bonds [25], The hydrogen bonding pattern in the racemic acid monohydrate crystal is very different the molecules are bound into columns that are linked up to form sheets [26]. 

Interesting nonlinear optical properties were shown by chiral helicenes (5) forming LB films with larger second-order susceptibility than corresponding racemic structures. Nanotubes based on cyclic peptides were reported to form membranes with ion-selective channels. 

The ligand tet b has the C-racemic structure (4.10) and gives the kinetically controlled blue trigonal bipyramidal copper(lI) complex (4.11) which isomerises in base to the red tetragonal complex. 

There are also 10 possible diastereoisomers of the planar Ni(rac-1,7-CTH) " system (Fig. 3). All are necessarily racemates. Structures r-1,7-1 and 2 are of the basic form I and are of C2 symmetry. The methyl groups attached to the asymmetric carbons are both equatorial in r-1,7-1 and both axial in r-1,7-2. Structures r-1,7-3 and 4 are of the basic forms II and III, respectively, and have Ci symmetry. In each of these structures one methyl group attached to an asymmetric carbon is axial and one is equatorial. 

Every racemic structure having Cj symmetry and every meso structure of Ci symmetry has both of those methyl groups attached to the asymmetric carbons either axial or equatorial. Both methyl groups attached to the asymmetric carbons in structures having Ci symmetry can be axial or equatorial, or one may be axial and one equatorial. 

There are 10 racemic structures possible for the Ni(rac-1,4-CTH) system and these are shown in Fig. 4. Structures r-1,4-1 and 4 have the basic... 

Taking into account the consequent racemic structure of the active sites one could expect to be able to discriminate the two enantiomeric faces of prochiral a-olefins. This was proven by Zambelli et al30.3l on the basis of NMR studies of polypropylene and ethylene/propylene copolymers, which showed that the chirality of the sites was mainly responsible for the isotactic specific polymerization. 

Sketch of the racemic and chiral structures of the SmCP phase of achiral banana-shape molecules in antiferroelectric (at E = 0) and ferroelectric (at E > Etj, 5 V/ om) states. Left column Racemic structure, in which the chirality alternates in the adjacent layers. The APE domains are synclinic with coexisting opposite tilt directions. Right column chiral structure, in which the adjacent layers have the same handedness, but domains of different chirality coexist. The shading illustrates the bent or tilted shape of the molecules (brighter parts are closer to the reader). R (L) is the chirality descriptor corresponding to right (left(-handed layer conformations. 

High resolution H- and C-NMR of polypropylene and of ethylene-propylene copolymers as well as determination of enantiomers distribution in polymers from racemic a-olefins demonstrated that the catalytic complex is responsible for the isotactic enchainment. This implies the chiral (racemic)structure of the catalyst which at present seems to be adequately proved as discussed in the two following sections. 

The first naturally occurring prostaglandin was synthesized by Beal, Babcock and Lincoln of the Upjohn laboratories. Starting with 3-ethoxy-2-cyclopentenone XIII d,1-dihydro-prostaglandin Ei ethyl ester XVII was obtained as one isomer of a complex mixture of isomeric racemates. Structure and stereochemistry of XVII was proved by a number of physical, chemical... 

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