Chiral subunit

Brunner et al. [26] synthesized and applied so-called dendrizymes in enan-tioselective catalysis. These catalysts are based on dendrimers which have a functionalized periphery that carries chiral subunits, (e.g. dendrons functionalized with chiral menthol or borneol ligands). The core phosphine donor atoms can be complexed to (transition) metal salts. The resultant dendron-enlarged 1,2-diphosphino-ethane (e.g. 16, see Scheme 17) Rh1 complexes were used as catalysts in the hydrogenation of acetamidocinnamic acid to yield iV-acetyl-phenylalanine (Scheme 17) [26]. A small retardation of the hydrogenation of the substrate was encountered, pointing to an effect of the meta-positioned dendron substituents. No significantly enantiomerically enriched products were isolated. However, a somewhat improved enantioselectivity (up to 10-11% e.e.) was... 

The distinction between the various configurations of monocentric, tetrahedral molecules is dependent on the ability to differentiate between all four ligands. In systems with more than one asymmetric C-atom these can be regarded as the chiral subunits of the total configuration. If there is sufficient flexibility one can expect 2n configurations with n different asymmetric C-atoms. If there are asymmetric C-atoms of the same kind, as in the isomers of tartaric acid, a lower number of distinguishable configurations is encountered. 

The n asymmetric C-atoms of molecules with rigid polycentric skeletons cannot be treated as independent chiral subunits of the overall configuration. Further, the number of conceivable configurations... 

Apart from cyclic or acyclic transition state geometry further distinctions of diastereoselec-tion have to be made with respect to the way in which the chiral center is attached to the reactive site. The term auxiliary control is used if a chiral subunit, e.g., an alcohol or an amine, is fixed covalently to the unsaturated substrate and then removed by bond cleavage after the addition. In contrast, if the stereogenic center remains part of the molecule after the addition, the term substrate control is applied (these definitions are given in Section A. 1.). 

In addition to the classification of liquid chromatographic enantioseparation methods by technical description, these methods could further be classified according to the chemical structure of the diverse CSPs. The chiral selector moiety varies from large molecules, based on natural or synthetic polymers in which the chirality may be based on chiral subunits (monomers) or intrinsically on the total structure (e.g., helicity or chiral cavity), to low molecular weight molecules which are irreversibly and/or covalently bound to a rigid hard matrix, most often silica gel. 

As discussed in Section 3.1.6.1., natural biopolymers are useful chiral selectors, some of which are readily available they are constructed from chiral subunits (monomers), for instance, from L-amino acids or D-glucose. If synthetic chiral polymers of similar type are to be synthesized, appropriate chiral starting materials and subunits, respectively, must be found. Chiral polymers with, for example, a helical structure as the chiral element, are built using a chiral catalyst as chirality inducing agent in the polymerization step. If the chirality is based on a chiral subunit, the chirality of the polymer is inherent, whereas if the polymer is constructed from chiral starting materials, chiral subunits are formed which lead to chirally substituted synthetic polymers that in addition may order or fold themselves to a supramolecular structure (cf. polysaccharides). 

Our group has devoted two articles for the study of systems where the chiral subunits are bonded to a boron or to a metallic atom (Scheme 3.28). In both cases, the weakness of the bonds formed can be easily broken. In the first case, a series of bisdiphenylborates and the effect of the fluoro substitution on the relative stability of the homo vs. heterochiral complexes have been studied by means of DFT calculations (B3LYP/6-31G ) [29]. In addition, the corresponding isoelectronic structures, in which the boron has been substituted by a carbon or a positively charged nitrogen, have been considered. In all the cases, the homo and heterochiral complexes present D2 and S4 symmetries, respectively. 

Chirality is a very general symmetry concept, and its consequences are not limited to the optical properties of systems. An electrical conductor for instance may be chiral because of several reasons. The material may crystallize in a chiral space group, like tellurium or /9-manganese [30], or be composed of chiral subunits like chiral conducting polymers [31] and Langmuir-Blodgett films [32] or vapors [33] of chiral molecules. Even if the material itself is nonchiral, it may still be formed into a chiral shape, like a helix. In all these cases, the conductor can exist in two enantiomeric forms. 

Optical activity can also be induced on achiral azobenzene units when they are incorporated into the polymer main chains through covalent bonding to chiral subunits (34-37 in Fig. 14) [88-92]. Axially chiral amino- or carboxy-substituted biphenyl and binaphthyl and tartaric acid derivatives were used as the optically... 

Cellulose is also a chiral polymer consisting of chiral subunits, D-p-glucose, a number of chiral environments created within the polymer including the areas (or cavities) formed between adjacent glucose units, and the spaces (or charmels) between the polysaccharide chains. Chiral recognition can take place through attractive interactions between the solute and the D-p-glucose units or by inclusion into the cavities or channels. Both these interactions take place on the type II CSPs. The commercially available type II CSPs are listed in Table 2,... 

Proteins are complex polymers of high molecular weight that are composed of chiral subunits (L-amino adds). These biopolymers play a variety of different roles in a biological system, including the uptake and transport of drug substances. One of the key aspects of the uptake and transport mechanisms is the reversible binding of small molecules to the protein and this process is often stereospectfic. 

Naemura, K. Ogasahara. K. Hirose, K. Tobe. Y. Preparation of homochiral azophenolic crown ethers containing l-phenylethane-1.2-diol and 2,4-dimethyl-3-oxapentane-l,5-diol as a chiral subunit Enantiomer recognition behaviour towards chiral 2-aminoetlianol derivatives. Tetrahedron Asymmetry 1997. 8 (1). 19-22. 

II. Pairs of chiral subunits of opposite absolute configuration... 

The chirality of compounds like 1-3 is entirely based on the chirality of the derivatizing reagents. However, due to their nonplanar shape, calixarenes offer numerous additional possibilities for producing chiral host molecules, which are not based on a chiral subunit but on the absence of a plane of symmetry or an inversion center in the molecule as a whole. In other words, opening of the macrocyclic structure would lead to an achiral linear molecule. Below we attempt to present a systematic overview of such inherently chiral calixarenes. 

O-alkylation of the parent spherand calixarenes leads to derivatives in which the biphenyl subunits are fixed in a certain configuration, as in usual atropisomers. Although such a molecule then contains chiral subunits it may nevertheless be regarded as an inherently chiral molecule. Opening of the macrocycle would lead to a linear molecule in which (at least for small residues R) racemization would be... 

Figure 11.4 e.pJd.p. [ " Clcompounds through coupling of a racemic f CItarget to a chiral subunit of the target substance followed by HPLC separation of the resulting diasteromers... 

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