Subunit achiral

All of the methods in categories 2 to 5 have been designed and developed on the basis of the achiral enolate structure. The methods in category 1 involve chiral enolates where the enolate subunit (achiral on its own) covalently links to a chiral element. In contrast to these methodologies, the next section describes a novel asymmetric induction based on dynamic chirality of the enolate structure. 

Another saturated tetrahydrofuryl core has found application as a component of liquid crystals. Cholesteric liquid crystal polymers are useful as photostable UV filters in cosmetic and pharmaceutical preparations for the protection of human epidermis and hair against UV radiation, especially in the range 280-450nm <2000DEP19848130>. Fused bifuran 81 is a suitable monomer for the preparation of these desired polymers as it contains the requisite characteristics of having more than one chiral, bifunctional subunit type which is capable of forming a cholesteric liquid crystal phase with a pitch of <450 nm. It also contains an achiral aromatic or cycloaliphatic hydroxyl or amino carboxylic acid subunit, achiral aromatic or cycloaliphatic dicarboxylic acids, and/or achiral aromatic or cycloaliphatic diols or diamines. Polymers prepared from suitable monomers, such as diol 81, can also be used as UV reflectors, UV stabilizers, and multilayer pigments. 

With this information at hand, Maruoka and coworkers developed a new C2-symmetric chiral quaternary ammonium bromide 12 which incorporated an achiral, conformationally flexible biphenyl subunit (Scheme 5.8) [12]. 

Supramolecular systems on the basis of dimeric and oligomeric porphyrinoids are varied depending upon the chirality of the supramolecular counterparts, type of the covalent linkage, and number of the porphyrin subunits. Firstly, we shall examine examples of the assemblies consisting of achiral and racemic bis-porphyrinoid hosts and chiral guests. 

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... 

The chirality of the compounds discussed so far, especially of those in Section 2 is caused entirely by the chirality of their substituents. Due to their nonplanar shape, calixarenes offer various additional possibilities to produce chiral derivatives, the chirality of which is not based on a chiral group or subunit but on the absence of a symmetry plane, an inversion center, or an alternating axis in the molecule as a whole. This means, that opening of the macrocyclic structure would lead to an achiral linear molecule. Such molecules may therefore be called inherently chiral which should not be confused with the term intrinsically chiral,110 A graph whose chirality is independent of its embedding in the three-dimensional space is intrinsically chiral, while the inherent chirality defined above is due to the three-dimensional structure. 

Chiral allylic indium reagents and achiral aldehydes. Chiral indium reagents have been generated by the oxidative metallation of allylic bromides, which bear remote stereogenic subunits. For example, the 1,4-asymmetric induction in the indium-mediated coupling of allyl bromides 298 with benzaldehyde affords the syn-adducts as the major products (Table 10-35) [199]. The stereoselectivity of the reaction improves with more sterically encumbered allyl bromides. 

C-O-C groups rather than connected with the CH2 subunits of the dendritic branches. The VCD results support the explanation of the observed induced circular dichroism effect as a consequence of the chiral arrangement of the inherently achiral benzene rings within the dendritic branches that was induced by the chiral center. 

Let us assume that a rotaxane is formed from an axle with two different stoppers and a wheel that bears a directionality defined by the sequence of atoms in the macrocycle. Both of these subunits are achiral, both are identical to their mirror images. Nevertheless, the corresponding rotaxane exists in two enantiomeric forms, although no chiral center is present in the molecule (Fig. 4). It is the molecular topology that makes these species chiral, and consequently, this phenomenon was coined "topological chirality.. It also exists for catenanes, knots, and other mechanically bound molecules. Shown in Fig. 4 is a detailed structure of such a rotaxane. The wheel contains one sulfonamide group that provides the directionality of... 

The origin of optical activity in molecules often reduces to the question of how the molecule acquires the electronic properties expected of a chiral object when it is formed from an achiral object. Most often an achiral molecule becomes chiral by chemical substitution. In coordination compounds, chirality commonly arises by the assembly of achiral units. So it is natural to develop ideas on the origins of chiral spectroscopic properties from the interactions of chirally disposed, but intrinsically achiral, units. Where this approach, an example of the independent systems model, can be used, it has obvious economic benefits. Exceptions will occur with strongly interacting subunits, e.g., twisted metal-metal-bonded systems, and in these cases the system must be treated as a whole—as an intrinsically chiral chromophore. ... 

To prevent interference by ChiroCLEC, the acyl carrier intended for phosphine activation (the mixed mesitoate anhydride 212) was placed on an insoluble solid support where it can be accessed by the soluble phosphine, but not by the insoluble ChiroCLEC. Under three phase conditions, interference was prevented because the phosphine does not activate vinyl pivalate, the acyl donor intended for activation by ChiroCLEC in the form of an activated ester 214 nor does the activated acylphosphonium species 213 come into contact with the ChiroCLEC. Potential destruction of the lipase catalyst is thereby avoided, and the enantio-complementary activated intermediates convert the racemic alcohol R,S)-7 into the solid phase-bound ester 215 and the soluble pivalate 216 with excellent enantioselectivity. This is a proof-of-concept experiment that demonstrates the most difficult application, the case where two similar catalytic reactions are conducted in parallel. Furthermore, the experiment demonstrates PKR with the incorporation of achiral subunits to achieve enantiodivergence, and achieves product separation by simple filtration. 

CD may sometimes be used to deduce the strxicture of a system. This is only really viable when the system can be considered as a collection of chromo-phores (spectroscopically well-defined subunits of a molecule) each of which is only slightly perturbed by the rest of the system. In the rest of this section we shall consider the coupling of two intrinsically achiral chromophores that are chirally oriented with respect to one another. A range of applications of this theory is given in (4). We have to treat eda/mdf transitions separately from... 

The equation describing the octant rule dependence of the dynamic coupling induced CD of an transition is derived as follows (4, 7). One first divides the molecule into an achiral chromophore A composed of the C=0 bond and the two carbon atoms to which it is joined (Figure II). The rest of the molecule is divided into chromophores, C,. which are subunits of the molecule that do not exchange electrons with the rest of the molecule. For a molecule where (apart... 

Returning to Figure 4.33. Note that (a) possesses a mirror plane passing through its center. Molecules that enjoy such internal symmetry and are thus achiral are called meso one-half is the mirror image of the other. Since the descriptor meso refers to any achiral member of a set of diastereomers that also includes at least one chiral member, it should be clear that the ( )-isomer is also meso, as it possesses a point of symmetry. That is, a line, drawn from the center of the molecule in a given direction for some specific distance, will encounter the identical structural subunit when drawn in the opposite direction at the same distance. Finally, it should also be clear to you that this diastereomer, while chiral, is not asymmetric it has an axis of symmetry and asymmetric objects have no symmetry. This diastereomer is dissymmetric. (For a thorough discussion see Eliel, E. Elements of Stereochemistry, John Wiley Sons, New York, 1969.)... 

The Nicolaou strategy is based on RCM [14] as the final ring dosing step in order to establish the 16-membered macrolide. The linear subunit of C1-C12 was obtained by an intermolecular aldol reaction, in which an a-chiral aldehyde was coupled with an achiral enolate, yielding the desired aldol product as a 2 3 ratio of diastereomers 8 and 9. 

ROA is about one order of magnitude larger and one can measure vibrations which cannot be observed in solutions or neat liquids. But one has to be careful not to observe artifacts produced by the linear birefringence of the crystals. An interesting class of crystals are the cubic crystals of the sodium halogenates. These are composed of achiral subunits that form a... 

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