Sense of chirality

At the bottom of Fig. 5, the fourth shell is shown from two directions. Note the spiral of atoms that are emphasized by a dark grey. This spiral can be wound around any of the five-fold axes from tip to tip. Similar spirals exist in the other layers, too. Each layer can be envisioned to consist of five such spirals of atoms. For each layer, there is also the stereoisomer with the opposite sense of chirality. 

The sense of chirality transfer from chiral titanium derivatives takes the expected... 

From the atomic to the macroscopic level chirality is a characteristic feature of biological systems and plays an important role in the interplay of structure and function. Originating from small chiral precursors complex macromolecules such as proteins or DNA have developed during evolution. On a supramolecular level chirality is expressed in molecular organization, e.g. in the secondary and tertiary structure of proteins, in membranes, cells or tissues. On a macroscopic level, it appears in the chirality of our hands or in the asymmetric arrangement of our organs, or in the helicity of snail shells. Nature usually displays a preference for one sense of chirality over the other. This leads to specific interactions called chiral recognition. 

In general, the same sense of chiral induction is obtained with either geometrical stereoisomer, which facilitates the use of (E/Z)-isomeric mixtures. An exception to this was recently reported by Heller and Bomer [56d]. Remarkably, hydrogenation of methyl (Z)-/3-acetylamino pentenoate with [(S,S)-Et-DuPhosRh (COD)]BF4 at 1 bar gave the (R)-enantiomer of product in 31% ee, whereas the same reaction at 30 bar resulted in an inversion of configuration and the (S)-product in 77% ee. 

Various substituted unsaturated acylphosphonates participate in highly dias-tereoselective and enantioselective cycloadditions with vinyl ethers, Eqs. 177 and 178. It is intriguing to note that catalysts [(.V,.Y)-f-Bu-box]Cu (OTf)2 (269c) and [(.V,.S )-Ph-box]Cu (OTf>2 (269d) possessing the same sense of chirality afford opposite antipodes of the cycloadduct in comparable selectivities. Cyclopentadiene was found to react with acylphosphonates to give a mixture of the normal Diels-Alder adduct and the inverse electron demand hetero-Diels-Alder adduct (35 65), Eq. 179. This result may be contrasted with crotonylimide, which furnishes the normal demand Diels-Alder adduct exclusively. 

Liu S, Pestano JPC, Wolf C (2008) Enantioselective fluorescence sensing of chiral alpha-amino alcohols. J Org Chem 73 4267-4270... 

On examining this compound (Sect. IV-B), we found an enantiomorphic difference between the ring ligands of C(l). It can be used to derive a priority order for the proximal carbon atoms of C(l). By provisionally assigning them different priorities one can determine which preference order induces the R, and which the S, configuration at C(4). This induced sense of chirality is mere... 

Cahn et al. (4) thought it beneficial to have two methods available for specifying the sense of chirality of the biaryls, as examples of either axial chirality or conformational chirality. As we have merged both into a single class—the line of stereoisomerism—any justification for two modes of description has vanished. Except for the modifications envisaged earlier, the procedures of Section 4 of the Sequence Rule (4) seem well suited to describe the isomerism of the line. Thus the terms MIP would become the general descriptors of the chiral line of torsion. 

Some impressive levels of asymmetric induction were noted in the following examples (eq. [68], Table 30). From the cases studied, it is readily apparent that the steric requirements of the Rj Ugand at the resident asymmetric center in aldimine chelate 90 exerts a profound effect on the degree of asymmetric induction. Earher studies (80) offer a precedent for the suggestion that the aldolate complexes 93 and 94 are the penultimate precursors to the major and minor diastereomeric /3-lactams 91 and 92, respectively (Scheme 17). The pericyclic transition state 95 (M = TiL ) could well explain the sense of chirality transfer (82). 

The chiral A/ -propionyl-2-oxazolidones (32 and 38) are also useful chiral auxiliaries in the enantioselective a-alkylation of carbonyl compounds, and it is interesting to observe that the sense of chirality transfer in the lithium enolate alkylation is opposite to that observed in the aldol condensation with boron enolates. Thus, whereas the lithium enolate of 37 (see Scheme 9.13) reacts with benzyl bromide to give predominantly the (2/ )-isomer 43a (ratio 43a 43b = 99.2 0.8), the dibutylboron enolate reacts with benzaldehyde to give the (3R, 25) aldol 44a (ratio 44a 44b = 99.7 0.3). The resultant (2R) and (25)-3-phenylpropionic acid derivatives obtained from the hydrolysis of the corresponding oxazolidinones indicated the compounds to be optically pure substances. 

An interesting example exists of variation of diastereoselectivity, due both to the nature and sense of chirality of the electrophile, and to the configuration of the 2-pyrrolidinemethanol auxiliary6. Thus, the use of epoxide 16 as electrophile leads to an unexpected reversal of the diastereoselectivity relative to that observed when the corresponding O-protected iodohydrin 10 is employed. Both of the electrophiles are chiral and therefore reaction of each with the enantiomers of 1-(l-oxopropyl)-2-pyrrolidinemethanol leads to different diastereoselectivities due to the fact that there is a matched and a mismatched pair of reactions. 

Yields, melting points, and specific rotations of nitrodiol monoacetates which were prepared by procedure C described above by using PLE-powder2 instead of purified enzyme. Entries 1 and 3 in Table 1 refer to runs following the above procedure, for all other cases the self made crude PLE-powder was used. The configuration and the sense of chirality of the products of entries 1,3 and 4 were determined by x-ray crystal structure analysis of the camphanic esters, those of the other are inferred by analogy and by NMR comparison. The open chain compounds (entries 1 and 2) were obtained using TES buffer at pH 6.5... 

Fig. 4.76 A dendrimer with flat dendrons with a propeller-like arrangement, in which correlated intramolecular conformational motion leads to inversion of the sense of chirality Owing to a partial steric intermeshing, when one of the dendrons is inverted the other three are also constrained to un-...

Both the cis- and the trans-disubstituted spiranes resulted, in different ratios, depending on the reaction conditions. Clearly, the trans spiranes are chiral. The first conjugate addition to the Michael acceptors 75a-c is intermolecular in nature and defines the sense of chirality at the first chiral center. Subsequent intramolecular ring closure to the spiranes 76 defines the cis or trans configuration of the product. When cyclohexane-1,3-dione (74a) was reacted with dibenzalacetone (75a) in the presence of ca 5 mol% (—)-quinine (3a, Scheme 4.3), a 2.5 1 trans/cis mixture resulted, with the trans isomer 76 having optical purity of ca 30% (Scheme 4.37) [61] (the absolute configuration of the predominant enantiomer was not assigned). 

The design of the peptide implies that interaction of the catalyst with its substrate relies heavily on hydrogen bonding. Initial studies indeed revealed that, in particular, N-acyl amino alcohols such as 25 and ent-25 were efficiently differentiated whereas both enantiomers of l-(l-naphthyl)ethanol were acetylated at identical rates [28]. Catalyst 23b, shown in Scheme 12.12, was the most efficient from a series of ten peptides. For best performance, proper matching of the sense of chirality of all three chiral amino acids is necessary, and the type of amino acid present at the carbon terminus enables further tuning (for example, L-Phe was found to be better than, e.g., L-Val, selectivity factor 21) [29]. 

Figure 4. A translating spinning cylinder. The polar vector in the rotation-translation (screw displacement) corresponds to the direction of translation and the axial vector to the direction of spin. Time reversal (7) does not change the sense of chirality of homomorphous systems (a) and (b) in terms of the helicity generated by the product of the two vectors, (a) and (b) are both right-handed. Space inversion (P) of (a) yields a left-handed system (c), the enantiomorph of (a). Time reversal of (a), followed by rotation of (b) by 180° (Rn) about an axis perpendicular to the cylindrical axis, yields (d), a homomorph of (a). Space inversion of (d) brings us back to (c).

To visualize an object with Dm symmetry, imagine a cylinder whose outside is covered with n slanted striations, as illustrated at the top of Figure 8. The two constructions shown (D symmetry) are enantiomorphs whose sense of chirality is related to the way in which the striations are slanted. As n approaches infinity, the symmetry of the constructions approaches Z) in the limit, infinitely many C2 axes are embedded in a plane perpendicular to the C axis. This is the symmetry of a stationary cylinder undergoing a twisting motion, as indicated by the arrows on the cylinders at the bottom of Figure 8, and of an axial tensor of the second rank.41 It is also the helical symmetry of a nonpolar object undergoing a screw displacement, that is, of an object whose enantiomorphism and sense of chirality are T-invariant. 

The chirality of objects such as scalene triangles and oriented circles in R2 (Figure 1) and helices in R3 (Figure 3) is a property shared by both enantiomorphs the difference between them is their sense of chirality. In what follows, we shall for simplicity use the term configuration to stand for sense of chirality. Two enantiomorphs are thus said to have opposite configurations. 

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