Chiral groups

Secondary amines having one oi two chiral groups attached to the nitrogen atom are prepared from boronic esters by their conversion into alkyldichlotobotanes, followed by treatment with organic azides (518). The second chiral group can be derived from an optically active azide. 

As seen in Table 2, many of the chiral tubules with d = 1 have large values for M for example, for the (6,5) tubule, M = 149, while for the (7,4) tubule, M = 17. Thus, many 2tt rotations around the tubule axis are needed in some cases to reach a lattice point of the ID lattice. A more detailed discussion of the symmetry properties of the non-symmorphic chiral groups is given elsewhere in this volume[8. ... 

The mechanistic analogy to the Streckcr synthesis becomes obvious in the addition of the isocyanide to the imine to produce the a-amino nitrilium intermediate. Since all four components are involved in this step, it might be expected that every chiral component (chiral groups R1, R2, R3, R4) contributes to diastereofacial differentiation in the nucleophilic attack on the imine. However, in peptide syntheses by four-component condensation5, the chiral isocyanide or a chiral carboxylic acid component has only limited influence on the diastereoselectivity of the a-amino amide formation5. 

Induction of asymmetry into the /J-lactam-forming process was inefficient with acyclic imines having chiral groups on the nitrogen [19] but efficient with rigid, cyclic chiral imines (Table 3). One of these was used as a chiral template to produce highly functionalized quaternary systems (Eq. 5) [34]. 

There are two basic procedures that have been successfully used for the separation of isomers. The first is to add a chiral agent to the mobile phase such that it is adsorbed, for example, on the surface of a reverse phase, producing a chirally active surface. This approach has been discussed on page (38) in chapter 2. The alternative is to employ a stationary phase that has been produced with chiral groups bonded to the surface. 

In addition to the substituents listed in Scheme 1, chiral groups may be introduced and unsymmetrically substituted amidinate anions are also possible. The amidinate anions may also contain additional functional groups, or two such anions can be linked with or without a suitable spacer unit. Yet another variety comprises the amidinate ligands being part of an organic ring system. All these aspects will be covered in the present review. 

It is often possible to convert an achiral compound to a chiral compound by (1) addition of a chiral group (2) running an asymmetric synthesis, and (3) cleavage of the original chiral group. An example is conversion of the achiral 2-pentanone to the chiral 4-methyl-3-heptanone (50). In this case, >99% of the product was the (5) enantiomer. Compound 49 is called a chiral auxiliary because it is used to induce asymmetry and then is removed. 

Zareba et al. [165] described the crystal structure of the chiral 4-(l-methyl-heptyloxycarbonyl)-phenyl 4-heptyloxytolane-4 -carboxylate (C7-tolane) which shows monotropic antiferroelectric and ferroelectric phases. The single-crystal X-ray analysis of this compound shows that the crystal has a smectic-like layer structure composed of largely bent molecules where the chain of the chiral group is almost perpendicular (86°) to the core moiety. Within the layers, the molecules are tilted. The central tolane group of the molecule is roughly planar. 

Among /1-thiosubstituted organophosphorus compounds bearing chiral groups, phosphono methyl thiazolines (Sect. 2.2.1, Scheme 8) and o-sulfanyl aryl phos-phonamides or phosphinoxides (Sect. 3.3, schemes 20 and 21) have already been mentioned. As a complement to this, some recent synthesis of non racemic /1-sulfinyl phosphines and phosphonates and thiazolidinyl phosphonates are reported below. Moreover, some chiral )8-thio-substituted phosphines have been used as metal ligands in asymmetric catalysis and are listed in Sect. 5.3. 

Mixtures of a nematic liquid crystal (LC or LC ) with small quantities of gold nanoparticles coated with alkylthiolates (<5 wt%) including an alkylthiolate functionalized with a chiral group have been studied (Figure 8.29) [72]. All mixtures show nematic mesophases with transition temperatures and phase stability very similar to those oftheliquid crystal precursors LC or LC. The introduction ofachiral center into the mixtures (mixtures of Au ) produce chiral nematic mesophases. A similar result is obtained in mixtures of Au and LC doped with the chiral dopant (s)-Naproxen. 

The classical method, which was followed to prepare the first example of an optically pure chiral organotin compound, is characterized by the use of a auxiliary chiral group necessary to convert the racemic mixture of enantiomers into a mixture of diastereomers which are then separated by a suitable physical method and converted back into the separated enantiomers by splitting off the chiral auxiliary group. This last step is sometimes difficult to achieve 34 ). 

Separation of Diastereomeric Organotin Compounds Followed by the Cleavage of the Auxiliary Chiral Group... 

Table 3. Synthesis of optically active organotin compounds RR R"SnL from one of the diastereomeric compounds bearing the auxiliary chiral group R RR R SnLR -> RR R Sn—L ( optically pure optical purity unknown)... 

H, 13C, 117Sn and 119Sn NMR and polarimetry. Coordination of the tin atom to the amino group in the y-position stabilizes molecules of general structure 35. The chiral group R of compound 36 (see below this table), for example, induces chirality at the Sn centre. A slow racemization of the Sn centre takes place in solution, especially when the y-amino group is attached to a flexible substituent. See also next entry. ... 

Dipolarophiles D4. 1,3-Dipolar cycloaddition between acrylonitrile (D4) and chiral nonracemic nitrones is a key step in an efficient synthetic route to isoxa-zolidinyl analogs of thiazofurin (540) (Scheme 2.253). Opposite diastereofacial induction was observed when the chiral group was placed at either the carbon or the nitrogen atom of the nitrone function (753). 

Control of Main-Chain Helicity by Position of Remote Chiral Group... 

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