Chiral templates, conjugate

Several conjugate addition reactions have been used for the synthesis of amino acids, as seen in chapter three. It is easy to prepare conjugated esters and using chiral auxiliary or a chiral template, conjugate addition can lead to amino acids with high diastereoselectivity and enantioselectivity. 

Similiar to albomycin and ferrichromes, the linear tripeptide of A -hydroxy-A -acetyl-ornithine can be used as the simplest chiral template for artificial siderophores. It can be used alone as a tripeptide like for 80, can be conjugated to a hydrophobic fatty acid 81132 pj. p j.j longer peptide 82, 83, and 85. ... 

Any of the wide variety of chiral molecules available from nature or other sources can, in principle, be used in a synthesis. Scheme 10.20 shows Srikrishna s synthesis of (-)-neopupukean-lO-one (237) from the chiral template (l )-carvone (233). Formation of the enolate anion at C6 and 1,4-addition to the conjugated... 

The success in diastereoselective Lewis acid-mediated conjugate radical additions using chiral oxazolidinones led to the development of enantioselective variants. The achiral template selection was based on literature precedents for rotaraer control s-cis vs s-trans) and the requirement for a bidentate Lewis acid chiral ligand combination for obtaining selectivity (Eq. (13.51)). 

Table 8.2 shows the retention behavior of two enantiomers of Phe-Phe. Although CyD itself is a chiral compound, the polymer/silica-gel conjugate prepared without template (Non-imp) discriminates between these enantiomers poorly. When L-Phe-L-Phe is used as template, however, L-Phe-L-Phe is retained more strongly than D-Phe-D-Phe. With the conjugate prepared in the presence of D-Phe-D-Phe, D-Phe-D-Phe is retained much more strongly than its enantiomer. 

There are various methods to synthesize polymer nanostructures, i.e., template synthesis, chiral reactions, self-assembly, interfacial polymerization and electrospinning. Recent developments in conducting polymer nanotubes and nanofibers were summarized by Long et al. Different preparation methods, physical properties, and potential applications of one-dimensional nanostructures of conjugated polyaniline (PANI), pol5 3nrole (PPy) and poly (3, 4-ethylenediox3d hiophene) (PEDOT) were discussed. 

Chiral smectic C materials have mostly been designed to have a high Pg and a low viscosity to enable their use in ferroelectric devices. The stracture of chiral smectic C materials is sirmmarised by a template structure in Figure 6.4 which also shows the types of moieties and their combination. The first S material used to establish the viability of a device was DOBAMBC (7a) in 1974. The use of ester and ether moieties provides the necessary lateral dipoles for molecular tilting and the use of the conjugated cinnamate... 

Lewis acid promoted reactions of silicon enolates, /.e., silyl enol ethers and ketene silyl acetals with various electrophiles have yielded a wealth of novel and selective synthetic methods. This combination of reagents has been used in the past to perform such reactions as aldol-condensations with aldehydes and acetals, imine-condensations, conjugate additions to a,P-enones, alkylations, electrophilic aminations, and Diels-Alder/cyclocondensations. Our own interest in this field has involved the use of titanium tetrachloride to promote the reaction of ketene silyl acetals with non-activated imines as an efficient route to P-lactams. This reaction has been applied to the asymmetric synthesis of P-lactams via a chiral imine-titanium tetrachloride template. We have also found that both ketene silyl acetals and vinylketene silyl acetals oxidativelly dimerize or cross-couple, in the presence of titanium tetrachloride to conveniently yield various diesters . Our present study concerns reactions of vinylketene silyl acetals with non-activated imines and vinylimines promoted by titanium and zirconium tetrachlorides. 

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