Backbone achiral

Subsequently, these catalysts were evaluated in the enantioselective desymmetri-sation of achiral trienes, and three distinct trends in catalyst selectivity were found. Firstly, catalysts 56a-b with two phenyl moieties on the backbone of the A -heterocycle exhibited higher enantioselectivity than those with a fused cyclohexyl group as the backbone 55a-b. Secondly, mono-ort/io-substituted aryl side chains induced greater enantioselectivity than symmetrical mesityl wing tips. Thirdly, changing the halide ligands from Cl to I" increased the enantioselectivity. As a result, catalyst 56b turned out to be the most effective. For example, 56b in the presence of Nal was able to promote the desymmetrisation of 57 to give chiral dihydrofuran 58 in up to 82% conversion and 90% ee (Scheme 3.3). 

In 2008, Grisi et al. reported three ruthenium complexes 65-67 bearing chiral, symmetrical monodentate NHC ligands with two iV-(S)-phenylethyl side chains [74] (Fig. 3.26). Three different types of backbones were incorporated into the AT-heterocyclic moiety of the ligands. When achiral triene 57 was treated with catalysts 65-67 under identical reaction conditions, a dramatic difference was observed. As expected, the absence of backbone chirality in complex 65 makes it completely inefficient for inducing enantioselectivity in the formation of 58. Similarly, the mismatched chiral backbone framework of complex 66 was not able to promote asymmetric RCM of 57. In contrast, appreciable albeit low selectivity (33% ee) was observed when the backbone possessed anti stereochemistry. 

Although many questions are still open, peptide nucleic acids are easier to synthesize via simple reaction routes than is natural RNA. The PNAs have another important advantage they are achiral and uncharged, i.e., they contain no chiral centres in the polymeric backbone (see Sect. 9.4). Unfortunately, however, they do not fulfil all the necessary conditions for molecular information storage and transfer. Thus, the search for other possible candidates for a pre-RNA world continues. 

In recent years, there is no doubt that BINOL is one of the most extensively studied motifs. Incorporating a chiral binol unit into the chiral or achiral backbone constitutes a straightforward way in which to generate new chiral ligands [109]. 

Thus, an achiral promoter for the Cr(0)-mediated [6-1-2] cycloaddition was recently disclosed by Rigby. Here, Cr(0) is attached to the polymeric backbone via immobilized triphenylphosphine [111]. 

Miller and co-workers have taken a totally different approach to design an efficient catalyst for enantioselective acylation. Their strategy relied on the use of a pep-tide-based backbone incorporating a 3-(l-imidazolyl)-(5)-alanine unit as the catalytic core. Upon treatment with an achiral acyl source these biomimetic enantioselective acyl transfer catalysts allow the formation of an acyl imidazolium ion in proximity to the chiral environment generated by the folding of the peptide [3, 159-174]. 

Connon and co-workers synthesized a small library of novel axially chiral binaphthyl-derived bis(thio)ureas 152-165 and elucidated the influence of the steric and electronic characteristics of both the chiral backbone and the achiral N-aryl(alkyl) substituents on catalyst efficiency and stereodifferentiation in the FC type additions of indole and N-methylindole to nitroalkenes (Figure 6.50) [315]. 

The most stable elements of secondary structure of peptides and proteins are turns, helices, and extended conformations. Within each of these 3D-structures the most commonly found representatives are (3-turns,a-helices, and antiparallel (3-sheet conformations, respectively. y-TurnsJ5 310-helices, poly(Pro) helices, and (3-sheet conformations with a parallel strand arrangement have also been observed, although less frequently. Among the many types of (3-turns classified, type-I, type-II, and type-VI are the most usual, all being stabilized by an intramolecular i <— i+3 (backbone)C=0 -H—N(backbone) H-bond and characterized by either a tram (type-I and type-II) or a cis (type-VI) conformation about the internal peptide bond. In the type-I (3-turn a helical i+1 residue and a quasi-helical 1+2 residue are found, whereas in the type-II (3-turn the i+1 residue is semi-extended and the 1+2 residue is also quasi-helical but left-handed. This latter corner position may be easily occupied by the achiral Gly or a D-amino acid residue. 

Asymmetric hydrogenation of alkenes is efficiently catalysed by rhodium complexes with chiral diphosphite and diphosphoramidite ligands derived from BINOL or diphenylprolinol. Choice of a proper achiral backbone is crucial.341 Highly enantioselective hydrogenation of A-protected indoles was successfully achieved by use of the rhodium catalyst generated in situ from [Rh(nbd)2]SbF6 (nbd = norborna-2,5-diene)... 

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