Chirality reaction-induced

The importance of the o-hydroxyl moiety of the 4-benzyl-shielding group of R,R-BOX/o-HOBn-Cu(OTf)2 complex was indicated when enantioselectivities were compared between the following two reactions. Thus, the enantioselectivity observed in the reaction of O-benzylhydroxylamine with l-crotonoyl-3-phenyl-2-imi-dazolidinone catalyzed by this catalyst was 85% ee, while that observed in a similar reaction catalyzed by J ,J -BOX/Bn.Cu(OTf)2 having no hydroxyl moiety was much lower (71% ee). In these reactions, the same mode of chirality was induced (Scheme 7.46). We believe the free hydroxyl groups can weakly coordinate to the copper(II) ion to hinder the free rotation of the benzyl-shielding substituent across the C(4)-CH2 bond. This conformational lock would either make the coordination of acceptor molecules to the metallic center of catalyst easy or increase the efficiency of chiral shielding of the coordinated acceptor molecules. 

Ligands for catalytic Mukaiyama aldol addition have primarily included bidentate chelates derived from optically active diols,26 diamines,27 amino acid derivatives,28 and tartrates.29 Enantioselective reactions induced by chiral Ti(IY) complex have proved to be one of the most powerful stereoselective transformations for synthetic chemists. The catalytic asymmetric aldol reaction introduced by Mukaiyama is discussed in Section 3.4.1. 

With 55 as the chiral auxiliary, good enantioselectivity can be obtained in reactions between aldehydes and esters. In the case of phenyl thioacetate, as depicted in Scheme 3-21, an aldol reaction induced by 55a or 55b can also give acceptable stereoselectivity. 

The use of tartrates as chiral auxiliaries in asymmetric reactions of allenyl bor-onic acid was first reported by Haruta et al.69 in 1982. However, it was not for several years that Roush et al.,70 after extensive study, achieved excellent results in the asymmetric aldol reactions induced by a new class of tartrate ester based allyl boronates. 

Jorgensen s group reported the aza Diels-Alder reactions in the presence of several chiral catalysts.52 They found that chiral bis(oxazoline) ligands 81, 83, 103, 104, and 105, which were effective in asymmetric oxo hetero Diels-Alder reactions, induced the aza Diels-Alder reaction of a-imino ester with Danishefsky s diene with only poor to moderate enantioselectivity. Selected results are listed in Scheme 5-40. 

Chiral //-hydroxylester 61 cannot be satisfactorily obtained through the reaction between a prochiral ketone and the enolate. It can, however, be synthesized via the chiral ligand-induced asymmetric Reformatsky reaction of ketones (Scheme 8-21). 

Pericas and coworkers173 studied the endo selective reactions of 1-alkoxy-l,3-butadienes and 1-alkoxy-l,3-octadienes with maleic anhydride. They found that the trans-2-phenyl-cyclohexan-l-ol and 3-exo-(neopentyloxy)isobornan-l-ol based chiral dienes induced the highest facial selectivities. The relative transition state energies for the formation of the different diastereomers were calculated using semi-empirical methods (AMI). 

SCHEME 6. Enantioselective Diels-Alder reaction induced by a chiral aluminium-containing Lewis acid88... 

Stereospecific nucleophilic substitution in chiral ion—dipole complexes. Chiral molecules can be discriminated in the Cl source of a CIMS instmment by specific ion-molecule reactions induced by chiral reagent gas. This method has been applied with success to distinguish between enantiomeric and diastereomeric forms of menthols ((lR,2S,5R)-(— )-14, (lS,2R,5S)-(+)-14, and (lR,2R,5S)-(—)-14 in Scheme 11) through the nucleophilic displacement of their hydroxyl group by (5)-2-amino-l-butanol Self-protonation of As... 

Scheme 77 illustrates some examples of the enantioselective reactions induced by chiral Ni catalysts (187). Atropisomeric binaphthyl or ter-naphthyl compounds can be obtained in high ee with these catalysts. The stereochemistry of the reactions may be determined kinetically by the diastereomeric diorganonickel(II) intermediates. These intermediates have a chiral propeller structure that undergoes little epimerization because of steric hindrance. 

Most asymmetric catalyses are termolecular reactions. To obtain a sufficient asymmetric bias, the reactant and/or substrate must be placed in a chiral environment induced by the catalyst. Perhaps one of the most reliable mechanisms for transmitting stereochemical information is the in situ formation of reactive intermediates in which the chiral catalyst and reactant are covalently bound. Under some conditions, the inter-... 

Asymmetric Pummerer rearrangement is a very attractive reaction as previously described. In particular, the reactions induced by SKA work well, and may be synthetically exploited in many cases. The results described here demonstrate that the stereoselective a-deprotonation of the sulfoxide is a prerequisite process for asymmetric induction in the Pummerer reaction. Since many kinds of synthetic and enzymatic preparative methods of optically pure sulfoxides have been developed, the present Pummerer-type reaction will be applicable to many other chiral sulfoxides with one a-substituent, chiral vinylsulfoxides and chiral co-carbamoylsulfox-ides, thus leading to enantioselective syntheses of many new bioactive compounds in the near future. 

Whether any or parts of the aforementioned models apply to the results on polarized secondary electron induced chiral reactions is difficult to say. The situation is considerably more complex since it involves reactive scattering and the substrate itself is magnetic. Hopefully, additional experimental and theoretical efforts will lead to a greater understanding of the processes involved. 

This class of photosensitized enantiodifferentiations includes a variety of irreversible photodecomposition and photorearrangement reactions induced by energy or electron transfer from chiral sensitizers to racemic substrates. 

Carter C, Fletcher S, Nelson A (2003) Towards phase-transfer catalysts with a chiral anion inducing asymmetry in the reactions of cations. Tetrahedron Asymmetry 14 1995-2004... 

Octahedral Co(III) complexes with ethylenediamine (en) are chiral. Working on related rotaxanes, Yamanari and Shimura studied the stereoselectivity of the stoppering reaction induced by the chirality of the cyclodextrin.186 When a-CDX and a racemic mixture of Co(III) precursor complexes were used, they observed that the (A,A) configuration at the stoppers was predominant. More... 

The next milestone appeared in the 1950s in the context of the development of asymmetric reactions. Various stereochemical reactions induced by facial discrimination of the carbonyl group have always been pivotal in this field. Cram s rule inspired an explosion of studies on diastereoselective reactions followed by enan-tioselective versions. The recent outstanding progress in the non-linear effect of chirality or asymmetric autocatalysis heavily relies on the carbonyl addition reactions. Thanks to these achievements, natural products chemistry has enjoyed extensive advancement in the synthesis of complex molecules. It is no exaggeration to say that we are now in a position to be able to make any molecules in as highly selective a manner as we want. 

In the past years, only a few reports have dealt with a chiral auxiliary induced diastereoselective aza MBH reaction. In 1994, Kiindig et al. explored the reaction of methyl acrylate and acrylonitrile with enantiopure planar chiral o substituted Cr(CO)3. Under the catalysis of DABCO, the corresponding aza MBH adducts were obtained in good yields. Removal of the metal provided chiral amines in high yields and enantiomeric excesses [12]. Later on, Aggarwal ef al. used enantiopure Nsulfinimines in the aza MBH reaction with methyl acrylate in the presence of 3 hydroxyquinuclidine (3 HQD) and Lewis add. The desired adducts, functionalized P sulfonated amino acid derivatives, were obtained with good diastereoselectivities (Scheme 13.4) [13]. 

First attempts to achieve silyl enol ethers ) are known since the late 195O s when hydrosilylation-type procedures were applied to a,d-unsaturated ketones is—i is) based upon the observation by Duffaut and Galas ) that simple ketones are able to add trichlorosilane (2) under UV irradiation. These hydrosilylation reactions were widely expanded and intensively studied ). a,/3-unsaturated ketones react via 1,4-addition ) to silyl enol ethers ) affecting only the conjugated double bonds. It is worth mentioning that the employment of chiral catalysts induces an asymmetrical reaction " ) (Scheme 24). 

When L-amino acids (with the exception of proline) catalyzed the formose reaction, an excess of D-glyceraldehyde formed. In contrast, without any special catalyst, an equal mixture of the d- and L-glyceraldehyde formed. The reaction conditions were prebiotic. Furthermore, addition of small amounts of water increased the enantiomeric excess to more than 90% (Breslow et al. 2010). An intriguing mechanism of chiral induction takes place when chiral L-amino acids (such as the ones found in the Murchison meteorite) were used as basic catalysts in the formose reaction induced about 10% ee of D-threose (Pizzarello and Weber 2004). Stereo-selective syntheses of pentose sugars occur under realistic prebiotic conditions when LL-dipeptides catalyzed the formose reaction (Pizzarello and Weber 2010). 

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