Asymmetric conformational flexibility

It is obvious, fipom some of the series of substituents in Table 7-11, that the steric constant reflects intuitive notions of group size, and Taft " pointed out close parallels with van der Waals radii. For asymmetrical groups a single van der Waals radius cannot be defined, and the situation is complicated by conformational flexibility. Nevertheless, empirical relationships can be established, and Kutter and Hansch gave Eq. (7-55),... 

Along another line of work in our group (S,S)-l,4-bis(dimethylamino)-2,3-dimethoxy butane (DDB), which had been used as cosolvent in asymmetric synthesis [113], was tested as a core moiety for a dendritic amine catalyst. The conformationally flexible DDB-core, which has been synthesized in five steps from diethyl tartrate was coupled with different branches to give dendritically expanded diamines 90-92 (molecular weight 3800 Da) [114] (Fig. 32). 

In chiral ligand 24, the two cyclopentane rings restrict the conformational flexibility of the nine-membered ring, and the four stereogenic centers in the backbone dictate the orientation of the four R-phenyl groups. Scheme 6 15 shows the application of Rh-24 in the asymmetric hydrogenation of dehy-droacylamino acids. 

A combination of axially chiral C2-symmetric binaphthol 85 with Zr(OBu-f)4 and TBHP represented a novel access to asymmetric BV oxidation. The system works under stoichiometric conditions and leads to the formation of the zirconium species 113, responsible for the activation of ketone and, likely, of the peroxide . As an example, the BV oxidation of 114 afforded the lactones 115 and 116 in a ratio 1 5 with 84% and 14% ee, respectively (equation 80). Asymmetric inductions are preserved also if one of the two chiral diols coordinated to zirconium is replaced with conformationally flexible biphenols . 

Asymmetric epoxidation, dihydroxylation, aminohydroxylation, and aziridination reactions have been reviewed.62 The use of the Sharpless asymmetric epoxidation method for the desymmetrization of mesa compounds has been reviewed.63 The conformational flexibility of nine-membered ring allylic alcohols results in transepoxide stereochemistry from syn epoxidation using VO(acac)2-hydroperoxide systems in which the hydroxyl group still controls the facial stereoselectivity.64 The stereoselectivity of side-chain epoxidation of a series of 22-hydroxy-A23-sterols with C(19) side-chains incorporating allylic alcohols has been investigated, using m-CPBA or /-BuOOH in the presence of VO(acac)2 or Mo(CO)6-65 The erythro-threo distributions of the products were determined and the effect of substituents on the three positions of the double bond (gem to the OH or cis or trans at the remote carbon) partially rationalized by molecular modelling. 

The three-step procedure described here, using inexpensive, commercially available starting materials and the chiral auxiliary SAMP, Illustrates the synthetic utility of the "SAMP-/RAMP-hydrazone method".18 It is remarkable that the classical electrophilic substitution of a conformationally flexible, acyclic ketone 1 (S)-4 occurs with virtually complete asymmetric... 

Being inspired by Maruoka s results with the C2-symmetric binaphthyl-derived quaternary ammonium salt [21], Lygo and colleagues designed a quaternary ammonium salt 23, comprising conformationally flexible biphenyl units and commercially available chiral secondary amines [22], A library of 40 quaternary ammonium salt was synthesized and evaluated for their catalytic efficiency in the asymmetric alkylation of... 

All of complexes 41a-d were significantly less enantioselective than complex 33. In the case of complexes 41a,b, this may be due to conformational flexibility within the five-membered ring. For complex 41c, both methyl groups are forced to adopt axial positions on the cyclohexane ring, and this may suggest that the unfavorable influence of axially located substituents extends beyond the ethylene diamine unit. Finally, in complex 41d the cyclohexane ring is in a boat rather than a chair conformation, and this or the phenyl substituents may account for the low level of asymmetric induction observed. 

Increasing the asymmetric unit from a mono- to a dinucleotide obviously increases the conformational flexibility of the nucleic acid structure. When the two nucleotides are only minor variants of one another, such as the differences between them are one of degree rather than kind, the resulting structure would still be of the same kind as that of the parent polymononucleotide helix. On the other hand, if the two nucleotides appear in quite different conformational domains, this could eventually lead to unusual secondary structures. For exampls, we have recently obtained from the synthetic DNA poly d(GC) poly d(GC) diffraction patterns which can be interpreted in terms of a left-handed helix... 

The four-membered analogues, phosphetanes, are also attractive for application as chiral ligands in asymmetric catalytic systems, especially because phosphetanes have a rigid structure, restrict the conformational flexibility, and can enhance the efficiency of the chiral transfer in the catalytic process. Hence, many efforts in the development of catalytic chemistry using chiral phosphetane have been performed <1998CCR755>. 

The purpose of this review is to examine the recent progress in the fi< of asymmetric photoreactions involving a chiral reactant, and to discuss the fact< that control the diastereodifferentiation. In the excited state as in the groi state, the level of asymmetric induction is strongly related to the conformation flexibility of the reactants. When carried out in the solid state, in a confin cavity of zeolites or in supramolecular scaffoldings, restrictions of the mobil occur, and photoreactions can become highly stereoselective. These aspects asymmetric induction will be covered in separate chapters, and this review v consider only diastereoselective photoreactions carried out in solution. ... 

Introduction. Chiral phosphines have played a crucial role in the development of catalytic asymmetric reactions. In particular, the coordination of a resolved, chiral phosphine to a transition metal center has been exploited to produce highly enan-tioselective catalysts for a variety of catalytic processes. Although there are many chiral monodentate and bidentate chiral phosphines available, (l/ ,2S, 4/ ,5S)-2,5-dimethyl-7-phenyl-7-phosphabicyclo[2.2.1 jheptane provides the advantage of a rigid-ified ring system that reduces the conformational flexibility present in many other phosphine ligands. 

Numerous reviews have appeared which are relevant to aromatic photochemistry and are also of general interest and application these include accounts of the chemistry of excited-state complexes by Davidson, the photochemical electron-transfer reactions of ethylenes and related compounds by Mattes and Farid, photosolvolyses and photoreactions involving carbo-cations by Cristol and Bindel, conformational flexibility and photochemistry by Wagner, asymmetric photochemistry in solution by Rau, and the photochemistry of iminium salts and related heteroaromatic systems by Mariano." The photochemistry of fragrance materials has been considered by Shibamoto and Mihara, and in their second article they deal with aromatic compounds and phototoxicity. ... 

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