Hydrogen bonding agent, chiral

Recent interest in the use of N-unsubstituted 2-quinolones stems from the fact, that they coordinate effectively to chiral lactam-based templates via two hydrogen bonds. The prototypical template to be used in photochemical reactions is compound 115, which can be readily prepared from Kemp s triacid [108]. The template is transparent at a wavelength X > 290 nm, and can be nicely used in stoichiometric amounts for enantioselective photochemical and radical reactions [109]. Conditions which favor hydrogen bonding (nonpolar solvent, low temperature) are required to achieve an efficient association of a given substrate. The intramolecular [2 + 2]-photocycloaddition of 4-alkylquinolone 114 proceeded in the presence of 115 with excellent enantioselectivity, and delivered product 116 as the exclusive stereoisomer (Scheme 6.41) [110]. Application of the enantiomer ent-115 ofcomplexing agent 115 to the reaction 111 —> 112 depicted in Scheme 6.40 enabled enantioselective access to (+ )-meloscine [111]. 

Taking into account the basic considerations outlined in the previous section, we reasoned that a chiral complexing agent, which needs to be applicable in photochemistry, requires a general binding motif. In addition, it appeared necessary to employ directed interactions in such a template rather than an unspecific complexation as encountered in chiral solvents or larger supramolecular entities. Hydrogen bonds appealed... 

Y. Dobashi and S. Hara, Extended scope of chiral recognition applying hydrogen bond association in nonaqueous media (R,R)-V,V -disopropyltartaramide (DIPTA) as a widely applicable resolving agent, J. Am. Chem. Soc. 107 (1985), 3406. 

Chiral Solvating Agent NEA is an effective chiral solvating agent for NMR determination of enantiomeric purity. The combination of enantiomerically pure NEA (3-5 mol excess) and racemic solute causes the NMR spectra of the diastereomerically solvated enantiomers to differ. Since NEA is an efficient hydrogen-bond acceptor, it solvates better if the solute is a hydrogen-bond donor. (R)-(+)-NEA has been used to determine the enantiomeric purity of a variety of substrates. ... 

Chiral stationary phases have received the most attention. Here, a chiral agent is immobilized on the surface of a solid support. Several different modes of interaction can occur between the chiral resolving agent and the solute." In one type, the interactions are due to attractive forces such as those between ir bonds, hydrogen bonds, or dipoles. In another type, the solute can fit into chiral cavities in the stationary phase to form inclusion complexes. No matter what the mode, the ability to separate these very closely related compounds is of extreme importance in many fields. Figure 32-16 shows the separation of a racemic mixture of an ester on a chiral stationary phase. Note the excellent resolution of the R and S enantiomers. 

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