Enantioselection sense

Asymmetric conjugate addition of dialkyl or diaryl zincs for the formation of all carbon quaternary chiral centres was demonstrated by the combination of the chiral 123 and Cu(OTf)2-C H (2.5 mol% each component). Yields of 94-98% and ee of up to 93% were observed in some cases. Interestingly, the reactions with dialkyl zincs proceed in the opposite enantioselective sense to the ones with diaryl zincs, which has been rationalised by coordination of the opposite enantiofaces of the prochiral enone in the alkyl- and aryl-cuprate intermediates, which precedes the C-C bond formation, and determines the configuration of the product. The copper enolate intermediates can also be trapped by TMS triflate or triflic anhydride giving directly the versatile chiral enolsilanes or enoltriflates that can be used in further transformations (Scheme 2.30) [110],... 

Marchelli used the copper(II) complex of histamine-functionalized P-cy-clodextrin for chiral recognition and separation of amino acids [27]. The best results were obtained for aromatic amino acids (Trp). Enantioselective sensing of amino acids by copper(II) complexes of phenylalanine-based fluorescent P-cyclodextrin has been recently published by the same author [28, 29]. The host containing a metal-binding site and a dansyl fluorophore was shown to form copper(II) complexes with fluorescence quenching. Addition of d- or L-amino acids induced a switch on of the fluorescence, which was enantioselective for Pro, Phe, and Trp. This effect was used for the determination of the optical purity of proline. 

So far aziridination reactions have, in some ways, had more in common with cyclopropanation reactions (see Section 9.1) than with epoxidation reactions. Nevertheless, the aziridination reaction is more synthetically akin to epoxidation, and on that basis, is included in the present chapter. Aziridines maybe prepared by nitrene transfer to alkenes or by carbene transfer to imines and both approaches have been performed in an enantioselective sense using enantiomerically pure metal-based catalysts. 

Prasad, B., Kumar, D., Madhuri, R., and Tiwari, M. P. (2011). Metal ion mediated imprinting for electrochemical enantioselective sensing of 1-histidine at trace level. Biosens. Bioelectron. 28,117-126. 

Fig. 2 Principal mechanisms of enantioselective sensing involving quenching processes...

Fig. 3 (a) Fluorescent cyclodextrins used for enantioselective sensing of amino adds (b) enhancement of fluorescence by addition of proline to Cu((S)-l) (reprinted with permission from Elsevier from [87]) (c) quenching of fluorescence by addition of Cu(AA>2 to the free cyclodextrin (S)-l in box A the signal is strongly dependent on the concentration of Cu(AA)2 and in box B it is mainly dependent on the stereochemistry of the analyte, (reprinted from [90] with permission by the Royal Society of Chemistry)... 

This approach is particularly interesting because it requires very low concentrations of the sensor, and it allows one to perform enantioselective sensing in water on very low quantity of unmodified amino acids, which are insoluble in most organic solvents used in the vast majority of sensory systems described in the literature. 

The combination of enantioselective selectors with fluorophore residues allowed one to use other preorganized scaffolds for enantioselective sensing. For example, using diamine-9,9-dimethylxanthenes modified with dansyl-leucine (26 Fig. 7) it was possible to obtain the enantioselective quenching of carbamoyl lactic acid, which was attributed to a PET mechanism [83]. 

Fig. 12 Enantioselective sensing by conformational changes in the sensors, (a) Mechanism of sensing in fluorophore-bearing cyclodextrins by interaction with a guest (b) water soluble imidazolium containing BINOL macrocycles 38, 39 (c) tris- and bis(oxazolmyl)phtaiols (40, 41) (d) l,8-bis(9,9 -diacridyl)naphthalene derivative 42 (e) Stem-Vobner plot of 42 upon interaction with enantiomers of a-halo acids (left 2-chloropropanoic acid, right 2-bromo-3-methylbutanoic acid), showing nonlinear response (from [85] reproduced by permission of The Royal Society of Chemistry)...

Fig. 14 Examples of enantioselective sensors based on excimer formation, (a) Dimeric pyrene-containing y-cyclodextrins and their mechanism of enantioselective sensing upon interaction with a guest molecule (b) cleft-like sensors based on excimer modulation (c) naphthol containing...

Mei X, Wolf C (2004) Enantioselective sensing of chiral carboxylic acids. J Am Chem Soc 126 14736-14737... 

Corradini R, Paganuzzi C, Marchelli R et al (2007) Fluorescent cyclodextrins bearing metal binding sites and their use for chemo- and enantioselective sensing of amino acid derivatives. J Incl Phenom Macrocycl Chem 57 625-630... 

Costero AM, Llaosa U, Gil S et al (2009) Enantioselective sensing of dicarboxylates. Influence of the stoichiranetry of the complexes on the sensing mechanism. Tetrahedron... 

Pagliari S, Corradini R, Galavema G et al (2000) Enantioselective sensing of amino acids by copper(II) complexes of phenylalanine-based fluorescent p-cyclodextrins. Tetrahedron Lett 41 3691-3695... 

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