L-abrine

In contrast, investigation of the effect of ligands on the endo-exo selectivity of the Diels-Alder reaction of 3.8c with 3.9 demonstrated that this selectivity is not significantly influenced by the presence of ligands. The effects of ethylenediamine, 2,2 -bipyridine, 1,10-phenanthroline, glycine, L-tryptophan and L-abrine have been studied. The endo-exo ratio observed for the copper(II)-catalysed reaction in the presence of these ligands never deviated more than 2% from the endo-exo ratio of 93-7 obtained for catalysis by copper aquo ion. 

Table 3.4. Enantiomeric excess and reaction times of the copper(L-abrine)-catalysed Diels-Alder reaction of3.8cwith3.9in different solvents at 0 C.

Figure 3.4. TJV-vis absorption spectra of 3.10c in water and in water containing 3.0 mM of Cu (glycine) complex, 3.0 mM of Cu(N-methyl-Ftyrosine) and 3.0 mM of Cu(L-abrine).

The influence of a large number of oc-amino acids on the values of and k at have been determined. These a-amino acids included glycine, L-valine, L-leucine, L-phenylalanine, L-tyrosine, L-tryptophan, NOrmethyl-L-tryptophan (L-abrine), N-methyl-L-tyrosine, N,N-dimethyl-L-tyrosine and p -me thoxy-N-me thyl -L -phenyl al anine. 

We have investigated the effect of solvents on the enantioselectivity. It turned out that water (74% ee) favours the enantioselectivity of the Cu (L-abrine) catalysed Diels-Alder reaction between Ic and 2 as compared to chloroform (44% ee), ethanol (39% ee), THF (24% ee) and acetonitrile (17% ee). The... 

The complex obtained from commercially available chiral a-amino acids (AA) with Cu + ion induces asymmetry in the Diels-Alder reaction of 31 (R = H) with 32. By using 10% Cu(II)-AA (AA = L-abrine) the cycloaddition occurs e/iJo-stereoselectively in 48 h at 0°C with high yield and with acceptable enantioselectivity ee = 1A%). This is the first example of enantioselective Lewis-acid catalysis of an organic reaction in water [9b]. 

Copper-complexes prepared with other type of N-chelating ligands have been also prepared and evaluated as catalysts for the Diels-Alder reaction. Eng-berts et al. [103] studied enantioselective Diels-Alder reaction of 3-phenyl-l-(2-pyridyl)-2-propen-l-one with cyclopentadiene in water (Scheme 39). By using coordinating chiral, commercially available a-amino-adds and their derivatives with copper salts as catalysts, they obtained the desired product with yields generally exceeding 90%. With L-abrine (72 in Scheme 39) as chiral moiety, an enantiomeric excess of 74% could be achieved. Moreover, the catalyst solution was reused with no loss of enantioselectivity. 

TABLE 7.3 Solvent Effect on Enantioselectivity Catalysis by Copper (L-abrine) as the Lewis Acid... 

Lewis-acid catalysis (Table 7.2) was observed for Cu, Ni " , Co, and Zn " ", with Cu being the best catalyst (both strongest binding and most efficient in accelerating the reaction with CPD). Varying the solvent (Table 7.3) in the catalysis by Cu ions with L-abrine as a ligand illustrates a large increase in enantioselectivity in water. So water promotes enantioselectivity. ... 

Abrus precatorius L. China L-abrine, precatorine, squalene, hypaphorine, trigonelline, cycloarterenol, 5- 3-cholanic acid.33 Antiemetic, an expectorant, parasiticide. 

In water Co(II), Ni(II), Zn(II), and, particularly, Cu(II) nitrates accelerate the reaction shown in Sch. 42 by a factor of up to 79 000 [170]. When the reaction is performed with the copper catalyst and in the presence of the a-amino acid ligands L-tyr-osine or L-abrine product ee of 74 % are obtained. Other ligands result in lower asymmetric induction (0-67 % ee) [171]. 

L = bis(hydroxymethyldihydroxyoxazolyl)pyridine = 1,4-bis(diphenylphosphanyl)butane L = (S,S)-2,6-bis-(4-isopropyl-2-oxazolin-2yl)pyridine PS-PEG = polystyrene-poly(ethyleneglycol) L-abrine = N-(a)-methyl-L-... 

Arene-arene interactions between the pyridine part of the substrate and L-abrine, which are enforced in water through the hydrophobic effect, also havebenefidal effects on reaction rate and enantioselectivity (Table 3.2). 

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