Selection enantioselectivity

L2909>. An organocatalytic addition of 2-trimethylsilyloxyfuran to aldehydes using 10 mol% of l,3-bis(3-(trifluoromethyl)phenyl)urea provided adducts with modest threo selectivity <06TL8507>. A syn-selective, enantioselective, organocatalytic vinylogous Mukaiyama-Michael addition of 2-trimethylsilyloxyfuran to (E)-3-... 

Although this chapter describes advances in this field, it does not exclusively select enantioselective transformations. Although some of the reactions presented here feature poor catalytic turnover, and require stoichiometric (or even excess) quantities of catalyst, we chose to include such examples because of the scope of the concept, the possible recovery of the catalyst, and the hope that further optimization would allow the use of these systems in substoichiometric catalyst loadings. 

Hydrogenation catalysts. A number of new optically active phosphine ligands have been developed for selective enantioselective reduction of N=C bonds of amino acid precursors, including DPCB(l)1, the methylene homolog (2)2 of dipamp, and the novel ligand 3, in which the source of chirality is a Re atom.5 A ligand 4 has been used as a Rh(I) complex for hydrosilylation.4... 

Interestingly, the chiral titanium complex derived from 6-Br-BINOL affords higher cis selectivity, enantioselectivity, and catalytic activity than the parent BINOL-Ti catalyst in the hetero Diels-Alder reactions of 1-methoxydienes with glyoxylate, but not with bromoacrolein (Sch. 49) [130]. 

These results are consistent with the chelated transition states depicted in Scheme 18. Steric interactions between the substituent and the carboxamide favor (AC) for ( )-allylic ethers. The R -substi-tuent of a (Z)-allylic ether, though less affected by this interaction, still experiences a certain degree of steric strain in the anti transition state (AB) thus diminishing anti selectivity. Enantioselectivity is controlled by the substituents R and R on the pyrrolidine ring. As pictured in Scheme 18, bonding occurs preferentially on the face of the enolate anti to R. For the diastereomeric secondary allylic ethers (Table 21, entries 8) transition state (AB) represents the matched arrangement for R = H and R = alkyl, whereas (AC) is matched for R = alkyl and R = H. The former arrangement would lead to an ( )-pro-duct and the latter to a (Z)-product. 

Scheme 16. Exo- and en do-selective enantioselective Diels-Alderase antibodies...

Nitrilase, nitrile hydratase Nitrile hydrolysis Chemo- and regio-selectivity Enantioselectivity, no commercial enzyme... 

Because free-enzyme porcine pancreatic lipase had much lower activity in SCCO2 than in organic solvents, the authors decided instead to test immobilized lipases. Macroporous resin-supported lipase from M. miehei had fair activity but poor enantioselectivity. Porcine pancreatic lipase immobilized on supports had the greatest activity if the support was highly hydrophilic Sephadex G-25 and Bio-gel P6 were selected. Enantioselectivity was 83% for the (5)-gycidyl butyrate at 25-30% conversion, comparable to results in organic solvents. Several chiral alcohols were studied by Cernia et al. (80) as substrates for kinetic resolution by esterification catalyzed by silica-supported lipase from Pseudomonas sp. [Eq. (5)] ... 

The selectivity, enantioselectivity, and sensitivity of a method can be improved by using it as detection system in flow injection analysis and/or sequential injection analysis systems.261... 

Enantioselective intramolecular cascade Mizoroki-Heck reactions have been shown to proceed with moderate to good selectivity via the cationic manifold. There are surprisingly few enantioselective examples, given the wide array of transformations known for cr-alkylpalladium intermediates in racemic or diastereoselective reactions. All of the nongroup-selective, enantioselective, cascade, intramolecular Mizoroki-Heck reactions reported to date involve formation of one quaternary centre. A substantial advance would be to expand the range of transformations available for the a-alkylpalladium species and... 

Temperature has a major impact on retention selectivity (enantioselectivity), resolution, and column efficiency for chromatographic enantiomer separations and can provide valuable information about solute conformational changes, the stationary phase transitions, as well as the chromatographic retention. 

Enzyme mimic A reacting molecule or system that shows at least some of the catalytic features of a ttue enzyme significant catalysis, substrate selectivity, enantioselectivity, turnover, etc. 

Scheme 38.3 exo-Selective enantioselective Diels-Alder reactions. 

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