Synthesis, resolution, and absolute configuration determination of a vicinal amino alcohol with axial chirality. Application to the synthesis of new box and pybox ligands.

New racemic vicinal amino alcohol derivatives with 4-benzylidenecyclohexane skeleton and axial chirality have been prepared. A preparatively easy and efficient protocol for resolution of the N-benzoylamino alcohol is described. Using a 250 × 20 mm (L × ID) Chiralpak® IA column, and the appropriate mixture of n-hexane/ethanol/chloroform as eluent, both enantiomers of N-benzoylamino alcohol 3 are obtained with >99% enantiomeric excess (ee) by successive injections of a solution of the racemic sample in chloroform. The obtained axially chiral vicinal amino alcohol is used to synthesize structurally novel bisoxazoline ligands in high yields.

High-performance liquid chromatographic enantioseparation of unusual amino acid derivatives with axial chirality on polysaccharide-based chiral stationary phases.

The successful enantioseparation of axially chiral amino acid derivatives containing a cyclohexylidene moiety on an analytical and semipreparative scale was achieved for the first time by HPLC using polysaccharide-based chiral stationary phases. Racemic methyl N-benzoylamino esters, easily obtained by methanolysis of the corresponding 5(4H)-oxazolones, were subjected to chiral HPLC resolution using chiral stationary phases based on immobilized 3,5-dimethylphenylcarbamate derivatives of amylose (Chiralpak(®) IA column) or cellulose (Chiralpak(®) IB column). The behaviour of both selectors under different elution conditions was evaluated and compared. The amylose column showed better performance than the cellulose column for all enantiomers tested. The semipreparative resolution of axially chiral amino acid derivatives with different side chains has been achieved on a 250mm×20mm ID Chiralpak(®) IA column using the appropriate mixture of n-hexane/chlorofom/ethanol as eluent by successive injections of a solution of the sample in chloroform. Using this protocol up to 120mg of each enantiomer of the corresponding axially chiral amino acid derivative were obtained from 300mg of racemate. [(Sa)-2a, 105mg; (Ra)-2a, 60mg, [(Sa)-2b, 105mg; (Ra)-2b, 90mg, [(Sa)-2c, 120mg; (Ra)-2c, 100mg].

Stereochemical outcome of copper-catalyzed C-H insertion reactions. An experimental and theoretical study.

The combination of chiral preparative HPLC separation, VCD measurements, and theoretical calculations allows the unambiguous determination of the absolute configuration of the conformationally flexible products of copper-catalyzed carbene insertion reactions. DFT calculations were used to predict the stereochemical outcome of the copper-bis(oxazoline)-catalyzed C-H insertion reaction between methyl diazophenylacetate and tetrahydrofuran and also to predict the absolute configuration of the major stereoisomers derived from the same reaction with different cyclic ethers. These predictions were verified experimentally through NMR and VCD spectroscopy and allowed rationalization of the stereochemical outcome of these reactions without further derivatization of the products, which can be prblematic under certain conditions as described herein.

L- and D-proline adsorption by chiral ordered mesoporous silica.

Chiral ordered mesoporous silica (COMS) was synthesized in the presence of amino acid proline by combining tetraethyl orthosilicate and quaternized aminosilane silica sources. The as-prepared materials were activated by calcination or microwave chemical extraction to remove the organic templates. The powder X-ray diffraction and N2 adsorption characterization revealed in COMS the structural and textural features of MCM-41-type silica. The chirality of the material was disclosed by mixed and separate L- and D-proline adsorption on the COMS prepared with L-proline (L-Pro-COMS) and D-proline (d-Pro-COMS). It was found that the maximum L-proline and D-proline adsorption capacities on L-Pro-COMS were ca. 2.3 and 0.6 mmol/g, respectively, while the adsorption of D-proline was higher than that of l-proline on d-Pro-COMS. Finally, both activation routes yielded enantioselective silicas able to separate proline racemate.

Switch in regioselectivity of epoxide ring-opening by changing the organometallic reagent.

The regio- and stereoselective ring-opening of a 2-(2'-oxiranyl)-1,2,3,6-tetrahydropyridine using organometallic reagents is reported. The choice of the organometallic reagent determines the formation of either 2-[(R)-1-hydroxyalkyl]- or 2-[(S)-2-hydroxy-1-alkyl]-1,2,3,6-tetrahydropyridines. The formation of 2-[(S)-2-hydroxy-1-alkyl]-1,2,3,6-tetrahydropyridines is a rare example of epoxide ring-opening with retention of configuration. The process has been applied to the asymmetric synthesis of ß-(+)-conhydrine and to the formal synthesis of (2S,2'R)-erythro-methylphenidate from a common precursor. Extension of the structural diversity of the process has allowed the synthesis of several ß-(+)-conhydrine analogs.

Recent advances in enantioselective organocatalyzed anhydride desymmetrization and its application to the synthesis of valuable enantiopure compounds.

Recent years have witnessed increasing interest in the field of asymmetric organocatalysis. In particular, efforts in this field have been devoted to the use of small organic molecules in asymmetric processes based on enantiotopic face discrimination and, only recently, efforts have also been devoted to asymmetric organocatalytic desymmetrization of prochiral substrates-a process based on enantiotopic group discrimination. This critical review documents the advances in the use of organocatalysis for the enantioselective desymmetrization of achiral and meso anhydrides and its application to the synthesis of valuable compounds as reported until 2010 (134 references).

Enantioselective C-H carbene insertions with homogeneous and immobilized copper complexes.

The efficiency of chiral bis(oxazoline)- and azabis(oxazoline)-copper complexes in the enantioselective carbene insertion into C-H bonds of cyclic ethers in homogeneous phase strongly depends on the structure of the substrate. The immobilization on laponite clay by electrostatic interactions not only allows the recovery and reuse of the heterogeneous catalysts, but in some cases also improves enantioselectivity and overall chemoselectivity, making possible reactions that do not take place or lead to low yields in solution, even with the commonly used Rh(2)[S-DOSP](4) catalyst.