1.Vanadium-catalyzed asymmetric oxidation of alpha-hydroxy esters using molecular oxygen as stoichiometric oxidant.
Radosevich AT1, Musich C, Toste FD. J Am Chem Soc. 2005 Feb 2;127(4):1090-1.
A vanadium-catalyzed method for the oxidative kinetic resolution of alpha-hydroxyesters, using oxygen as the terminal oxidant, is described. The catalyst is generated in situ from vanadium(V) tri-iso-propoxyoxide in combination with a tridentate ligand derived from 3,5-di-tert-butylsalicylaldehyde and (S)-tert-leucinol. The reaction allows for the enantioselective synthesis of both aromatic and aliphatic secondary alcohols, including those containing olefins and alkynes.
2.Diastereo- and enantioselective cyclopropanation with chromium fischer carbene complexes: alkenyl oxazolines as useful achiral and chiral substrates.
Barluenga J1, Suárez-Sobrino AL, Tomás M, García-Granda S, Santiago-García R. J Am Chem Soc. 2001 Oct 31;123(43):10494-501.
The cyclopropanation reaction of chromium Fischer carbene complexes with alkenyl oxazolines has been studied in both racemic and enantioselective fashions. The oxazolinyl group acts as both electron-acceptor substituent and chiral auxiliary. Achiral (4,4-dimethyloxazolin-2-yl)alkenes derived from trans-crotonic and trans-cinnamic acids 2a,b undergo the cyclopropanation reaction to give 4a-d,g with excellent diastereoselectivity (trans/cis ratio between 93:7 and >97:3), while those derived from acrylic and metacrylic acids 2c,d give the cyclopropanes 4e,f,h with much lower selectivity (trans/cis ratio between 68:32 and 83:17). The homogeneous catalytic hydrogenolysis of 4 leads in a selective manner to 5 or 6, depending on the nature of the R3 substituent. The removal of the oxazoline moiety is achieved by carboxybenzylation/hydrolysis and ester reduction, yielding monoprotected 1,4- and 1,3-diols 9 and 11, respectively. The alkenes derived from enantiopure (S)-valinol and (S)-tert-leucinol 3 led to cyclopropanes trans-12 with high relative and absolute stereocontrol.
3.Enantioseparation of racemic N-acylarylalkylamines on various amino alcohol derived tau-acidic chiral stationary phases.
Ryoo JJ1, Kim TH, Im SH, Jeong YH, Park JY, Choi SH, Lee KP, Park JH. J Chromatogr A. 2003 Feb 14;987(1-2):429-38.
Five tau-acidic chiral stationary phases (CSPs), CSP 4, CSP 5, CSP 6, CSP 7 and CSP 8, were prepared by connecting the N-(3,5-dimethylbenzoyl) derivative of (R)-alaninol, (S)-leucinol, (1S,2R)-ephedrine and (S)-tert-leucinol and the O-(3,5-dinitrobenzoyl) derivative of (R)-phenylglycinol to silica gel through a carbamate or urea linkage. The CSPs were applied to the resolution of various racemic N-acyl-1-naphthylaminoalkanes by chiral HPLC, and the chromatographic resolution results were compared with those of previously reported CSPs (CSP 2, CSP 3), which are derived from N-(3,5-dinitrobenzoyl)-(1S,2R)-norephedrine and N-(3,5-dinitrobenzoyl-(R)-phenylglycinol. Based on a comparison of the resolution results for each CSP, the role of each functional group on the five chiral selectors is explained.
