| 1. |
- Berglund, Per, et al.
(författare)
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Switched enantiopreference of Humicola lipase for 2-phenoxyalkanoic acid ester homologs can be rationalized by different substrate binding modes
- 1999
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Ingår i: Tetrahedron. - 0957-4166 .- 1362-511X. ; 10:21, s. 4191-4202
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Tidskriftsartikel (refereegranskat)abstract
- Humicola lanuginosa lipase was used for enantioselective hydrolyses of a series of homologous 2-phenoxyalkanoic acid ethyl esters. The enantioselectivity (E-value) of the enzyme changed from an (R)-enantiomer preference for the smallest substrate, 2-phenoxypropanoic acid ester, to an (S)-enantiomer preference for the homologous esters with longer acyl moieties. The E-values span the range from E=13 (R) to E=56 (S). A molecular modeling study identified two different substrate-binding modes for each enantiomer. We found that the enantiomers favored different modes. This discovery provided a model that offered a rational explanation for the observed switch in enantioselectivity. (C) 1999 Elsevier Science Ltd. All rights reserved.
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| 2. |
- Hallman, K., et al.
(författare)
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Polymer-bound bis(oxazoline) as a chiral catalyst
- 2001
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Ingår i: Tetrahedron. - 0957-4166 .- 1362-511X. ; 12:10, s. 1475-1478
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Tidskriftsartikel (refereegranskat)abstract
- A chiral bis(oxazoline) was grafted on ArgoGel and used in the palladium-catalysed substitution of (+/-)-1,3-diphenyl-2-propenyl acetate with dimethyl malonate. The enantioselectivity was the same as that observed when the analogous monomeric catalyst was used (94-95% e.e.), despite the fact that the C-2 symmetry of the ligand was affected when coupled to the polymer. The polymer-supported catalyst could be recycled several times after removal of precipitated Pd(0). The polymer-bound bis(oxazoline) was also applied in a zinc-catalysed Diels-Alder reaction but lower selectivity and reactivity than the monomer was observed.
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| 3. |
- Hedenström, Erik, et al.
(författare)
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Do enzymes recognise remotely located stereocentres? : Highly enantioselective Candida rugosa lipase-catalysed esterification of the 2- to 8-methyldecanoic acids
- 2002
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Ingår i: Tetrahedron. - 0957-4166 .- 1362-511X. ; 13:8, s. 835-844
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Tidskriftsartikel (refereegranskat)abstract
- Several racemic methyl decanoic acids have been synthesised and successfully resolved in esterification with 1-hexadecanol at aw=0.8 in cyclohexane using immobilised Candida rugosa lipase (CRL) as the catalyst. The enantiomeric ratios (E=2.8-68) obtained were surprisingly high even when the methyl group was as remotely located as in 8-methyldecanoic acid (E=25). Interestingly, the lipase shows enantiopreference for the S-enantiomer when the methyl group is located on even numbered carbons i.e. for the 2-,4-,6- and 8-methyldecanoic acids and to the R-enantiomer when the methyl group is located on uneven numbered carbons i.e. for the 3-,5- and 7-methyldecanoic acids.
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| 4. |
- Holmén, Anders, et al.
(författare)
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Direct determination of the absolute configuration of a cyclic thiolsulfinate by VCD spectroscopy
- 2003
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Ingår i: Tetrahedron. - : Pergamon Press. - 0957-4166 .- 1362-511X. ; 14:15, s. 2267-2269
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Tidskriftsartikel (refereegranskat)abstract
- Experimental and calculated vibrational circular dichroism (VCD) spectra of the enantiomers of the conformationally rigid thiolsulfinate, naphtho[1,8-cd]-1,2-dithiole 1-oxide 1, obtained by chromatographic resolution of the racemate, were in excellent agreement, showing a (+)-(S)-1/(-)-(R)-1 relationship.
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| 5. |
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| 6. |
- Lake, F., et al.
(författare)
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Ti-mediated addition of diethylzinc to benzaldehyde. The effect of chiral additives
- 2001
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Ingår i: Tetrahedron. - 0957-4166 .- 1362-511X. ; 12:5, s. 755-760
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Tidskriftsartikel (refereegranskat)abstract
- In the presence of a chiral tridentate bissulfonamide, the titanium-mediated addition of diethylzinc to benzaldehyde gave alkylated products ranging from the (R)-enantiomer, formed with an e.e. of 26%, to the (S)-enantiomer, formed in 72% e.e. The enantioselectivity was also affected by the presence of additional chiral mono- and bidentate ligands, with the reactions proceeding via complexes containing the chiral sulfonamide and the additive. The: addition of (1R,2R)-2,2-diphenyl-,12-ethanediamine and (1S,2S)-1.3-diphenyl-1,2-ethanediamine gave the (S)-product with e.e. of 49% and the (R)-product with 16% e.e., respectively, whereas without additives the (R)-product was obtained in 26% e.e, In the presence of (1R,2R)-1,2-diphenyl-1,2-ethanediamine only (i.e. without the chiral sulfonamide), the (S)-product formed with a 3% e.e.
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| 7. |
- Liu, Rong, et al.
(författare)
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Chemoenzymatic preparation of (2S,3S)- and (2R,3R)-2,3-butanediols and their esters from mixtures of d,l- and meso-diols
- 2001
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Ingår i: Tetrahedron. - 0957-4166 .- 1362-511X. ; 12:5, s. 771-778
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Tidskriftsartikel (refereegranskat)abstract
- An efficient method of preparing the pure enantiomers of 2,3-butanediol front commercially available mixtures of the d,l- and meso-isomers was developed. It furnished (2S,3S)-2.3-butanediol with > 99% e.e. and a >9.5/0.5 diastereomeric ratio and (2R,3R)-2,3-butanediol in 95% e.e. and >9.5/<5 diastereomeric ratio.
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| 8. |
- Lutsenko, S., et al.
(författare)
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Microwave-mediated ruthenium-catalyzed asymmetric hydrogen transfer
- 2001
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Ingår i: Tetrahedron. - 0957-4166 .- 1362-511X. ; 12:18, s. 2529-2532
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Tidskriftsartikel (refereegranskat)abstract
- Ru-catalyzed hydrogen transfer from propan-2-ol to acetophenone under microwave conditions using monotosylated (R,R)-diphenylethylenediamine as the chiral source afforded (R)-1-phenylethanol in >90% yield and 82% e.e. within 9 min, while use of ephedrine or norephedrine gave the same compound in high yield with 70 and 46% e.e., respectively. t-Butylphenylketone was reduced to (R)-2,2-dimethyl-1-phenyl-1-propanoI under the same conditions in close to quantitative yield, although with low enantioselectivity.
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| 9. |
- Oxelbark, Joakim, 1970-, et al.
(författare)
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A comparison of silica and porous graphitic carbon as support materials for a chiral selector in HPLC
- 2002
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Ingår i: Tetrahedron. - : Pergamon Press. - 0957-4166 .- 1362-511X. ; 13:20, s. 2235-2240
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Tidskriftsartikel (refereegranskat)abstract
- The tartaramide based chiral selector 2 was synthesized, coated onto porous graphitic carbon (PGC), and evaluated as a chiral stationary phase for HPLC. Its performance was compared to the silica based sorbent 1 containing the same chiral moiety. The retention and separation characteristics of the two columns were found to be correlated, but separation factors and column efficiencies were constantly lower on the PGC column. Coating of PGC as a novel and simple means to evaluate chiral selectors was evaluated.
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| 10. |
- Risberg, E., et al.
(författare)
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Enantioselective addition of organolithium reagents to a 2H-azirine
- 2002
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Ingår i: Tetrahedron. - 0957-4166 .- 1362-511X. ; 13:18, s. 1957-1959
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Tidskriftsartikel (refereegranskat)abstract
- 3-(2-Naphthyl)-2H-azirine was used as a model substrate in the first enantioselective addition of organolithium reagents to an azirine. Various organolithium reagents have been used together with different chiral ligands.
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