| 1. |
- Engström, Karin, et al.
(författare)
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Directed evolution of an enantioselective lipase with broad substrate scope for hydrolysis of α-substituted esters
- 2010
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 132:20, s. 7038-7042
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Tidskriftsartikel (refereegranskat)abstract
- A variant of Candida antarctica lipase A (CalA) was developed for the hydrolysis of α-substituted p-nitrophenyl esters by directed evolution. The E values of this variant for 7 different esters was 45−276, which is a large improvement compared to 2−20 for the wild type. The broad substrate scope of this enzyme variant is of synthetic use, and hydrolysis of the tested substrates proceeded with an enantiomeric excess between 95−99%. A 30-fold increase in activity was also observed for most substrates. The developed enzyme variant shows (R)-selectivity, which is reversed compared to the wild type that is (S)-selective for most substrates.
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| 2. |
- Engström, Karin, et al.
(författare)
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Enantioselective Kinetic Resolution of p-Nitrophenyl 2-Phenylpropanoate by a Variant of Candida antarctica Lipase A Developed by Directed Evolution
- 2010
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society. - 0002-7863 .- 1520-5126. ; 132:20, s. 7038-7042
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Tidskriftsartikel (refereegranskat)abstract
- A variant of Candida antarctica lipase A (CalA) was developed for the hydrolysis of α-substituted p-nitrophenyl esters by directed evolution. The E values of this variant for 7 different esters was 45−276, which is a large improvement compared to 2−20 for the wild type. The broad substrate scope of this enzyme variant is of synthetic use, and hydrolysis of the tested substrates proceeded with an enantiomeric excess between 95−99%. A 30-fold increase in activity was also observed for most substrates. The developed enzyme variant shows (R)-selectivity, which is reversed compared to the wild type that is (S)-selective for most substrates.
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| 3. |
- Franzén, Johan, et al.
(författare)
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Stereoselective palladium-catalyzed carbocyclization of allenic allylic carboxylates
- 2003
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 125:46, s. 14140-14148
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Tidskriftsartikel (refereegranskat)abstract
- Palladium(0)-catalyzed reaction of allene-substituted allylic carboxylates 3-8 employing 2-5 mol % of Pd(dba)(2) in refluxing toluene leads to the carbocyclization and elimination of carboxylic acid to give bicyclo[4.3.0]nonadiene and bicyclo[5.3.0]decadiene derivatives (12-17). The carbon-carbon bond formation is stereospecific, occurring syn with respect to the leaving group. Addition of maleic anhydride as a ligand to the above-mentioned procedures changed the outcome of the reaction, and under these conditions 3-5 afforded cycloisomerized products 21-23. The experimental results are consistent with a mechanism involving oxidative addition of the allylic carboxylate to Pd(0) to give an electron-deficient (pi-allyl)palladium intermediate, followed by nucleophilic attack by the allene on the face of the pi-allyl opposite to that of the palladium atom. Furthermore, it was found that the Pd(dba)(2)-catalyzed cyclization of the trans-cycloheptene derivative (trans-8) can be directed to give either the trans-fused (trans-17) or the cis-fused (cis-17) ring system by altering the solvent. The former reaction proceeds via a nucleophilic trans-allene attack on the (pi-allyl)palladium intermediate, whereas the latter involves a syn-allene insertion into the allyl-Pd bond of the same intermediate. The products from the carbocylization undergo stereoselective Diels-Alder reactions to give stereodefined polycyclic systems in high yields.
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| 4. |
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| 5. |
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| 6. |
- Åberg, Jenny B., et al.
(författare)
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CO Assistance in ligand exchange of a ruthenium racemization catalyst: identification of an acyl intermediate
- 2009
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society. - 0002-7863 .- 1520-5126. ; 131:27, s. 9500-9501
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Tidskriftsartikel (refereegranskat)abstract
- An acyl intermediate in the activation of eta(5)-(Ph(5)Cp)Ru(CO)(2)Cl by t-BuOK was identified by means of in situ FT-IR measurements and NMR spectroscopy. This strongly supports the conclusion that the ligand exchange takes place via CO assistance, i.e., that the activation occurs via nucleophilic attack by tert-butoxide on one of the CO ligands. The tert-butoxycarbonyl intermediate shows stretching vibrations at 1933 and 1596 cm(-1), corresponding to the CO and COOt-Bu groups, respectively. In the (13)C NMR spectrum, the CO group appears at 209.5 ppm and the COOt-Bu group at 208.7 ppm. The NMR assignments were confirmed by density functional theory calculations. The subsequent alcohol-alkoxide exchange is also thought to take place via CO assistance. However, no intermediate in that step could be detected.
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| 7. |
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