Complementary regioselective esterification of non-reducing oligosaccharides catalyzed by different hydrolases

The enzymatic transesterification of several tri- and tetrasaccharides with vinyl laurate is described. The lipases from Candida antarctica B (Novozym 435) and Thermomyces lanuginosus (Lipozyme TL IM) and the alkaline protease from Bacillus licheniformis (subtilisin Carlsberg) have been used with each carbohydrate to obtain different regioisomers. By using the sugars in their amorphous form, complete solubility is achieved in the reaction media (tert-butanol/pyridine mixtures for the lipases and pyridine for the protease) and high isolated yields of the corresponding monoesters are obtained. Good to excellent regioselectivity is observed for all the enzymes, showing a final complementary picture respect to the primary hydroxyls of the oligosaccharides studied.     

General Aspects and Optimization of Enantioselective Biocatalysis in Organic Solvents: The Use of Lipases [New Synthetic Methods (76)]

Enantioselective biocatalysis in nonaqueous media is becoming increasingly important in preparative synthetic chemistry. This article discusses (1) the general catalytic properties of enzymes in nonaqueous environments, (2) the basic principles that govern lipase-catalyzed enantioselective esterification and transesterification reactions in organic media for the preparation of optically active acids and alcohols, (3) the determination of kinetic and thermodynamic parameters, and (4) the quantitative analysis of published data.     

Dynamic kinetic resolution of 2-methyl-2-nitrocyclohexanol: Combining the intramolecular nitroaldol (Henry) reaction & lipase-catalysed resolution

Efforts to combine the intramolecular nitroaldol reaction with lipase-catalysed resolution of the resulting nitroaldol adduct in a one-pot dynamic kinetic resolution (DKR) are described. Significant challenges were encountered in the combination of the two systems. trans-2-Methyl-2-nitrocyclohexyl acetate (±)-3b was isolated in excellent enantiopurity (>98% ee) via a sequential DKR sequence where the lipase-mediated resolution and base-mediated interconversion of 2-methyl-2-nitrocyclohexanol 2 were effected alternately, demonstrating the feasibility of this approach initially. Further work showed, for the first time, evidence that a DKR-type system is possible for 2. Reaction engineering allowed the design of a sequential one-pot reaction system which furnished the products with excellent enantioselectivity, and good diastereoselectivity.     

The effect of the migrating group structure on enantioselectivity in lipase-catalyzed kinetic resolution of 1-phenylethanol

We have studied the effects of the acyl moiety on the enantioselectivity of three lipases: Candida antarctica B, Pseudomonas cepacia and Candida cylindracea, frequently used in kinetic resolutions by acylation or hydrolysis. The size of the acyl group was examined using various enol esters during the transesterification of 1-phenylethanol and the hydrolysis of the corresponding phenylethylesters. C. antarctica-B lipase showed the highest selectivity in the transesterification of 1-phenylethanol with isopropenyl and vinyl acetate, vinyl decanoate, vinyl laurate, (E > 200). The esters 1-phenyl -ethyl-acetate, decanoate and laurate are also hydrolyzed with high selectivities (E > 150) with CAL-B. The results can be correlated to the three-dimensional form of each lipase. The effect of the migrating group on the reactivity and selectivity of the lipases are discussed for both reactions.     

Quantitative Screening of Hydrolase Libraries Using pH Indicators: Identifying Active and Enantioselective Hydrolases

Rapid and quantitative screening in 96-well microplates can identify active and enantioselective hydrolases. Hydrolysis of esters releases a proton, which can be detected with pH indicators by colorimetry (figure). Using pure enantiomers, we measured the initial rates of enzyme-catalyzed hydrolysis. The relative initial rate approximates the enantioselectivity. This screening greatly speeds up selection of the best hydrolase for a synthesis.     

Michael additions of primary and secondary amines to acrylonitrile catalyzed by lipases

The present Letter details our findings on the lipase-catalyzed Michael reactions between primary or secondary amines and acrylonitrile. Several lipases were evaluated, and good results were obtained leading to the formation of Michael adducts in shorter reaction times than the uncatalyzed reactions.     

Lipase-catalyzed amidation of carboxylic acid and amines

The amidation reaction is of a very particular interest, especially in the pharmaceutical industry and always requires the activation of the acid with a large excess of reactants. Therefore, a large amount of waste is generated. In order to reduce the environmental impact of such reaction, we have developed enzymatic amidation conditions which are compatible with a wide range of amines and acids, in particular with the biologically relevant lipoic acid. Water is the only by-product generated during this reaction thus a very high atom economy is obtained. In addition, we have shown that the lipase can be recovered and reused several times without a significant loss of activity.     

On the enzymatic hydrolysis of methyl 2-fluoro-2-arylpropionates by lipases

The enzymatic hydrolysis of methyl 2-fluoro-2-arylpropionates was performed using lipases from Candida rugosa and Candida cylindracea (OF-360). A careful analysis of the reaction products revealed that racemic 2-hydroxy-2-arylpropionic acid and traces of 2-arylacrylic acid are formed, in addition to the expected 2-aryl-2-fluoropropionic acid. The presence of powerful electron-releasing groups in the aromatic ring of the substrate increase the amount of 2-hydroxypropionic acid. A mechanistic hypothesis has been formulated according to which the enzyme facilitates the elimination of fluoride ion from the hydrolysed acid with the formation of an α-carboxy-stabilized carbocation which provides 2-hydroxypropionic acids by nucleophilic attack of H₂O and 2-arylacrylic acids by a β-elimination process.     

Formation of chiral tertiary homoallylic alcohols via Evans aldol reaction or enzymatic resolution and their influence on the Sharpless asymmetric dihydroxylation

Enantioenriched tertiary homoallylic alcohol derivatives (S)-2c and (S)-2a were obtained via Evans aldol methodology and enzymatic resolution of racemic tertiary acetate 2e, respectively. In order to study asymmetric 1,3-induction of the stereogenic center present in 2, congener (R)-2a as well as its O-protected derivatives (R)-2b-d were submitted to Sharpless asymmetric dihydroxylation to yield the diastereomeric 1,2,4-triol derivatives (2R,4R)- and (2S,4R)-3a-d, revealing that neither the substrate nor the Sharpless catalyst exert any stereocontrol. Similar observations were made for the less bulky alkynyl-substituted derivative 12b. However, by using a directed dihydroxylation, the anti product (2R,4R)-3a was favored.     

A chemoenzymatic approach to (+)-pilocarpine

A short synthesis for the alkaloid (+)-pilocarpine has been developed. Key step of this synthesis is a chemoenzymatic resolution utilizing the lipase AP.