Conformational effects on lipase-mediated acylations of 2-substituted cyclohexanols

Lipase-mediated acetylations of trans- and cis-2-substituted cyclohexanols gave the corresponding (1R)-cyclohexyl acetates and (1S)-cyclohexanols in high yields and ee, but c-4-tert-butyl-c-2-ethenyl-r-1-cyclohexanol was unreactive owing to the steric interaction between the axial OH group and the axial H atoms at the 3- and 5-positions. In the cis-isomer the OH group occupies an equatorial position to bind to the lipase, and less bulky axial alkenyl and alkynyl groups might not so much prevent acetylations than an alkyl group.     

Lipase-catalyzed ester formation in organic solvents an easy preparative resolution α-substituted cyclohexanols

The commercial preparation “lipase My” catalyses ester formation from an alcohol and a fatty acid in organic solvents. Preparative resolution of α-substituted cyclohexanols, including menthol, was achieved under these conditions.     

Enzymatic resolution of methyl N-alkyl-azetidine-2-carboxylates by Candida antarctica lipase-mediated ammoniolysis

A facile method for the synthesis of optically active azetidine-2-carboxylic acid derivatives is presented. Racemic N-alkylated azetidine esters are resolved by lipase from Candida antarctica in an ammoniolysis reaction, and both the S-amide and the R-ester are obtained with excellent stereoselectivity.     

Formation of enantiopure 5-substituted oxazolidinones through enzyme-catalysed kinetic resolution of epoxides

Halohydrin dehalogenase from Agrobacterium radiobacter catalyzed the enantioselective ring opening of terminal epoxides with cyanate as a nucleophile, yielding 5-substituted oxazolidinones in high yields and with high enantiopurity (69-98% ee). This is the first example of the biocatalytic conversion of a range of epoxides to the corresponding oxazolidinones.     

Chemoenzymatic synthesis of calcilytic agent NPS-2143 employing a lipase-mediated resolution protocol

The kinetic resolution of chlorohydrin (±)-6 has been successfully carried out via a lipase-mediated transesterification with vinyl acetate in organic as well as ionic liquid media to yield (R)-alcohol 6 and (S)-acetate 7 in high enantioselectivity. An enantioconvergent synthesis has also been achieved by a Mitsunobu esterification of a mixture of (R)-alcohol 6 and (S)-acetate 7 in one pot to convert the (R)-alcohol 6 to (S)-acetate 7. (S)-Acetate 7 has been hydrolyzed by LiOH·H₂O to give epoxide (R)-2. This enantiopure epoxide has been used as a chiral precursor for the synthesis of calcilytic agent NPS-2143.     

Synthesis of 3-substituted 2-cyclohexenones through umpoled functionalization

A new protocol to obtain 3-substituted 2-cyclohexenones, was developed by reversing the chemical reactivity of 2-cyclohexenone. One-pot synthesis of 3-substituted 2-cyclohexenones can be achieved by treatment of 3-phenylthiosilyl enol ether with a mixture of t-BuLi/HMPA that allows hydrogen-selective exchange in presence of reactive electrophiles such as aldehydes, ketones and alkyl halides. This affords the corresponding product in moderate overall yield, after silyl enol ether cleavage and concomitant thiophenol elimination initiated with TBAF.     

Impact of variation of the acyl group on the efficiency and selectivity of the lipase-mediated resolution of 2-phenylalkanols

By tuning the steric properties of the acyl group to control the efficiency and selectivity of the resolution, 2-phenyl-1-propanol 1a was prepared by lipase-catalysed hydrolysis using a short-chain acyl group, with E-values of up to 66 (ee up to 95%). 2-Phenylbutan-1-ol 1b was similarly resolved (up to 86% ee) using the optimised conditions, while the ester of the more sterically demanding 3-methyl-2-phenylbutan-1-ol 1c proved resistant to enzymatic hydrolysis under these conditions.     

Kinetic resolution and double stereodifferentiation in catalytic asymmetric C-H activation of 2-substituted pyrrolidines

Dirhodium tetrakis(S-(N-dodecylbenzenesulfonyl)prolinate) (Rh(2)(S-DOSP)(4)) catalyzed decomposition of methyl aryldiazoacetates in the presence of 2-substituted pyrrolidines results in highly diastereoselective and enantioselective C-H insertions. These reactions can proceed with impressive levels of double stereodifferentiation and kinetic resolution, which allows for three stereocenters to be controlled during the C-H insertion step.     

Efficient preparation of enantiomerically pure (E)-4-(tributylstannanyl)but-3-en-2-ol via lipase-mediated resolution

The practical preparation of enantiomerically pure (E)-4-(tributylstannanyl)but-3-en-2-ol 1 from 3-butyn-2-ol 2 is reported. A modified Guibé's Pd-catalyzed hydrostannation of 2 provided the racemic γ-hydroxy vinylstannane 1 in a good yield. The enzymatic esterification of 1, with an inexpensive lipozyme, afforded (R)-3 and (S)-1 with very high enantiomeric excesses and chemical yields. This procedure is suitable for the multigram scale preparation of the potential chiral building blocks, (R)-1 and (S)-1.     

Kinetic resolution of 1,2-dihydroxy-3-ferrocenylpropane by sequential lipase-catalysed esterification

One-pot sequential esterification of 1,2-dihydroxy-3-ferrocenylpropane 1, catalysed by lipase from Pseudomonas cepacia, allowed kinetic resolution of the racemate, affording (−)-(R)-1-acetoxy-2-hydroxy-3-ferrocenyl-propane (−)-2, and (+)-(S)-1,2-diacetoxy-3-ferrocenylpropane (+)-3, in high chemical yield and enantiomeric excess.     

Enzymic optical resolution and absolute configuration of tricyclo[5.2.1.02,6]decadienones

Access to optically active endo-tricyclodecadienones 1 (X=CH₂) has been realized by (i) classical resolution of the diastereomeric ephedrine salts of 3 and (ii) pig liver esterase catalyzed kinetic resolution of 2.     

Lipase-catalyzed resolution of anti-6-substituted 1,3-dioxepan-5-ols

Several anti-6-substituted 1,3-dioxepan-5-ols were kinetically resolved using an immobilized lipase (Amano PS–C II) in toluene in the presence of vinyl acetate at 30 °C. This approach provided, in some cases, the alcohol and the acetate in high enantiomeric purity, depending on the nature of the substituent (R = N₃, SePh, I, OBn) and the acetal group (unsubstituted or dimethyl). The role of the size of substituents is also discussed. Enantiopure anti-6-substituted 1,3-dioxepan-5-ols are useful building blocks.     

Efficient kinetic resolution in the asymmetric hydrosilylation of imines of 3-substituted indanones and 4-substituted tetralones

Kinetic resolution of the N-methyl imines of 3-substituted indanones and 4-substituted tetralones could be accomplished by hydrosilylation with a chiral titanocene catalyst. N-Methyl imines of 4-substituted tetralones were resolved to yield, after hydrolysis of the unreacted starting materials, ketones with high ee's and the amine products with high diastereomeric and enantiomeric purity. The utility of this process was demonstrated in the synthesis of sertraline.     

Enzymatic resolution of 2-substituted tetrahydroquinolines. Convenient approaches to tricyclic quinoxalinediones as potent NMDA-glycine antagonists

Two approaches leading to the enantiomerically pure tricyclic quinoxalinedione class of NMDA-glycine antagonists using enzymatic resolutions are described. An intermediate, racemic methyl 1,2,3,4-tetrahydroquinoline-2-carboxylate 3, was resolved to (S)-3 in 97% ee and 47% yield (E=67) using -chymotrypsin. In an improved method, hydrolysis of another intermediate, racemic methyl 1,2,3,4-tetrahydroquinoline-2-acetate 4, with Novozym^® 435 provided the desired (S)-4 in high enantioselectivity and yield (93% ee, 50%, E=94).     

Hydrophobic cellulose nanopaper through a mild esterification procedure

Films of cellulose nanofibrils (CNF) (referred to as nanopaper) present a great potential in many applications due to the abundance, low environmental impact, excellent oxygen barrier properties and good mechanical performance of CNF. However, the strong hygroscopic character of the natural nanofibers limits their use in environments with high relative humidity. In this paper, we introduce a simple route for the esterification and processing of CNF with the aim of reducing their hydrophilicity, and producing hydrophobic cellulose nanopaper with reduced moisture sensitivity. The preparation steps of hydrophobic nanopapers involve vacuum filtration, solvent exchange from water to acetone, and reaction with anhydride molecules bearing different hydrophobic alkyl chains by hot pressing. Porous films having a surface area between 38 and 47 g/m² and pore sizes in the 3–200 nm range are obtained. This method preserves the crystalline structure of native cellulose, and successfully introduces hydrophobic moieties on CNF surface as confirmed by FTIR, XPS and elemental analysis. As a result, modified nanopapers have a reduced moisture uptake, both higher surface water contact angle and wet tensile properties as compared with reference non-modified nanopaper, thus illustrating the benefit of the modification for the use of cellulose nanopaper in humid environments.     

Kinetic resolution of C1-substituted oxabenzonorbornadienes by Ir-catalyzed asymmetric [2+2] cycloaddition reactions with arylacetylenes

The kinetic resolution of racemic C1-substituted oxabenzonorbornadienes was realized by iridium-catalyzed asymmetric [2+2] cycloaddition reaction with arylacetylenes. Cyclobutene products and unreacted C1-substituted oxabenzonorbornadienes were obtained with high to excellent enantiomeric purities.     

Synthesis of conformationally restricted acetylcholine analogues. Comparing lipase-mediated resolution with simulated moving bed chromatography of arylated β-hydroxy-pyrrolidines

The enantiodivergent synthesis of new, conformationally restricted acetylcholine analogues was accomplished using arylated endocyclic enecarbamates as key intermediates. Stereoselective hydroboration of the aryl enecarbamates provided the corresponding aryl-β-hydroxy-pyrrolidines. Acetylation, deprotection, followed by N-bismethylation, led to the desired betaine products. The kinetic resolution of the intermediate alcohols was performed by lipase-mediated hydrolysis of its acetate derivatives, resulting in excellent enantioselectivities (E > 100). An enzymatic enantiopreference predicted by the Kazlauskas’s model was confirmed following Riguera’s protocol. Finally, chromatographic resolution of racemic alcohol 5a was evaluated by semi-preparative scale chiral simulated moving bed chromatography and its performance compared with biocatalysis.