Heterofunctional control of regio- and enantioselectivity in rhodium-catalysed hydroboration of allylic systems

Aryl allylic sulfones were reacted with catecholborane (HBcat) in the presence of neutral and cationic Rh complexes modified with bidentate chiral ligands, to produce the branched heteroorganoboronate ester, as the main isomer with moderate enantioselectivity. The relative rate of the formation of the secondary regioisomer seems to be sensitive to the nature of the catalytic system and the electronic effects of the substrate.     

An efficient RNA-cleaving DNA enzyme that synchronizes catalysis with fluorescence signaling

DNA enzymes are single-stranded DNA molecules with catalytic capabilities that are isolated from random-sequence DNA libraries by "in vitro selection". This new class of catalytic biomolecules has the potential of being used as unique molecular tools in a variety of innovative applications. Here we describe the creation and characterization of an RNA-cleaving autocatalytic DNA, DEC22-18, that uniquely links chemical catalysis with real-time fluorescence signaling capability in the same molecule. A trans-acting DNA molecule, DET22-18, was also developed from DEC22-18 that behaves as a true enzyme with a k(cat) of approximately 7 min(-1)-a rate constant that is the second largest ever reported for a DNA enzyme. It cleaves a chimeric RNA/DNA substrate at the lone RNA linkage surrounded by a closely spaced fluorophore-quencher pair-a unique structure that permits the synchronization of the chemical cleavage with fluorescence signaling. DET22-18 has a stem-loop structure and can be conjugated with DNA aptamers to form allosteric deoxyribozyme biosensors.     

Enantioselective nucleophilic addition of organometallic reagents to quinoline: regio-, stereo- and enantioselectivity

Some 2-alkyl-1,2-dihydroquinoline and some 2-aryl-1,2-dihydroquinoline were obtained by enantioselective addition of methyl-, butyl-, phenyl- and 1-naphthyllithium on quinoline. Bisoxazolines were used as external chiral ligands, giving enantiomeric excess up to 79%. The ligand could be used in catalytic amounts without significant loss of enantioselectivity.     

A genetic selection system for evolving enantioselectivity of enzymes

As an alternative to screening in the directed evolution of enantioselective enzymes, a selection system has been implemented for a lipase-catalyzed hydrolytic kinetic resolution of a chiral ester.     

Carbonyl orientation determines regio- and enantioselectivity in 1,2-/1,4-reduction of an NAD model compound

An optically active, axially chiral NAD model compound(1) with a quinoline ring system was reduced by the chiral NADH model compound (4), affording a mixture of 1,2- and 1,4-dihydroquinolines. The carbonyl orientation governs the molecular arrangement in the transition state of the reaction and determines the regio- and enantioselectivity of the product.     

Fairly marked enantioselectivity for the hydrolysis of amino acid esters by chemically modified enzymes

The hydrolysis (deacylation) of enantiomeric substrates by the chemically modified enzymes decanoyl-alpha-chymotrypsin and decanoyl-trypsin was studied. Reaction activity for decanoyl-alpha-chymotrypsin was lower than that for the native enzyme, although intriguingly the enantioselectivity was markedly enhanced as compared with the native enzyme. In particular, the apparently complete enantioselective catalysis was attained for the hydrolytic cleavage of p-nitrophenyl N-dodecanoyl-D(L)-phenylalaninates. The enhancement of enantioselectivity, however, was not observed for decanoyl-trypsin. These results suggest that the chemically modified alpha-chymotrypsin by addition of hydrophobic groups has promoted enantioselectivity for the hydrolysis of hydrophobic esters.     

ATP-dependent allosteric DNA enzymes

Effector-activated ribozymes that respond to small organic molecules have previously been generated by appending binding species (aptamers) to ribozymes. In order to determine if deoxyribozymes can similarly be activated by effector molecules, we have appended an anti-adenosine aptamer to a selected deoxyribozyme ligase. The resultant constructs are specifically activated by ATP. Optimization of the joining region resulted in ligases that are activated up to 460-fold by ATP. The selected deoxyribozyme catalyzes ligation largely via a templating mechanism. Effector activation is surprisingly achieved by suppression of the rate of the background, templated ligation reaction in the absence of the effector molecule, probably by misalignment of the oligonucleotide substrates. This novel allosteric mechanism has not previously been observed for nucleic-acid catalysts and is rare even in protein catalysts.     

Enantioselective O- and N-nitroso aldol synthesis of tin enolates. Isolation of three BINAP-silver complexes and their role in regio- and enantioselectivity

The selective generation of three different silver-BINAP catalysts has been achieved via the proper combination of metal/ligand ratio and/or choice of silver salt. Indeed, the X-ray crystallographic study has been used to gain insight into the structure of the catalyst. After the evaluation of each species, 1:1 (AgX.(R)-BINAP) complex was found to be the real active species in the O-selective nitroso aldol reaction. On the other hand, a systematic survey of solvent has shown that the optimal 2:1 (AgX.(R)-BINAP) complex is the effective catalyst in the N-selective pathway. Thus, a new method of generation of the silver-BINAP catalyst and the synthetic transformations provide not only new insights into the developing area of catalytic enantioselective nitroso aldol synthesis but also clear guidance for the design of an effective catalyst.     

The origin of regio- and enantioselectivity in the Rh/chiral 1,4-diene-catalyzed addition of phenylboronic acid to enones: insights from DFT

A density functional theory study of the addition of phenylboronic acid to cyclohexenone catalyzed by chiral 1,4-diene-Rh(I) catalyst reveals that 1,4-addition is thermodynamically preferred. The enthalpy-driven enantioselection occurs during the carborhodation step and not the enone binding step, as previously proposed. The chiral ligand selectively destabilizes the disfavored transition state by making it "more early".     

Novel concurrent redox cascades of (R)- and (S)-carvones enables access to carvo-lactones with distinct regio- and enantioselectivity

Within this study, we investigated a one-pot enzymatic redox cascade composed of different enoate reductases (5 EREDs from diverse bacterial origins) and various Baeyer-Villiger monooxygenases (4 BVMOs) with complementary regioselectivity that enabled access to six out of eight carvo-lactone stereoisomers starting from readily available natural carvones. Applicability of this two-step cascade was demonstrated by preparative scale experiments yielding up to 76% of the desired chiral carvolactone.     

Modified DNA aptamer that binds the (R)-isomer of a thalidomide derivative with high enantioselectivity

A thalidomide-binding aptamer was produced by systematic evolution of ligands by exponential enrichment from a library of non-natural DNA in which thymidine had been replaced with a modified deoxyuridine bearing a cationic functional group via a hydrophobic methylene linker at the C5 position. The additional functional group in the modified DNA aptamer could improve stability against nucleases and increase the binding affinity to thalidomide. The selected aptamer could recognize thalidomide enantioselectively, although a racemic thalidomide-attached gel was used for the selection. Surface plasmon resonance and fluorescence titration studies revealed that the selected modified DNA aptamer and a truncated version bound with an (R)-thalidomide derivative with high enantioselectivity, but not with the (S)-form. The modified group in the DNA aptamer is indispensable for the interaction with thalidomide, as the corresponding natural type DNA bearing the same base sequence showed no binding affinity with (R)- nor (S)-thalidomide. Computational sequence analysis suggested that the selected apatamer (108 mer) could fold into a three-way junction structure; however, truncation of this aptamer (31 mer) revealed that the thalidomide-binding site is a hairpin-bulge region that is a component of one of the arms of the three-way junction structure. The Kd value of the truncated 31 mer aptamer for binding with the (R)-thalidomide derivative was 1.0 microM estimated from fluorescence titration study. The aptamer that can recognize a single enantiomer of thalidomide will be useful as a biochemical tool for the analysis and study of the biological action of thalidomide enantiomers.     

Directed evolution of hybrid enzymes: Evolving enantioselectivity of an achiral Rh-complex anchored to a protein

The concept of utilizing the methods of directed evolution for tuning the enantioselectivity of synthetic achiral metal-ligand centers anchored to proteins has been implemented experimentally for the first time.     

Studies on the regio- and stereoselectivity of halohydroxylation of 1,2-allenyl sulfides or selenides

It was observed that the halohydroxylation of 1,2-allenyl sulfides or selenides with Br2 (CuBr2 or NBS) or I2 and water demonstrated a fairly good regioselectivity (i.e., the C=C bond that is remote from the S or Se atom was halohydroxylated with the halogen atom connecting to the middle carbon atom and the hydroxyl group connecting to the non-S terminal carbon or Se-substituted terminal carbon atom of the allene moiety), leading to the synthesis of synthetically important 3-organosulfur or seleno-2-haloallylic alcohols. The stereoselectivity depends on the nature of X+ and S or Se, showing a Z-selectivity with the matched Lewis acid-base pair.     

Enhancing protease enantioselectivity by ionic liquids based on chiral- or ω-amino acids

Ionic liquids (ILs) carrying anions of chiral- or ω-amino acids were prepared. The enzymatic hydrolysis of phenylalanine methyl ester was studied in aqueous solutions of these ILs. These ILs were found capable of stabilizing the protease activity and enantioselectivity at low concentrations. Interestingly, higher ees and yields of l-phenylalanine were generally observed in ILs based on d-amino acids rather than in those derived from l-isomers. The reason could be that d-amino acids are more kosmotropic than l-isomers. Meanwhile, the IL–D₂O solution was able to further enhance the enzymatic resolution, when comparing with that in an IL–H₂O system.     

Allylcyanation of alkynes: regio- and stereoselective access to functionalized di- or trisubstituted acrylonitriles

Allyl cyanides are found to add across alkynes in the presence of a nickel catalyst prepared from Ni(cod)2 and P(4-CF3-C6H4)3 in situ to give variously functionalized di- or trisubstituted acrylonitriles in highly stereoselective manners possibly via a pi-allylnickel species as an intermediate. alpha-Siloxyallyl cyanides also react at the gamma-position of a cyano group with both internal and terminal alkynes having various functional groups to give silyl enol ethers, which give the corresponding aldehydes or ketones upon hydrolysis.     

Enhanced enantioselectivity of Bacillus coagulans in the hydrolysis of 1,2-O-isopropylidene glycerol esters by thermal knock-out of undesired enzymes

The enantioselective hydrolysis of different (RS)-1,2-O-isopropylidene glycerol esters has been achieved with whole cells of Bacillus coagulans NCIMB 9365 furnishing the (S)-alcohol as the major enantiomer. The reaction is catalysed by a thermostable cell-bound carboxylesterase and improvement of the enantioselectivity has been achieved by heat treatment of the whole cells, which causes the knock-outs a non-enantioselective competing enzyme. Thermally-treated cells hydrolysed (RS)-1,2-O-isopropylidene glycerol esters with high enantioselectivity, the highest enantiomeric ratio (80–100) being observed for the benzoate. The biocatalyst displayed good stability and could be re-used after filtration for 12 cycles before showing significant loss of activity; repeated biotransformation batches allowed the recovery of 9.55 g/L of enantiomerically pure (S)-isopropylideneglycerol benzoate starting from 24.0 g/L of the racemic mixture.     

Asymmetric synthesis of l-proline regio- and stereoselectively labelled with deuterium

A synthesis of l-proline where all of the ring methylenes are stereoselectively labelled with deuterium is described. A catalytic deuteration of protected 3,4-dehydro-l-proline using transition metal catalyst followed by RuO₄-oxidation gave a [3,4-D₂]pyroglutamic acid derivative. A syn-selective deuteration of the aminal derived from the pyroglutamate with Et₃SiD–BF₃·OEt₂ furnished (2S,3S,4R,5S)-[3,4,5-D₃]proline. The present procedure is also applied to the synthesis of the corresponding (2S,3S,4R,5R)-isomer.