Α-keto Esters Research Articles

Significantly improved thermostability of a reductase CgKR1 from Candida glabrata with a key mutation at Asp 138 for enhancing bioreduction of aromatic α-keto esters

The keto ester reductase from Candida glabrata, designated as CgKR1, is a highly versatile biocatalyst with a broad substrate spectrum. Its substrate preference was altered by rational design of the active pocket for bioreduction of aromatic α-keto esters in our previous work. However, its practical application is still hindered by its poor thermostability and high biocatalyst loading. In this work, random mutagenesis followed by a saturation mutagenesis was performed aiming to improve its thermostability. Variants M5 (I99Y/D138N/G174A) and M6 (D138N) exhibited an obvious increase in T5015 from 41.8°C of wild-type to 53.4°C and 48.8°C, respectively, indicating the important role of residue 138 for its thermostability. The homologous three-dimensional structures of the wild-type and variant M6 (D138N) revealed that the increased thermostability might be attributed to the increased hydrogen bonds that asparagine forms with other polar amino acids around it. Combination of the most thermostable variant M5 and the substrate specificity-altered variant M1 (F92L/F94V) yielded variant M7 (F92L/F94V/I99Y/D138N/G174A), with not only higher activity toward aromatic α-keto esters, but also an increased T5015 value of 54.6°C. This variant M7 corresponds to an obvious increase in half-life from 2.16min of wild-type to 182min at 50°C. The substrate of methyl ortho-chlorobenzoylformate (CBFM) at 0.5M (100g/L) could be stoichiometrically converted to the product by 1g/L lyophilized cell of variant M7, with 4g/L lyophilized powder of BmGDH for cofactor regeneration within 10h, while the wild-type enzyme gave only 84% conversion even the reaction time was extended to 24h.

Enzymatic synthesis of chiral 2-hydroxy carboxylic acids

Optically active 2-hydroxy carboxylic acids are widely used not only as key building blocks for the synthesis of various organic compounds, but also as chiral resolving reagents. This review focuses on the recent advances in enzymatic preparation of enantiomerically pure 2-hydroxy carboxylic acids, which mainly includes two approaches: (dynamic) kinetic resolution of racemic 2-hydroxy carboxylic acids and derivatives by hydrolase enzymes, and asymmetric reduction of α-keto acids and esters by carbonyl reductases.

ChemInform Abstract: Asymmetric Aldol Reaction Catalyzed by a Chiral Phosphine—Silver Complex.

A catalytic asymmetric aldol reaction of alkenyl trihaloacetates or a γ,δ-unsaturated δ-lactone with aldehydes or an α-keto ester was achieved by using a 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl·silver trifluoromethanesulfonate complex as the chiral precatalyst and N,N-diisopropylethylamine as the base precatalyst in the presence of methanol. Optically active α-alkyl β-hydroxy ketones with enantioselectivities of up to 95 % ee were diastereoselectively obtained in moderate to high yields through the in situ generated chiral silver enolates.

Benzothiazoline: versatile hydrogen donor for organocatalytic transfer hydrogenation.

CONSPECTUS: The asymmetric reduction of ketimines is an important method for the preparation of amines in optically pure form. Inspired by the biological system using NAD(P)H, Hantzsch ester has been extensively employed as a hydrogen donor in combination with chiral phosphoric acid for the transfer hydrogenation of ketimines to furnish amines with high to excellent enantioselectivities. We focused on 2-substituted benzothiazoline as a hydrogen donor in the phosphoric acid catalyzed transfer hydrogenation reaction of ketimines for the following reasons: (1) benzothiazoline is readily prepared just by mixing 2-aminobenzenethiol and aldehyde, (2) both reactivity (hydrogen donating ability) and enantioselectivity would be controlled by tuning the 2-substituent of benzothiazoline, and (3) benzothiazoline can be stored in a refrigerator under inert atmosphere without conceivable decomposition. Both the 2-position of benzothiazoline and the 3,3'-position of phosphoric acid are tunable in order to achieve excellent enantioselectivity. Benzothiazoline proved to be useful hydrogen donor in combination with chiral phosphoric acid for the transfer hydrogenation reaction of ketimine derivatives to afford the corresponding amines with high to excellent enantioselectivities by tuning the 2-substituent of benzothiazoline. Ketimines derived from acetophenone, propiophenone, α-keto ester, trifluoromethyl ketone, and difluoromethyl ketone derivatives proved to be suitable substrates. Benzothiazoline could be generated in situ starting from 2-aminobenzenethiol and aromatic aldehyde in the presence of ketimine and chiral phosphoric acid and successfully worked in the sequential transfer hydrogenation reaction. The reductive amination of dialkyl ketones also proceeded with high enantioselectivities. Use of 2-deuterated benzothiazoline led to the formation of α-deuterated amines with excellent enantioselectivities. The kinetic isotope effect (kH/kC = 3.8) was observed in the competitive reaction between H- and D-benzothiazoline, which explicitly implies that the cleavage of the C-H (C-D) bond is the rate-determining step in the transfer hydrogenation reaction. Benzothiazoline yielded products with higher enantioselectivity in the transfer hydrogenation reaction of ketimines, particularly ketimines derived from propiophenone derivatives, than Hantzsch ester. DFT study elucidated the mechanism, as well as the difference in selectivity, between benzothiazoline and Hantzsch ester. The chiral phosphoric acid activates ketimines and benzothiazoline by means of the Brønsted acidic site (proton) and the Brønsted basic site (phosphoryl oxygen), respectively, to accelerate the hydride transfer reaction.

A phosphetane catalyzes deoxygenative condensation of α-keto esters and carboxylic acids via P(III)/P(V)═O redox cycling.

A small-ring phosphacycle is found to catalyze the deoxygenative condensation of α-keto esters and carboxylic acids. The reaction provides a chemoselective catalytic synthesis of α-acyloxy ester products with good functional group compatibility. Based on both stoichiometric and catalytic mechanistic experiments, the reaction is proposed to proceed via catalytic P(III)/P(V)═O cycling. The importance of ring strain in the phosphacyclic catalyst is substantiated by an observed temperature-dependent product selectivity effect. The results point to an inherent distinction in design criteria for organophosphorus-based catalysts operating via P(III)/P(V)═O redox cycling as opposed to Lewis base (nucleophilic) catalysis.

Stereo-controlled synthesis of polyheterocycles via the diene-transmissive hetero-Diels-Alder reaction of β,γ-unsaturated α-keto esters.

We describe the stereoselective synthesis of polyring-fused heterocyclic compounds based on diene-transmissive hetero-Diels-Alder reactions utilizing β,γ-unsaturated α-keto esters. This protocol involves the initial endo- or exo-selective Diels-Alder (DA) reactions with electron-rich dienophiles, methylenation of the ester carbonyl groups with the Tebbe reagent, and a stereoselective second DA reaction with electron-deficient dienophiles. The use of enantioselective DA reactions in the initial reaction enables access to chiral polyring-fused heterocyclic compounds with multiple chiral centres.

Diastereo- and enantioselective conjugate addition of α-keto esters to nitroalkenes: Complete switch in the enantioselectivity by tuning the metal center or rigidity of the ligand

A series of efficient catalytic systems has been developed to control the dual enantioselectivity of the conjugate addition of α-keto esters to nitroalkenes. The use of the chiral diamine (1S,1’S)-1,1′-biisoindoline as a chiral ligand with either (Cu(OAc)2·H2O or Ni(OAc)2·4H2O as the catalyst provide facile access to the respective enantiomers resulting from the conjugate addition reaction with high levels of enantioselectivity (94% ee vs 93% ee). Furthermore, the use of Cu(OAc)2·H2O as the metal center allowed the enantioselectivity of the conjugate addition reaction to be switched through the tuning of the rigidity of the chiral diamine ligand (94% ee vs 94% ee).

Enantioconvergent synthesis of functionalized γ-butyrolactones via (3 + 2)-annulation.

A dynamic kinetic resolution of β-halo α-keto esters in an asymmetric homoenolate reaction is described. A chiral N-hetereocyclic carbene catalyzes the a3 → d3-umpolung addition of α,β-enals to racemic α-keto esters, forming γ-butyrolactones with three contiguous stereocenters. The addition occurs with high regio-, diastereo-, and enantiocontrol. This methodology constitutes an intermolecular DKR process to set three stereocenters during the key bond forming event.

Mechanistic Studies on Alcoholysis of α-Keto esters

In the course of our studies on the solvent effect inphotochemistry of phenacyl carboxylates, we had a chanceto prepare and work with 2-methylphenacyl phenylgly-oxylate (1a), which we found it very unstable in methanoleven at standing in ambient temperature. The resultingproducts of the decomposition turned out to be α-hydroxy-2-methylacetophenone and methyl phenyl-glyoxylate, meth-anolysis products of 1a (Scheme 1). The seemingly simpleconversion, however, turned out to have a twist in reactionpathways according to our further investigation into thechemical change. The reaction occurred by way of a hemi-acetal of the keto carbonyl of 1a. Such a reaction pathwayfor the alcoholysis of α-keto esters has not been documentedin the literature to the best of our knowledge. Here we reportour preliminary findings on the unprecedented reactionpathway.A hint that the above reaction may not have occurred bydirect methanolysis of the ester group was given to us whena

Highly enantioselective synthesis of 3,4-dihydropyrans through a phosphine-catalyzed [4+2] annulation of allenones and β,γ-unsaturated α-keto esters.

A phosphine-catalyzed novel [4+2] annulation process was devised employing allene ketones as C2 synthons and β,γ-unsaturated α-keto esters as C4 synthons. In the presence of an L-threonine-derived bifunctional phosphine, 3,4-dihydropyrans were obtained in high yields and with virtually perfect enantioselectivities. The synthetic value of the dihydropyran motif was demonstrated by a concise preparation of an anti-hypercholesterolemic agent.

A biomimetic method to synthesise indolo[3,2-a]carbazoles

A simple and biomimetic synthetic strategy for indolo[3,2-a]carbazoles has been developed. Our approach involved the efficient conversion of 2-(3′-indolyl)tryptophan derivatives into the corresponding α-keto esters and the subsequent aromatic cyclisation of these intermediates to construct the characteristic heteroaryl-condensed carbazole core.

Diastereoselective synthesis of functionalized spirocyclopropyl oxindoles via P(NMe2)3-mediated reductive cyclopropanation.

A P(NMe2)3-mediated reductive cyclopropanation reaction of α-keto esters or amides with isatin-derived alkenes has been developed, providing efficient and diastereoselective synthesis of highly functionalized spirocyclopropyl oxindoles bearing two all-carbon quaternary centers. This reaction also represents a complementary and nonmetal-involving protocol for the challenging cyclopropanation of electron-deficient alkenes.

Dynamic kinetic asymmetric cross-benzoin additions of β-stereogenic α-keto esters.

The dynamic kinetic resolution of β-halo α-keto esters via an asymmetric cross-benzoin reaction is described. A chiral N-heterocyclic carbene catalyzes the umpolung addition of aldehydes to racemic α-keto esters. The resulting fully substituted β-halo glycolic ester products are obtained with high levels of enantio- and diastereocontrol. The high chemoselectivity observed is a result of greater electrophilicity of the α-keto ester toward the Breslow intermediate. The reaction products are shown to undergo highly diastereoselective substrate-controlled reduction to give highly functionalized stereotriads.

Phosphine-Catalyzed Annulation Reactions of 2-Butynoate and α-Keto Esters: Synthesis of Cyclopentene Derivatives

We have developed phosphine-catalyzed annulation reactions for the synthesis of highly substituted cyclopentene derivatives from 2-alkynoate and α-keto esters. These transformations involve carbon–carbon bond cleavage of α-keto esters. Preliminary mechanistic studies suggest that, in addition to facilitating carbon–carbon bond formation, the phosphine catalyst plays a role in promoting methanolysis.

ChemInform Abstract: Catalytic Asymmetric Claisen Rearrangement of Gosteli‐Type Allyl Vinyl Ethers: Total Synthesis of (‐)‐9,10‐Dihydroecklonialactone B.

The enantioselective synthesis of (−)-9,10-dihydroecklonialactone B is described. The catalytic asymmetric Claisen rearrangement of a Gosteli-type allyl vinyl ether was utilized to afford an acyclic α-keto ester building block endowed with functionality amenable to the preparation of the carbocyclic target molecule by suitable postrearrangement transformations: A highly diastereoselective Corey–Bakshi–Shibata reduction of a β-chiral α-keto ester and a reductive homologation of an α-hydroxy ester. A transprotection tactic by a chemoselective intramolecular 6-exo-trig iodoetherification enabled regioselective ring-closing alkene metatheses to afford the 5- as well as the 14-membered ring, however, with mixed success in terms of E/Z selectivity.

Versatile iridicycle catalysts for highly efficient and chemoselective transfer hydrogenation of carbonyl compounds in water.

Cyclometalated iridium complexes are shown to be highly efficient and chemoselective catalysts for the transfer hydrogenation of a wide range of carbonyl groups with formic acid in water. Examples include α-substituted ketones (α-ether, α-halo, α-hydroxy, α-amino, α-nitrile or α-ester), α-keto esters, β-keto esters and α,β-unsaturated aldehydes. The reduction was carried out at substrate/catalyst ratios of up to 50 000 at pH 4.5 and required no organic solvent. The protocol provides a practical, easy and efficient way for the synthesis of β-functionalised secondary alcohols, such as β-hydroxyethers, β-hydroxyamines and β-hydroxyhalo compounds, which are valuable intermediates in pharmaceutical, fine chemical, perfume and agrochemical synthesis.

Synthesis of a Common Cyclopentanoid Building Block for the Total Synthesis of Jatropha-5,12- and -6(17),11-dienes by Uncatalyzed Intramolecular Carbonyl­ Ene (ICE) Reaction

We report the scalable synthesis of a common cyclopentanoid building block for 15-hydroxyjatropha-5,12- and -6(17),11-dienes in 11 steps. Key to the synthesis is an uncatalyzed intramolecular carbonyl ene (ICE) reaction of an e,ζ-unsaturated α-keto ester.

Facile regio- and stereoselective metal-free synthesis of all-carbon tetrasubstituted alkenes bearing a C(sp3)-F unit via dehydroxyfluorination of Morita-Baylis-Hillman (MBH) adducts.

Highly E-selective all-carbon tetrasubstituted alkenes with a C(sp(3))-F unit have been synthesized through a dehydroxyfluorination of Morita-Baylis-Hillman (MBH) adducts which can be readily prepared from α,β-unsaturated carbonyl compounds and α-keto esters. A variety of subsequent transformations afforded monofluoromethyl substituted heterocycles in high yields.

Cyclopentanoids by uncatalyzed intramolecular carbonyl ene (ICE) reaction of α-keto esters.

The uncatalyzed intramolecular carbonyl ene (ICE) reaction of substituted ε,ζ-unsaturated α-keto esters to terpenoid-related building blocks has been studied. We found a beneficial effect of a silyl substituent at the ene segment on the kinetics of the ICE reaction. A generalizable and scalable synthesis of ε,ζ-unsaturated α-keto esters from allylic alcohols was developed.

Organocatalytic synthesis of optically active β-branched α-amino esters via asymmetric biomimetic transamination.

This paper describes an efficient asymmetric biomimetic transamination of α-keto esters with a quinine-derived chiral base as the catalyst, giving a variety of β-branched α-amino esters in 50-96% yield and 87-95% ee.