In Situ Acetaldehyde Synthesis for Carboligation Reactions

The enzyme 4-oxalocrotonate tautomerase (4-OT) can promiscuously catalyze various carboligation reactions using acetaldehyde as a nucleophile. However, the highly reactive nature of acetaldehyde requires intricate handling, which can impede its usage in practical synthesis. Therefore, we investigated three enzymatic routes to synthesize acetaldehyde in situ in one-pot cascade reactions with 4-OT. Two routes afforded practical acetaldehyde concentrations, using an environmental pollutant, trans-3-chloroacrylic acid, or a bio-renewable, ethanol, as starting substrate. These routes can be combined with 4-OT catalyzed Michael-type additions and aldol condensations in one pot. This modular systems biocatalysis methodology provides a stepping stone towards the development of larger artificial metabolic networks for the practical synthesis of important chemical synthons.     

4-Piperidones from Enantiomerically Pure Enones. Synthesis of 6-alkylsubstituted 4-oxo-pipecolic acid derivatives

The cyclization of the enantiomerically pure α,β-unsaturated ketone 1 , an aliphatic aldehyde 3 and ammonia or benzylamine gives diastereoselectively cis-piperidones 4/4 ′. Although only modest yields were obtained, the presented cyclization has the advantage of being a single step reaction. The products 4 can serve as precursors for all-cis-4-hydroxy-piperidones 5 and novel pipecolic acid derivates 9 .     

Systematic screening for catalytic promiscuity in 4-oxalocrotonate tautomerase: enamine formation and aldolase activity

The enzyme 4-oxalocrotonate tautomerase (4-OT) is part of a catabolic pathway for aromatic hydrocarbons in Pseudomonas putida mt-2, where it catalyzes the conversion of 2-hydroxy-2,4-hexadienedioate(1) to 2-oxo-3-hexenedioate(2). 4-OT is a member of the tautomerase superfamily, a group of homologous proteins that are characterized by a β-α-β structural fold and a catalytic amino-terminal proline. In the mechanism of 4-OT, Pro1 is a general base that abstracts the 2-hydroxyl proton of 1 for delivery to the C-5 position to yield 2. Here, 4-OT was explored for nucleophilic catalysis based on the mechanistic reasoning that its Pro1 residue has the correct protonation state (pK(a) ～6.4) to be able to act as a nucleophile at pH 7.3. By using inhibition studies and mass spectrometry experiments it was first demonstrated that 4-OT can use Pro1 as a nucleophile to form an imine/enamine with various aldehyde and ketone compounds. The chemical potential of the smallest enamine (generated from acetaldehyde) was then explored for further reactions by using a small set of selected electrophiles. This systematic screening approach led to the discovery of a new promiscuous activity in wild-type 4-OT: the enzyme catalyzes the aldol condensation of acetaldehyde with benzaldehyde to form cinnamaldehyde. This low-level aldolase activity can be improved 16-fold with a single point mutation (L8R) in 4-OT's active site. The proposed mechanism of the reaction mimicks that used by natural class-I aldolases and designed catalytic aldolase antibodies. An important difference, however, is that these natural and designed aldolases use the primary amine of a lysine residue to form enamines with carbonyl substrates, whereas 4-OT uses the secondary amine of an active-site proline as the nucleophile catalyst. Further systematic screening of 4-OT and related proline-based biocatalysts might prove to be a useful approach to discover new promiscuous carbonyl transformation activities that could be exploited to develop new biocatalysts for carbon-carbon bond formation.     

Regio- and Diastereoselective Conjugate Addition of Grignard Reagents to Chiral Fluoroalkyl α,β-Unsaturated N-tert-Butanesulfinyl Ketimines: Synthesis of Optically Active Fluorinated Enamines and Derivatives

A regio- and diastereoselective conjugate addition reaction of Grignard reagents to fluoroalkyl α,β-unsaturated N-tert-butanesulfinyl ketimines was disclosed. A range of different fluoroalkyls and Grignard reagents were well tolerated, giving up to 99% diastereomeric and regioisomeric ratios. This reaction provided a straightforward method for the synthesis of a variety of enantiomerically enriched α-fluorinated enamines and derivatives which are difficult to achieve with other methods.     

超声波辅助下KHSO4催化胺与α,β-不饱和化合物的Aza-Michael反应研究

超声波辅助条件下,水作溶剂,KHSO4有效的催化了脂肪族胺与α,β-不饱和化合物的Aza-Michael共轭加成反应,快速高收率地得到了Michael加成产物β-氨基化合物。该方法除了实验简单、反应速度快及选择性好外,还对环境友好,有助于绿色化学的进程。所有产物的结构均经质谱、红外光谱和核磁共振氢谱分析进行确证。     

Enhancement of the promiscuous aldolase and dehydration activities of 4-oxalocrotonate tautomerase by protein engineering

Double play: The enzyme 4-oxalocrotonate tautomerase (4-OT) catalyzes not only the initial cross-coupling of acetaldehyde and benzaldehyde to yield 3-hydroxy-3-phenylpropanal, but also the subsequent dehydration of this aldol compound to yield cinnamaldehyde as the final product. Mechanism-inspired engineering provided an active site mutant (F50A) with strongly enhanced aldol condensation activity.     

Biocatalytic Potential of the Epoxide Hydrolase from Agrobacterium radiobacter AD1 and a Mutant with Enhanced Enantioselectivity

Optically pure epoxides are useful synthons for a variety of biologically active compounds. The epoxide hydrolase obtained from Agrobacterium radiobacter AD1 hydrolyses racemic aryl epoxides with moderate and aliphatic epoxides with low enantioselectivity. The three‐dimensional structure of this enzyme indicates that two tyrosine residues interact with the epoxide oxygen. Mutating one of these, tyrosine 215, to a phenylalanine (Y215F) resulted in an enzyme with increased enantioselectivity towards aryl epoxides. The relatively strong decrease in activity towards the remaining enantiomers makes this enzyme a much better biocatalyst than the wild‐type enzyme for the preparation of optically pure (S)‐styrene oxide derivatives.     

Vilsmeier–Haack–Arnold formylations of aliphatic substrates with N-chloromethylene-N,N-dimethylammonium salts

The classical electrophilic substitution of activated aromatics with the Vilsmeier–Haack reagent N-chloromethylen-N,N-dimethylammonium chloride (Schemes 1 and 2) has been more recently extended to a great variety of aliphatic substrates, mainly due to the work of Arnold. In this review, a collection of representative examples for these henceforth called Vilsmeier–Haack–Arnold (VHA) formylation reaction of aliphatics is given: the reaction of polymethine cyanines, merocyanines, and other vinylogous iminium salts with the VHA reagent gives, after hydrolysis of the primary substitution products, trialdehydes such as triformylmethane (Scheme 3); VHA reaction with ene-diamines and diene-diamines yields N,N-dialkylaminomalonaldehydes and tetraaldehydes such as 1,1,2,2-tetraformylethane, respectively (Schemes 4 and 5); aldehyde acetals, enol ethers, and carboxylic acids deliver with VHA reagents 2-substituted malonaldehydes (Scheme 6), and α-amino acids give derivatives of the unstable aminomalonaldehyde (Scheme 7); alkenes and polyenes react with VHA reagents to give α,β-unsaturated or higher vinylogous aldehydes (Schemes 8 and 9), and alkenes with donor substituents yield alkylidene-malonaldehydes (Scheme 10); enolizable methyl and methylene ketones produce with VHA reagents 3-chlorovinylaldehydes (Scheme 11). Eventually, the VHA reagent can be used for the intermediate preparation of nucleophilic amino-chlorocarbenes (Scheme 12).     

A simple and efficient thia-Michael addition to α, β-unsaturated ketones catalyzed by Yb(OTf)3 in [bmim][BF4]

Yb(OTf)₃ in an ionic liquid [bmim][BF₄] has been described as an efficient catalyst for the thia-Michael addition of thiols to α,β -unsaturated ketones to give β -aryl-β-mercapto ketones in 82–94% yield and the catalyst along with ionic liquid was recycled and reused.     

Tuning Enzyme Activity for Nonaqueous Solvents: Engineering an Enantioselective “Michaelase” for Catalysis in High Concentrations of Ethanol

Enzymes have evolved to function under aqueous conditions and may not exhibit features essential for biocatalytic application, such as the ability to function in high concentrations of an organic solvent. Consequently, protein engineering is often required to tune an enzyme for catalysis in non-aqueous solvents. In this study, we have used a collection of nearly all single mutants of 4-oxalocrotonate tautomerase, which promiscuously catalyzes synthetically useful Michael-type additions of acetaldehyde to various nitroolefins, to investigate the effect of each mutation on the ability of this enzyme to retain its “Michaelase” activity in elevated concentrations of ethanol. Examination of this mutability landscape allowed the identification of two hotspot positions, Ser30 and Ala33, at which mutations are beneficial for catalysis in high ethanol concentrations. The “hotspot” position Ala33 was then randomized in a highly enantioselective, but ethanol-sensitive 4-OT variant (L8F/M45Y/F50A) to generate an improved enzyme variant (L8F/A33I/M45Y/F50A) that showed great ethanol stability and efficiently catalyzes the enantioselective addition of acetaldehyde to nitrostyrene in 40 % ethanol (permitting high substrate loading) to give the desired γ-nitroaldehyde product in excellent isolated yield (89 %) and enantiopurity (ee=98 %). The presented work demonstrates the power of mutability-landscape-guided enzyme engineering for efficient biocatalysis in non-aqueous solvents.     

手性铜试剂在羰基化合物加成反应中的应用

介绍了有机铜试剂具有价格低廉、毒性小、配位能力好等的优点,总结了有机铜试剂在不饱和羰基化合物的不对称共轭加成反应中的应用,特别是催化不饱和酯、硫酯、砜、醛和酮等α,β-不饱和羰基化合物的不对称共轭加成反应,提出了手性铜试剂催化不对称共轭加成反应在底物选择上仍然具有一定的局限性,该反应的底物主要集中于酮类化合物,而砜类和...     

Trans- α, β-ungesättigte Fettsäuren und deren Reduktionsprodukte mit Lithium-Aluminium-Hydrid und Diisobutyl-Aluminium-Hydrid

Trans-α,β-Unsaturated Fatty Acids and Products formed by their Reduction with Lithium-Aluminium-Hydride and Di-isobutyl-Aluminium-Hydride A method for the preparation of pure methyl esters of α,β-unsaturated fatty acids is described. The reduction of these esters with LiAlH₄ led to poor yields of α,β-unsaturated alcohols. Saturated alkanols and aldehydes were mainly formed. Di-isobutyl aluminium hydride gave near quantitative yields of α,β-unsaturated alcohols. The reaction was studied with the help of gas chromatography using strongly polar stationary phases.     

α,β-不饱和三氟甲基酮与非环状酮的杂Diels-Alder反应研究

探讨了α,β-不饱和三氟甲基酮与另一种非环状酮之间的杂Diels-Alder反应,通过对酸、酸的用量以及溶剂等因素的考察,确定了反应的最优条件。同时实现了不同结构的α,β-不饱和三氟甲基酮与非环状酮的杂Diels-Alder反应,以最高86%的收率获得了含三氟甲基的四氢吡喃酮衍生物,其结构经~1HNMR、~(13)CNMR表征。     

Fourier transform infrared studies on the thermal degradation of polyvinylimidazoles: Part I

Thermal degradation of polyvinylimidazoles on potassium bromide plate has been studied. At 300°C heat treatment, the occurrence of oxidation and dehydrogenation leads to the formation of saturated ketones and unconjugated C?C bonds in the polymer backbone, respectively. The combination of the two types of reactions also yields α,β-unsaturated keto compounds. Then at prolonged heat treatment at 300°C, cleavage of imidazole rings also occurs and α,β-unsaturated cyano structures are formed. At 400°C, a new class of compound, carbodiimides, is being produced while the amount of the α,β-unsaturated cyano compounds is greatly reduced. Similar to polyacrylonitrile, ladder-type structures for degraded PVI(1) and PVI(4) are proposed probable.     

α,β-不饱和酮与金属有机试剂的不对称共轭加成反应的研究进展

综述了近来α,β 不饱和酮与金属有机试剂的不对称共轭加成反应的研究进展。详细介绍了α,β 不饱和酮与金属试剂反应的配体及影响反应的条件。     

Protection of 4-Fluoroanisole from Aromatic Nucleophilic Photosubstitution by Cyclodextrins [1]

Aromatic nucleophilic photosubstitution reactions of 4-fluoroanisole with cyanide and water have been investigated in aqueous α-β-, and γ-cyclodextrin solutions. Although photoreactivity of the α- or β-cyclodextrin-complexed 4-fluoroanisole toward nucleophilic substitution is suppressed almost completely, none of the cyclodextrins had a measurable influence on the rate of 4-fluoroanisole fluorescence quenching by I⁻. In aqueous γ-cyclodextrin, the efficiency of photohydroxylation was increased slightly but the efficiency of photocyanation was unaffected. From Benesi-Hildebrand plots, the dissociation constants of the complexes of 4-fluoroanisole with α- and β-cyclodextrins were obtained. A good correlation between the relative quantum yields and the degree of complexation was found. Explanation for the lack of a marked effect on photosubstitution by γ-cyclodextrin is presented.     

Regioselective Cobalt‐Catalysed Hydrovinylation for the Synthesis of Non‐Conjugated Enones and 1,4‐Diketones

The highly regioselective cobalt‐catalysed 1,4‐hydrovinylation of terminal alkenes with 2‐trimethylsilyloxy‐1,3‐butadiene generates in a stereospecific fashion unsaturated E‐configured silyl enol ether intermediates that are suitable for diastereoselective Mukaiyama‐aldol reactions with bulky aliphatic aldehydes. The acidic hydrolysis of the enol ethers to γ,δ‐unsaturated ketones followed by ozonolysis can be used for the synthesis of various 1,4‐diketones and polycarbonyl derivatives. The 1,4‐diketones and polycarbonyl derivatives were successfully tested for the synthesis of some mono‐ and bis‐pyrrole derivatives. The γ,δ‐unsaturated ketones are useful building blocks (e.g., in natural product synthesis) and can be generated in a one‐pot procedure.     

Tuning the polarity of ADMET derived star-shaped polymers via thiol-ene chemistry

The synthesis of defined star-shaped polymers via head-to-tail acyclic diene metathesis (ADMET) polymerization is described, whereby di(trimethylolpropane)tetraacrylate (4-arm) and dipentaerythritol hexaacrylate (6-arm) served as core units and fatty acid derived 10-undecenyl acrylate as unsymmetric α,ω-diene monomer. The core-first approach was applied to synthesize stars having arms of ten or twenty monomer units exhibiting an α,β-unsaturated ester backbone. Subsequent post-polymerization modification of the α,β-unsaturated esters via base-catalyzed thia-Michael addition is demonstrated. For this, five different thiols were used in a simple and efficient procedure, without observing degradation of the polymer backbone. The polarity of these star-shaped polymers could thus be tuned by choosing different thiols for this modification, as it was shown by determining the octanol water partition coefficients of these polymers by high performance liquid chromatography (HPLC).     

Synthesis of Hydroxylated Biphenyl Derivatives Bearing an α,β-Unsaturated Ketone as a Lead Structure for the Development of Drug Candidates against Malignant Melanoma

A small collection of C₂-symmetric hydroxylated biphenyl derivatives featuring an α,β-unsaturated ketone as a lead structure was prepared, and the capacity of these compounds to act as antiproliferative agents against four human malignant melanoma cell lines was assayed. The prodrug approach was applied in order to improve the delivery of compounds into the cell by modulation of the phenolic hydroxy protecting group. The hydroxylated biphenyl structure bearing an α,β-unsaturated ketone and a phenolic-O-prenylated chain was found to facilitate the delivery of the molecule and interactions with biological targets. Four compounds showed antiproliferative activity resulting in IC₅₀ values in the range of 1.2 to 2.8 μM.     

An Unexpected Promiscuous Activity of 4-Oxalocrotonate Tautomerase: The cis-trans Isomerisation of Nitrostyrene

Serendipitous switch: While exploring cis-nitrostyrene as a potential electrophile in Michael-type addition reactions catalysed by the enzyme 4-oxalocrotonate tautomerase (4-OT), it was unexpectedly found that 4-OT catalyses the isomerisation of cis-nitrostyrene to trans-nitrostyrene (k(cat) /K(m) = 1.9×10(3) M(-1) s(-1) ).