Synthesis of l‐threitol‐based crown ethers and their application as enantioselective phase transfer catalyst in Michael additions

A few new l ‐threitol‐based lariat ethers incorporating a monoaza‐15‐crown‐5 unit were synthesized starting from diethyl l ‐tartrate. These macrocycles were used as phase transfer catalysts in asymmetric Michael addition reactions under mild conditions to afford the adducts in a few cases in good to excellent enantioselectivities. The addition of 2‐nitropropane to trans ‐chalcone, and the reaction of diethyl acetamidomalonate with β‐nitrostyrene resulted in the chiral Michael adducts in good enantioselectivities (90% and 95%, respectively). The substituents of chalcone had a significant impact on the yield and enantioselectivity in the reaction of diethyl acetoxymalonate. The highest enantiomeric excess (ee ) values (99% ee ) were measured in the case of 4‐chloro‐ and 4‐methoxychalcone. The phase transfer catalyzed cyclopropanation reaction of chalcone and benzylidene‐malononitriles using diethyl bromomalonate as the nucleophile (MIRC reaction) was also developed. The corresponding chiral cyclopropane diesters were obtained in moderate to good (up to 99%) enantioselectivities in the presence of the threitol‐based crown ethers.     

A Highly Enantioselective Access to Chiral 1‐(β‐Arylalkyl)‐1H‐1,2,4‐triazole Derivatives as Potential Agricultural Bactericides

A series of chiral 1‐(β‐arylalkyl)‐1H‐1,2,4‐triazole derivatives has been designed as potential antifungal agents. The target triazoles have been synthesized by using a chiral auxiliary as a controlling reagent. All of the compounds were obtained with high ee values, reaching up to 99%. Preliminary bioassay results have revealed that most of the synthesized compounds display significantly higher fungicidal activities against the species Fusarium oxysporium, Rhizoctonia solani, Botrytis cinereapers, Gibberella zeae, Dothiorella gregaria, and Colletotrichum gossypii than the commercial agent triadimefon. Moreover, some of the enantiomers have been found to display significant differences in activity.     

Green synthesis of enantiopure (S)‐1‐(benzofuran‐2‐yl)ethanol by whole‐cell biocatalyst

Optically active aromatic alcohols are valuable chiral building blocks of many natural products and chiral drugs. Lactobacillus paracasei BD87E6, which was isolated from a cereal‐based fermented beverage, was shown as a biocatalyst for the bioreduction of 1‐(benzofuran‐2‐yl) ethanone to (S)‐1‐(benzofuran‐2‐yl) ethanol with highly stereoselectivity. The bioreduction conditions were optimized using L. paracasei BD87E6 to obtain high enantiomeric excess (ee) and conversion. After optimization of the bioreduction conditions, it was shown that the bioreduction of 1‐(benzofuran‐2‐yl)ethanone was performed in mild reaction conditions. The asymmetric bioreduction of the 1‐(benzofuran‐2‐yl)ethanone had reached 92% yield with ee of higher than 99.9% at 6.73 g of substrate. Our study gave the first example for enantiopure production of (S)‐1‐(benzofuran‐2‐yl)ethanol by a biological green method. This process is also scalable and has potential in application. In this study, a basic and novel whole‐cell mediated biocatalytic method was performed for the enantiopure production of (S)‐1‐(benzofuran‐2‐yl)ethanol in the aqueous medium, which empowered the synthesis of a precious chiral intermediary process to be converted into a sophisticated molecule for drug production.     

Enantioselective addition of phenylacetylene to aldehydes catalyzed by polymer‐supported titanium(IV) complexes of β‐hydroxy amides

A series of polymer‐supported chiral β‐hydroxy amides and C₂‐symmetric β‐hydroxy amides have been synthesized and successfully used for the enantioselective addition of phenylacetylene to aldehydes. High yields (up to 93%) and enantioselectivities (up to 92% ee) were achieved by using polymer‐supported chiral β‐hydroxy amide 4b . The resin 4b is reused four times, giving the product with enantioselectivity 80% ee. Fortunately, it is found that this heterogonous system is suitable not only for aromatic aldehydes but also aliphatic aldehyde. Chirality, 2010. © 2009 Wiley‐Liss, Inc.     

Construction of Quaternary Stereocenters: Asymmetric α‐Amination of Branched Aldehydes Catalyzed by Monoimide Substituted Cyclohexane‐1,2‐Diamines

A highly efficient enantioselective α‐amination of branched aldehydes catalyzed by chiral imide monosubstituted 1,2‐diamine derivatives was reported to afford the quaternary stereogenic centers in excellent yields (up to 99%) and enantioselectivities (up to 97% ee). Chirality 25:668–672, 2013. © 2013 Wiley Periodicals, Inc     

A novel chiral aliphatic–aromatic diamine promoted direct,highly enantio‐ and diastereoselective Michael addition of cyclohexanone to nitroolefins under solvent‐free conditions

A series of new highly efficient chiral aliphatic–aromatic diamine catalysts have been designed and successfully applied to the asymmetric Michael addition of cyclohexanone with nitroolefins under solvent‐free conditions without any acidic additives. The desired adducts were obtained in high yields with excellent enantio‐ and diastereoselectivities of syn products (up to >99% ee, >99:1 dr). Chirality 2010. © 2010 Wiley‐Liss, Inc     

Optimizing the attractiveness of pheromone baits used for trapping the four‐eyed spruce bark beetle Polygraphus poligraphus

Bark beetles have caused extensive damage to forests in central Sweden during the past decade, and the four‐eyed spruce bark beetle, Polygraphus poligraphus, seems to be involved. However, its role in these bark beetle outbreaks is still not clear. The purpose of this study was to develop an efficient pheromone bait for P. poligraphus, which would make it possible to study the species more carefully and thereby contribute to protect exposed forests in an environmentally friendly way. Three field studies were conducted in 2015, 2016 and 2018 in Medelpad, county of Västernorrland, Sweden. The pheromone of P. poligraphus, (?)‐terpinen‐4‐ol, was tested at different release rates and in different enantiomeric purities, to find the most attractive formulation for the beetles. It was also tested in combination with racemic frontalin, a compound which has previously been shown to produce a synergistic effect together with (?)‐terpinen‐4‐ol of low enantiomeric purity; 52% ee. Other compounds, chosen based on responses from electroantennographic studies, were also tested in an attempt to find additional attractants and repellents for P. poligraphus. The most attractive treatment tested was enantiomerically pure (?)‐terpinen‐4‐ol (99% ee). When the enantiomeric purity was lower (50% ee), the trap catches was lowered to levels comparable to the catches for unbaited control traps. A strong synergistic effect with frontalin was observed for (?)‐terpinen‐4‐ol of low enantiomeric purity (50% ee) but not for the enantiomerically pure compound (99% ee). The release rate of (?)‐terpinen‐4‐ol (99% ee) was shown to be an important factor. For the combination of frontalin and (?)‐terpinen‐4‐ol (50% ee), the attraction seemed strongest when (?)‐terpinen‐4‐ol was released at a higher rate than frontalin. An interesting and novel result was that a repellent compound, α‐terpineol, was identified in our studies. Our results from field studies and electroantennography recordings also indicate that (+)‐terpinen‐4‐ol is a repellent for P. poligraphus.     

Functional characterization of salt‐tolerant microbial esterase WDEst17 and its use in the generation of optically pure ethyl (R)‐3‐hydroxybutyrate

The two enantiomers of ethyl 3‐hydroxybutyrate are important intermediates for the synthesis of a great variety of valuable chiral drugs. The preparation of chiral drug intermediates through kinetic resolution reactions catalyzed by esterases/lipases has been demonstrated to be an efficient and environmentally friendly method. We previously functionally characterized microbial esterase PHE21 and used PHE21 as a biocatalyst to generate optically pure ethyl (S)‐3‐hydroxybutyrate. Herein, we also functionally characterized one novel salt‐tolerant microbial esterase WDEst17 from the genome of Dactylosporangium aurantiacum subsp. Hamdenensis NRRL 18085. Esterase WDEst17 was further developed as an efficient biocatalyst to generate (R)‐3‐hydroxybutyrate, an important chiral drug intermediate, with the enantiomeric excess being 99% and the conversion rate being 65.05%, respectively, after process optimization. Notably, the enantio‐selectivity of esterase WDEst17 was opposite than that of esterase PHE21. The identification of esterases WDEst17 and PHE21 through genome mining of microorganisms provides useful biocatalysts for the preparation of valuable chiral drug intermediates.     

Direct asymmetric Michael addition of thioether‐based aryl sulfanyl‐propan‐2‐one to nitroalkenes catalyzed by a chiral amine‐thiourea bifunctional organocatalyst

Although the organocatalytic direct asymmetric Michael reactions of carbonyl compounds to nitroalkenes have been investigated intensely, the Michael reaction of the thioether‐based donors remains a rather undeveloped field. This work concerns the asymmetric Michael addition of aryl sulfanyl‐propan‐2‐one to nitroalkenes with benzoic acid as an additive, and chiral amine‐thiourea as a bifunctional organocatalyst. The reactions provided the highly functionalized chiral adducts with excellent enantioselectivities (up to 96% ee) and good yields. Moreover, the further transformed products exhibited excellent diastereoselectivity. Chirality, 2010. © 2010 Wiley‐Liss, Inc.     

Determination of enantiomeric excess of nipecotic acid as 1‐(7‐nitrobenzo[c][1,2,5]oxadiazol‐4‐yl) derivatives

For the enantiopure synthesis of novel chiral GABA uptake inhibitors, nipecotic acid ( 1 ) is an important key precursor. To characterize accurately the pharmacological activity of these interesting target compounds, the determination of the correct enantiomeric purity of nipecotic acid as the starting material is indispensable. In this report, a sensitive high‐performance liquid chromatography (HPLC) based method for the separation and quantitation of both enantiomers of nipecotic acid as 1‐(7‐nitrobenzo[c ][1,2,5]oxadiazol‐4‐yl) derivatives ( 5 ) on a Chiralpak ID‐3 column (Daicel, Illkirch, France) was established. UV/Vis‐detection at 490 nm was chosen to ensure a selective determination of even highly enantioenriched samples. Reliability was demonstrated by validation of specificity, linearity, lower limit of quantification (LLOQ), accuracy, and precision. By spiking highly enantiopure samples with small amounts of racemic rac ‐ 5 , it was proven that the established HPLC method is able to detect even slight changes in enantiomeric excess (ee) values. Thus, accurate determination of ee values up to 99.87% ee for (R )‐ 5 and 99.86% ee for (S )‐ 5 over a linear concentration range of 1– 1500 μM for (R )‐ 5 and of 1– 1455 μM for (S )‐ 5 could be demonstrated.     

Recyclable Merrifield resin‐supported organocatalysts containing pyrrolidine unit through A3‐coupling reaction linkage for asymmetric Michael addition

Merrifield resin‐supported pyrrolidine‐based chiral organocatalysts A ? D through A³‐coupling reaction linkage have been developed and found to be highly effective catalysts for the Michael addition reaction of ketones with nitrostyrenes. The reactions generated the corresponding products in good yields (up to 92%), excellent enantioselectivities (up to 98% ee), and high diastereoselectivities (up to 99:1 dr). In addition, the catalysts can be reused at least five times without a significant loss of catalytic activity and stereoselectivity. Chirality, 2010. © 2009 Wiley‐Liss, Inc.     

Resolution,determination of enantiomeric purity and chiral recognition mechanism of new xanthone derivatives on (S,S)‐whelk‐O1 stationary phase

The enantioresolution and determination of the enantiomeric purity of 32 new xanthone derivatives, synthesized in enantiomerically pure form, were investigated on (S ,S )‐Whelk‐O1 chiral stationary phase (CSP). Enantioselectivity and resolution (α and R_S) with values ranging from 1.41–6.25 and from 1.29–17.20, respectively, were achieved. The elution was in polar organic mode with acetonitrile/methanol (50:50 v/v ) as mobile phase and, generally, the (R )‐enantiomer was the first to elute. The enantiomeric excess (ee ) for all synthesized xanthone derivatives was higher than 99%. All the enantiomeric pairs were enantioseparated, even those without an aromatic moiety linked to the stereogenic center. Computational studies for molecular docking were carried out to perform a qualitative analysis of the enantioresolution and to explore the chiral recognition mechanisms. The in silico results were consistent with the chromatographic parameters and elution orders. The interactions between the CSP and the xanthone derivatives involved in the chromatographic enantioseparation were elucidated.     

Electrophilic [N—F]+ catalysed asymmetric allylation of (E)‐N,1‐diphenylmethanimine

New efficient catalysts based on electrophilic N‐fluoro quaternary ammonium salts are reported for catalytic allylation of (E)‐N,1‐diphenylmethanimine. The chiral version of the catalyst based on cinchonidine (F‐CD‐BF₄) shows high catalytic activity with approximately 94% ee and TOF (>800 h^?1). The F‐CD‐BF₄ is prepared from cinchonidine and Selectfluor by one‐step transfer fluorination.     

Axially chiral N‐(o‐aryl)‐4‐hydroxy‐2‐oxazolidinone derivatives from diastereoselective reduction of N‐(o‐aryl)‐2,4‐oxazolidinediones: Thermally interconvertible atropisomers via ring‐chain‐ring tautomerization

The reduction of the axially chiral N‐(o‐aryl)‐5,5‐dimethyl‐2,4‐oxazolidinediones by NaBH₄ yielded axially chiral N‐(o‐aryl)‐4‐hydroxy‐5,5‐dimethyl‐2‐oxazolidinone enantiomers having a chiral center at C‐4, with 100% diastereoselectivity as has been shown by their ¹H and ¹³C NMR spectra and by enantioselective HPLC analysis. The resolved enantiomeric isomers were found to interconvert thermally through an aldehyde intermediate formed upon ring cleavage via a latent ring‐chain‐ring tautomerization. It was found that the rate of enantiomerization depended on the size and the electronic effect of the ortho substituent present on the aryl ring bonded to the nitrogen of the heterocycle. Chirality 2010. © 2009 Wiley‐Liss, Inc.     

Enantioselective resolution of Rac‐terbutaline and evaluation of optically pure R‐terbutaline hydrochloride as an efficient anti‐asthmatic drug

Terbutaline is a β₂‐adrenoceptor agonist for the treatment of asthma and chronic obstructive pulmonary disease (COPD). Among the two isomers of terbutaline (TBT 2), R‐isomer was found to be the potent enantiomer in generating therapeutic effect, while S‐isomer has been reported to show side effects. In this study, R‐terbutaline hydrochloride (R‐TBH 6) was synthesized through chiral resolution from the racemic terbutaline sulfate (rac‐TBS 1) with 99.9% enantiomeric excess (ee) in good overall yield (53.6%). Further, R‐TBH 6 nebulized solution was prepared in half dosage of Bricanyl®, which is a marketed product of racemic terbutaline and evaluated in vitro aerosol performance and in vivo anti‐asthmatic effect on guinea pigs via. pulmonary delivery. From the investigation, it is evident that R‐TBH 6 nebulized solution of half dosage performed similar fine aerosol characteristics and anti‐asthmatic effect with Bricanyl®.     

Resolution of (±)‐β‐methylphenylethylamine by a novel chiral stationary phase for Pirkle‐type column chromatography

In this study, a new Pirkle‐type chiral column stationary phase for resolution of β‐methylphenylethyl amine was described by using activated Sepharose 4B as a matrix, L ‐tyrosine as a spacer arm, and an aromatic amine derivative of L ‐glutamic acid as a ligand. The binding capacities of the stationary phase were determined at different pH values (pH = 6, 7, and 8) using buffer solutions as mobile phase, and enantiomeric excess (ee) was determined by HPLC equipped with chiral column. The ee was found to be 47%. Chirality, 2010. © 2009 Wiley‐Liss, Inc.     

Synthesis and application of 3‐substituted (S)‐BINOL as chiral ligands for the asymmetric ethylation of aldehydes

A series of (S)‐BINOL ligands substituted at the 3 position with some five‐membered nitrogen‐containing aromatic heterocycles were effectively prepared and their catalytic abilities were evaluated in the asymmetric addition of diethylzinc to benzaldehyde in the presence of titanium tetraisopropoxide. Under the optimized reaction conditions, titanium complex of (S)‐3‐(1H‐benzimidazol‐1‐yl)‐1,1′‐bi‐2‐naphthol was found to be the most efficient catalyst for asymmetric ethylation of various aldehydes to generate the corresponding secondary alcohols in up to 99% yield and 91% ee. Chirality, 2010. © 2010 Wiley‐Liss, Inc.     

Green Asymmetric Synthesis: β‐Amino Alcohol‐Catalyzed Direct Asymmetric Aldol Reactions in Aqueous Micelles

The ability of chiral β‐amino alcohols to catalyze the direct asymmetric aldol reaction was evaluated for the first time in aqueous micellar media. A family of cheap and easily accessible β‐amino alcohols, obtained in one step from naturally occurring amino acids, was shown to successfully catalyze the asymmetric aldol reaction between a series of ketones and aromatic aldehydes. These aldol reactions furnished the corresponding β‐hydroxy ketones with up to 93% isolated yield and 89% ee. (S)‐2‐phenylglycinol and Triton X‐100 proved to be the best organocatalyst and surfactant, respectively. Chirality 25:119–125, 2013. © 2012 Wiley Periodicals, Inc.     

The resolution of acyclic P‐stereogenic phosphine oxides via the formation of diastereomeric complexes: A case study on ethyl‐(2‐methylphenyl)‐phenylphosphine oxide

As an example of acyclic P‐chiral phosphine oxides, the resolution of ethyl‐(2‐methylphenyl)‐phenylphosphine oxide was elaborated with TADDOL derivatives, or with calcium salts of the tartaric acid derivatives. Besides the study on the resolving agents, several purification methods were developed in order to prepare enantiopure ethyl‐(2‐methylphenyl)‐phenylphosphine oxide. It was found that the title phosphine oxide is a racemic crystal‐forming compound, and the recrystallization of the enantiomeric mixtures could be used for the preparation of pure enantiomers. According to our best method, the (R)‐ethyl‐(2‐methylphenyl)‐phenylphosphine oxide could be obtained with an enantiomeric excess of 99% and in a yield of 47%. Complete racemization of the enantiomerically enriched phosphine oxide could be accomplished via the formation of a chlorophosphonium salt. Characterization of the crystal structures of the enantiopure phosphine oxide was complemented with that of the diastereomeric intermediate. X‐ray analysis revealed the main nonbonding interactions responsible for enantiomeric recognition.     

A Simple Method for Resolution of Endo‐/Exo‐Monoesters of Trans‐Norborn‐5‐Ene‐2,3‐Dicarboxylic Acids Into Their Enantiomers

Separation of optical isomers obtainable from trans‐norborn‐5‐ene‐2,3‐dicarboxylic acid methyl and tert‐butyl monoesters was performed by crystallization of the respective salts prepared with (R)‐ and (S)‐1‐phenylethylamine. Starting from racemic endo‐monomethyl ester of trans‐norborn‐5‐ene‐2,3‐dicarboxylic acid, prepared by partial hydrolysis of the cyclopentadiene‐dimethyl fumarate adduct, the corresponding (2R,3R)‐endo‐monoester was isolated in 97% enantiomeric excess (ee) yield after seven repeated crystallizations from tetrachloromethane. Starting from exo‐mono‐tert‐butyl ester of the same acid, prepared by alcoholysis of the cyclopentadiene‐maleic anhydride adduct followed by isomerization, (2R,3R)‐exo‐monoester was isolated in >98% ee yield after four repeated crystallizations from ethanol. Crystallization of the acids from the mother liquor containing (S)‐1‐phenylethylamine yielded products with inverse stereochemical configuration. Chirality 27:151–155, 2015. © 2014 Wiley Periodicals, Inc.