Kinetic Resolution Research Articles

Palladium‐Catalyzed Enantioselective C−H Olefination to Access Planar‐Chiral Cyclophanes by Dynamic Kinetic Resolution

AbstractPlanar‐chiral cyclophanes have received increasing attention for drug discovery and catalyst design. However, the catalytically asymmetric synthesis of planar‐chiral cyclophanes has been a longstanding challenge. We describe the first Pd(II)‐catalyzed enantioselective C−H olefination of prochiral cyclophanes. The low rotational barrier of less hindered benzene ring in the substrates allows the reaction to proceed through a dynamic kinetic resolution. This approach exhibits broad substrate scope, providing the planar‐chiral cyclophanes in high yields (up to 99 %) with excellent enantioselectivities (up to >99 % ee). The ansa chain length scope studies reveal that the chirality of the cyclophanes arises from the bond rotation constraint of the benzene ring around the macrocycle plane, rather than the C−N axis. The C−H activation approach is also applicable to the late‐stage modification of bioactive molecules and pharmaceuticals.

Palladium-Catalyzed Enantioselective C-H Olefination to Access Planar-Chiral Cyclophanes by Dynamic Kinetic Resolution.

Planar-chiral cyclophanes have received increasing attention for drug discovery and catalyst design. However, the catalytically asymmetric synthesis of planar-chiral cyclophanes has been a longstanding challenge. We describe the first Pd(II)-catalyzed enantioselective C-H olefination of prochiral cyclophanes. The low rotational barrier of less hindered benzene ring in the substrates allows the reaction to proceed through a dynamic kinetic resolution. This approach exhibits broad substrate scope, providing the planar-chiral cyclophanes in high yields (up to 99%) with excellent enantioselectivities (up to >99% ee). The ansa chain length scope studies reveal that the chirality of the cyclophanes arises from the bond rotation constraint of the benzene ring around the macrocycle plane, rather than the C-N axis. The C-H activation approach is also applicable to the late-stage modification of bioactive molecules and pharmaceuticals.

Reshaping the Substrate Binding Pocket of β-Amino Acid Dehydrogenase for the Synthesis of Aromatic β-Amino Acids.

By reshaping the substrate-binding pocket of β-amino acid dehydrogenase (β-AADH), some variants were obtained with up to 2560-fold enhanced activity toward the model substrates (S)-β-homophenylalanine and (R)-β-phenylalanine. A few aromatic β-amino acids were prepared with >99% ee and high isolated yields via either kinetic resolution of racemates or reductive amination of the corresponding β-keto acids. This work expands the catalytic capability of β-AADHs and highlights their practical application in the synthesis of pharmaceutically relevant β-amino acids.

Flash Thermal Racemization for Chemoenzymatic Dynamic Kinetic Resolution of Chiral Amines

Flash Thermal Racemization for Chemoenzymatic Dynamic Kinetic Resolution of Chiral Amines

Chiral Brønsted Acid Catalysed Kinetic Resolution of Secondary and Tertiary 3‐Amido Allylic Alcohols

AbstractA synthetic method to prepare enantioenriched secondary and tertiary 3‐amido allylic alcohols efficiently that relies on their stereoablative enantioselective kinetic resolution (KR) catalysed by a chiral Brønsted acid is described. Achieved at room temperature, the stereoablative asymmetric KR protocol was shown to exhibit excellent functional group tolerance and provided product yields up to 50%, and ee and s values up to 99% and >200. The synthetic utility of the divergent catalytic protocol was exemplified by the 1 mmol scale preparation of one tertiary example and its reduction or bromination to give the corresponding 1,3‐amino alcohol and 2‐bromo‐1,3‐amino alcohol derivatives. The suggested asymmetric KR pathways provide rare instances of accomplishing enantioselective discrimination through the dehydration or imine‐enamine tautomerisation/retro‐aza‐aldol reaction of the respective 2° and 3° alcohol substrates.

Carbene-Catalyzed Direct O-Functionalization of Ketone: Atroposelective Access to Non-C2-Symmetric Binaphthyls.

Disclosed here is NHC-catalyzed direct intermolecular trapping of the ketone oxygen atom with the acyl azolium intermediate. The overall reaction is a dynamic kinetic resolution process that converts ketone to the corresponding enol ester with well-controlled axial chirality. Our reaction eventually affords non-C2-symmetric binaphthyl derivatives with important applications, such as in the area of asymmetric catalysis.

Lipase mediated new chemo-enzymatic synthesis of (RS)-, (R)-, and (S)-bunolol.

The β-adrenergic receptor blocking agents are an important class of drug molecules. The present study reports a new chemo and chemo-enzymatic synthetic process for (RS)-, (R)-, and (S)-bunolol, one of the potent β-adrenergic receptor blocker. In chemo-enzymatic process, CAL L4777 lipase was employed for enantioselective kinetic resolution to synthesize the enantiopure (R)-alcohol and (S)-ester from the corresponding racemic alcohol. Thereafter, the corresponding (R)-alcohol and deacylated (S)-ester were treated with tert-butylamine to produce (S)- and (R)-bunolol, respectively. In chemical approach, epichlorohydrin (RS-, R-, and S-) was used as a starting material via respective (RS)-, (S)-, and (R)-glycidyl ether as intermediates for synthesis of enantiomeric (RS)-, (R)-, and (S)-bunolol. In comparison between two approaches, it was found that the chemo-enzymatic process was more effective and resulted in enantiomeric excess of 98% with 35% yield.

Copper(I)-catalyzed diastereodivergent construction of vicinal P-chiral and C-chiral centers facilitated by dual “soft-soft” interaction

A copper(I)-catalyzed diastereodivergent addition of phosphinothioates (HP(S)ROR') to α,β-unsaturated thioamides is disclosed, which constructs vicinal P-chiral and C-chiral centers in generally high diastereo- and enantioselectivities. In this reaction, the kinetic resolution of HP(S)ROR' occurs, which affords (R)-HP(S)PhOMe in high enantioselectivity in the addition with (R,R)-Ph-BPE as the ligand. It is found through control experiment that dual “soft-soft” interaction, indicated by both 1H and 31P NMR experiments, is indispensable in the present reaction. The first “soft-soft” interaction between copper(I) catalyst and HP(S)ROR' enables facile deprotonation to generate nucleophilic [Cu]-SPROR' species. The second one between the [Cu]-SPROR' species and α,β-unsaturated thioamides facilitated the nucleophilic addition. Finally, both Michael adducts and (R)-HP(S)PhOMe are easily converted to synthetically useful compounds.

Rapid-Kinetics Degron Benchmarking Reveals Off-Target Activities and Mixed Agonism-Antagonism of MYB Inhibitors

The transcription factor (TF) MYB is a critical transcriptional dependency of acute myeloid leukemia (AML) where it has been a long-term focus of drug development efforts. However, development of transcriptional inhibitors is hampered by the lack of a generally accepted functional cellular readout to characterize their target specificity and on-target activity. We reasoned that the specificity and on-target activity of MYB inhibitors would be best evaluated in a rapid kinetics system where their immediate transcriptional effects are benchmarked against targeted MYB degradation. We began by engineering a chemical degron model and established the direct gene-regulatory functions of MYB in a nascent transcriptomics assay. We fused the C-terminus of MYB with an FKBP12 F36V (dTAG) domain and a fluorescent tag by a homozygous knock-in of the FKBP12 F36V-coding DNA sequence into the endogenous MYB locus in MV411 cells. The resulting fusion protein was nearly completely degraded after a 1-hour treatment with dTAG V-1, a highly specific VHL-engaging PROTAC. As expected, degradation of MYB resulted in a profound loss of cell viability, consistent with the effects of a genetic MYB knockout in AML cells. To establish MYB's direct gene-regulatory functions, we measured genome-wide rates of nascent mRNA synthesis by thiol (SH)-linked alkylation metabolic sequencing of RNA (SLAM-seq) after a 1-hour MYB degradation. Defining direct targets as those genes which displayed significant changes in transcription rates (FDR<0.0), we detected 450 genes directly regulated by MYB. Of these, 319 genes were downregulated and 131 genes were upregulated, indicating that MYB acts as a transcriptional activator and repressor of these genes, respectively. In parallel, we performed SLAM-seq in AML cells treated with six agents reported as MYB inhibitors: MYBMIM, celastrol, naphthol AS-E phosphate, mebendazole, plumbagin and all-trans retinoic acid (ATRA). For comparison, we treated MV411 cells with the BET bromodomain inhibitor JQ1, which has been reported to indirectly inhibit MYB by interfering with its expression and function. The MYB inhibitors displayed a dramatic variability in the number of dysregulated genes, varying from 19 (naphthol) to 1123 (MYBMIM). In contrast, JQ1 caused widespread and bimodal effects, altering the transcription rates of >2000 genes in both directions. On pairwise overlap, the MYB inhibitors captured a relatively minor portion of the direct MYB program (between 5-155, or 1-34%, of the 450 direct MYB targets), compared with 43% of the MYB program captured by JQ1. Nonetheless, the MYB inhibitors displayed a stronger specificity for MYB target genes compared to JQ1, because they elicited much narrower responses. Thus, while MYB inhibitors displayed strong enrichments for primary MYB targets, they did not attenuate the entire MYB transcriptional program, and significant portions of their activities appeared to be off-target. In addition, the inhibitors displayed bimodal effects on the transcription of MYB-regulated genes, further activating subsets of genes that were repressed by MYB degradation, and vice versa. These observations uncovered unexpected activities of MYB inhibitors as context-dependent mixed agonists-antagonists. We conclude that MYB has a narrow direct transcriptional program in AML cells, which is only partially captured by the existing MYB inhibitors, which display partial specificity for MYB targets and act as mixed agonists-antagonists. Our work will serve as a proof-of-principle demonstration of the use of rapid kinetic resolution of TF degron models for a more precise characterization of the target specificity and efficacy of TF-directed inhibitors. Benchmarking of TF modulators against degron models in nascent transcriptomics assays should be considered as an important criterion in their functional characterization.

Encapsulation of lipase in zeolitic imidazolate framework-8 induced by polyethyleneimine to form a honeycomb structure with enhanced activity

Embedding an enzyme in the metal-organic frameworks (MOFs) gives good protection to the fragile enzyme. However, this may also restrain the enzyme activity because of the decreased substrate accessibility. Encapsulation of lipase AK from Pseudomonas fluorescens for preparing the enzyme-MOF composite (AK@ZIF-8-PEI) was performed through a new strategy based on polyethyleneimine and enzyme induced in-situ growth of zeolitic imidazolate framework-8 (ZIF-8). Characterizations indicate that AK@ZIF-8-PEI has a honeycomb structure and the hierarchical porosity formed during the preparation, which provides adequate mass transfer channels for catalytic applications. Activity evaluation shows that specific activity of AK@ZIF-8-PEI is 8-fold than the commercial lipase powder. AK@ZIF-8-PEI is demonstrated as an efficient catalyst in kinetic resolution of α-naphthol enantiomers through enantioselective transesterification. Within 12 h, the conversion and substrate enantiomeric excess (ees) reaches 49.8 % and 96.4 %, achieving an improved resolution than previous researches.

Kilogram-Scale Preparation of the Amino Alcohol Fragment of Selgantolimod by Enzymatic Resolution of an α,α-Disubstituted Amino Ester

Kilogram-Scale Preparation of the Amino Alcohol Fragment of Selgantolimod by Enzymatic Resolution of an α,α-Disubstituted Amino Ester

Novel Kinetic Resolution of Thiazolo-Benzimidazolines Using MAO Enzymes.

The kinetic resolution of racemic 1H,3H-thiazolo[3,4-a]benzimidazoline (TBIM) heterocycles was achieved using E. coli whole cells expressing the MAO-N D11 enzyme. Several cosolvents were screened using TBIM 2a as the substrate. DMF was the best cosolvent, affording the pure enantiomer (+)-2a in 44% yield, 94% ee. The stereochemistry of TBIM was predicted by means of ab initio calculations of optical rotation and circular dichroism spectra. The reaction scope was investigated for 11 substituted (±) TBIM using an optimized protocol. The best yield and % ee were obtained for the nonsubstituted 2a. Among the substituted compounds, the 5-substituted-TBIM showed better % ee than the 4-substituted one. The small electron donor group (Me) led to better % ee than the electron-withdrawing groups (-NO2 and -CO2Et), and the bulky naphthyl group was detrimental for the kinetic resolution. Docking experiments and molecular dynamics (MD) simulations were employed to further understand the interactions between MAO-N D11 and the thiazolo-benzimidazoline substrates. For 2a, the MD showed favorable positioning and binding energy for both enantiomers, thus suggesting that this kinetic resolution is influenced not only by the active site but also by the entry tunnel. This work constitutes the first report of the enzymatic kinetic resolution applied to TBIM heterocycles.

Inverting the stereoselectivity of nitrilase toward racemic isobutylsuccinonitrile via modulating the key residues in the substrate binding pocket

A “polarity scanning” strategy was successfully performed in the present study to identify the critical residues for inverting the stereoselectivity of R-selective nitrilase as the product with S-type was the expected one. Mutant W187N exhibiting distinct stereoselectivity was generated with S-configuration (92.3 %, e.e). A combined mutant W187N/S189N demonstrated a further stereoselectivity improvement with S-selectivity (98.1 %, e.e) was finally acquired when the combination mutation was carried out regarding to site Trp187 and Ser189. Steered molecular dynamic (SMD) stimulation results indicated that the orientation of reacted β-cyano group in the substrate with distinct configurations switched dramatically, which resulted in the inverted stereoselectivity of nitrilase. The present study would not only deepen the theoretical basis for the kinetic resolution of dinitrile substrates by nitrilase, but also provide a case for other enzymes to synthesize pharmaceutical and chemical intermediates with enantio‑complementarity.

Organocatalytic Dynamic Kinetic Resolution Enabled Asymmetric Synthesis of Phosphorus-Containing Chiral Helicenes.

The catalytic asymmetric synthesis of phosphorus-containing helicenes remains a formidable challenge, presumably due to the lack of rational design of substrates, right choice of reactions together with highly effective catalysis systems. Herein, we disclosed an efficient and practical DKR-involving (dynamic kinetic resolution) cascade strategy toward synthesizing a novel family of phosphorus-installing helicenes by peptide-mimic phosphonium salt (PPS) catalysis. The sequential process of PPS-catalyzed phospha-Michael attack and copper salt-facilitated aromatization led to the formation of unprecedented phosphorus-containing oxa[5]helicene scaffolds. A wide variety of substrates bearing an assortment of functional groups were compatible with this protocol, furnishing the expected helical compounds in high yields and excellent stereoselectivities. Additionally, the helical products could be conveniently elaborated to promising phosphine ligands with perfectly retained helical chirality, which turned out to be highly efficient chiral ligands in transition metal-catalyzed reactions. These findings not only expand the current library of phosphorus-containing helicenes but also offer insights to explore other challenging scaffolds with molecular chirality.

Organocatalytic Dynamic Kinetic Resolution Enabled Asymmetric Synthesis of Phosphorus‐Containing Chiral Helicenes

AbstractThe catalytic asymmetric synthesis of phosphorus‐containing helicenes remains a formidable challenge, presumably due to the lack of rational design of substrates, right choice of reactions together with highly effective catalysis systems. Herein, we disclosed an efficient and practical DKR‐involving (dynamic kinetic resolution) cascade strategy toward synthesizing a novel family of phosphorus‐installing helicenes by peptide‐mimic phosphonium salt (PPS) catalysis. The sequential process of PPS‐catalyzed phospha‐Michael attack and copper salt‐facilitated aromatization led to the formation of unprecedented phosphorus‐containing oxa[5]helicene scaffolds. A wide variety of substrates bearing an assortment of functional groups were compatible with this protocol, furnishing the expected helical compounds in high yields and excellent stereoselectivities. Additionally, the helical products could be conveniently elaborated to promising phosphine ligands with perfectly retained helical chirality, which turned out to be highly efficient chiral ligands in transition metal‐catalyzed reactions. These findings not only expand the current library of phosphorus‐containing helicenes but also offer insights to explore other challenging scaffolds with molecular chirality.

Enantioselective synthesis of the bioactive flavanones lonchocarpol A and 6-(1,1-dimethylallyl)naringenin by catalytic asymmetric transfer hydrogenation

Practical enantioselective syntheses of the natural products lonchocarpol A and 6-(1,1-dimethylallyl)naringenin as well as their optical antipodes are reported. A rhodium-catalyzed asymmetric transfer hydrogenation of readily available racemic substrates with virtually complete kinetic resolution served as the key step. Solvent-controlled, clay-catalyzed [1,3] or [3,3] shifts of prenyl aryl ethers were used to install the C-6 prenyl or 1,1-dimethylallyl substituent of the target flavanones.

Palladium-Catalyzed Atroposelective Kinetic C-H Olefination and Allylation for the Synthesis of C-B Axial Chirality.

The direct C-H functionalization of 1,2-benzazaborines, especially asymmetric version, remains a great challenge. Here we report a palladium-catalyzed enantioselective C-H olefination and allylation reactions of 1,2-benzazaborines. This asymmetric approach is a kinetic resolution (KR), providing various C-B axially chiral 2-aryl-1,2-benzazaborines and 3-substituted 2-aryl-1,2-benzazaborines in generally high yields with excellent enantioselectivities (selectivity (S) factor up to 354). The synthetic potential of this reaction is showcased by late-stage modification of complex molecules, scale-up reaction, and applications.

Palladium‐Catalyzed Atroposelective Kinetic C−H Olefination and Allylation for the Synthesis of C−B Axial Chirality

AbstractThe direct C−H functionalization of 1,2‐benzazaborines, especially asymmetric version, remains a great challenge. Here we report a palladium‐catalyzed enantioselective C−H olefination and allylation reactions of 1,2‐benzazaborines. This asymmetric approach is a kinetic resolution (KR), providing various C−B axially chiral 2‐aryl‐1,2‐benzazaborines and 3‐substituted 2‐aryl‐1,2‐benzazaborines in generally high yields with excellent enantioselectivities (selectivity (S) factor up to 354). The synthetic potential of this reaction is showcased by late‐stage modification of complex molecules, scale‐up reaction, and applications.

Enhancing a Sphaerobacter thermophilus ω-transaminase for kinetic resolution of β- and γ-amino acids

Sphaerobacter thermophilus synthesizes an ω-transaminase (ω-TA) that allows the production of enantiomerically pure β-amino acids. To obtain ω-TA variants with a higher activity and more favorable properties for industrial use, we modified critical amino acid residues either in the catalytic center or in a previously proposed signature motif critical for aromatic β-amino acid ω-TAs. Seventeen different variants of this enzyme were generated and their activity was examined with four β-amino acids and one γ-amino acid, and compared with the wildtype’s activity. Among all variants, seven showed up to ninefold higher activity with at least one of the tested substrates. For most of these seven variants, the temperature optimum was even lower as in the wild type enzyme, with keeping a high temperature stability, making them more valuable for industrial purposes. Our results indicate that for the production of enantiomerically pure β-amino acids replacement of critical amino acid residues in the proposed signature motif of ω-TAs is a more effective strategy than modifying their catalytic center. Another finding was, that the proposed motif is not only suitable for aromatic amino acid ω-TAs, because some of the variants have a higher activity with β-alanine or β-leucine than with aromatic β-amino acids.Graphical

Crystal structure simulation of cationic cholesteric Liquid-Crystal polyesters. Non-Viral vectors

The crystal structure of six cationic cholesteric liquid crystal polyesters with choline, ammonium, and amide end groups are reported: PTOBEE-Choline [(C26H20O8)n-C5H13N]; PTOBDME-Choline [(C34H36O8)n-C5H13N]; PTOBEE-Ammonium [(C26H20O8)n-C5H13N]; PTOBDME-Ammonium [(C34H36O8)n-C5H13N]; PTOBEE-Amide (C26H19O9N)n and PTOBUME-Amide (C33H33O9N)n, have demonstrated non-viral vector properties in gene therapy by delivering DNA to the cell nucleus. They were synthesized by functionalization of precursor racemic polyesters via a condensation reaction between 4,4′-(terephthaloyl-di-oxydibenzoic chloride) (TOBC) and the racemic mixture of glycol (R-S-1,2-butanediol) or (R-S-1,2-dodecanediol), respectively. The resulting helical macromolecular structures exhibit unexpected optical activity and chiral morphology, previously attributed to the kinetic resolution of one preferable helical conformer of the possible four observed by NMR. Conformational analysis and energy minimization are performed on the four helical conformer monomeric models which are then polymerized to obtain helical chains. Three levels of chirality were observed in these polymer structures. Crystal models are built in triclinic primitive P1cells, with their molecular chains oriented parallel to the Z-axis (c lattice parameter equal to the helical pitch length), the cell parameters are obtained by performing geometry optimization tasks. A raw estimation of the relative weight of the four possible helical conformers on each synthetic cationic polymer has been performed on ideal mixtures considering their approximate known crystal structure with their experimental X-ray powder diffraction by Powder Quantitative Phase Analysis. The results would confirm the theory of the preferable recrystallization of a diastereoisomer, among the four possible, during the synthetic procedures. These findings shed light on the observed optical activity and provide insights into the crystal structure of these cationic polymers. The morphology of the cationic polymers is studied by ESEM in wet samples and crystals after drying the solvent.