High Stereoselectivity Research Articles

Squalene hopene cyclases and oxido squalene cyclases: potential targets for regulating cyclisation reactions

Squalene hopene cyclases (SHC) convert squalene, the linear triterpene to fused ring product hopanoid by the cationic cyclization mechanism. The main function of hopanoids, a class of pentacyclic triterpenoids in bacteria involves the maintenance of membrane fluidity and stability. 2, 3-oxido squalene cyclases are functional analogues of SHC in eukaryotes and both these enzymes have fascinated researchers for the high stereo selectivity, complexity, and efficiency they possess. The peculiar property of the enzyme squalene hopene cyclase to accommodate substrates other than its natural substrate can be exploited for the use of these enzymes in an industrial perspective. Here, we present an extensive overview of the enzyme squalene hopene cyclase with emphasis on the cloning and overexpression strategies. An attempt has been made to explore recent research trends around squalene cyclase mediated cyclization reactions of flavour and pharmaceutical significance by using non-natural molecules as substrates.

Organocatalytic stereoselective cationic polymerization of alkenyl vinyl ethers and post-functionalization via click chemistry

Vinyl polymer tacticity is critical for regulating the thermal and physical properties of materials. Stereoselective cationic polymerization of vinyl ethers has gained significant progress in recent years, while monomer scope beyond simple alkyl vinyl ethers to access functionalized isotactic polymer materials remains to be explored but highly desirable. Herein, we report the successful development of a stereoselective cationic polymerization of alkenyl vinyl ethers by employing confined Brønsted acid organocatalysis. This organocatalytic system shows high efficiency and stereoselectivity in the polymerization of several alkenyl vinyl ethers, as well as good compatibility to the diolefinic monomers without touching the C=C bonds in the alkenyl part during the cationic polymerization, thus allowing for further modification on these C-C double bonds after polymerization to construct functionalized, isotactic polymer materials. Further, a series of thermoplastic copolymers with tunable properties and variable and predictable contents of side-chain C=C bonds can be synthesized through the corresponding stereoselective copolymerization. Sequential post-polymerization modification of both isotactic homopolymers and copolymers by combining thiol-ene and sulfur(VI) fluoride exchange (SuFEx) click reactions is also demonstrated.

Rare-earth metal complexes supported by indolyl-based NCN-pincer ligand and their efficient catalysis for isoprene polymerization

Rare-earth metal dialkyl complexes [1-CH2N(CH3)2–3-CH2CH2N(CH3)2C8H4N]RE(CH2SiMe3)2 (RE = 1-Y, 2-Er, 3-Yb and 4-Lu) were synthesized by the well-designed indolyl-based proligand 1-CH2N(CH3)2–3-CH2CH2N(CH3)2C8H4NH with rare-earth metal trialkyl complexes (RE(CH2SiMe3)2(THF)2) underwent direct alkane elimination. Single-crystal X-ray analyses and the NMR spectroscopy revealed these dialkyl complexes were analogous mononuclear NCN-pincer geometry via CH activation of 2-sp2 indole. Employing these complexes, upon activation by aluminum alkyls and borate, initiated isoprene polymerization with high conversion and high stereoselectivity (98% conversion of 2000 equivalent of isoprene, 1,4-cis polymers up to 91.5%).

Development of Dynamically Chiral Helical Polymer Catalysts Based on Post-Polymerization Modification of Boronyl Pendants

Catalytic asymmetric synthesis is one of the most efficient methods for the preparation of chiral compounds with high optical purity. Although a variety of robust chiral scaffolds have been developed to achieve efficient chiral catalysts which show high catalytic activities and stereoselectivities, use of dynamic chiral scaffold is desired for the development of advanced chiral catalysts featuring stimuli-responsive properties such as chirality-switching and asymmetric amplification. We have developed chirality-switchable helical polymer ligands bearing monodentate phosphine pendants, which exhibit high enantioselectivities based on solvent-dependent dynamic screw-sense control of poly(quinoxaline-2,3-diyl)s (PQXs) having chiral side chains. However, the application has been limited to palladium-catalyzed asymmetric reactions. This article describes the development of helically chiral polymer catalysts based on post-polymerization modification of bornyl pendants (-B(OH)2) attached to the PQXs. Introduction of 2,2′-bipyridine pendants to chiral PQXs through Suzuki-Miyaura coupling at the boronyl pendants led to the development of chirality-switchable helical polymer ligands for copper-catalyzed asymmetric reactions. In addition, introduction of 4-aminopyridine pendants into the chiral PQXs provided chirality-switchable efficient asymmetric nucleophilic catalysts for kinetic resolution of secondary alcohols and asymmetric Steglich-type acyl rearrangements. Furthermore, by using the boronyl pendants as molecular recognition sites for screw-sense induction to achiral PQXs, a chiral-guest-responsive helical polymer ligand bearing monodentate phosphine pendants was developed for the palladium-catalyzed asymmetric reaction. Using an amino alcohol as a chiral additive, the screw-sense induction proceeded with asymmetric amplification, giving the corresponding product with higher optical purity than that of the chiral additive.

Cobalt-Mediated Radical Cyclizations: Stereoselective Synthesis of Cyclopentanes, Cyclohexanes, and Tetralins

A stereoselective method for assembling five- and six-membered, mono- and binuclear carbocyclic compounds by intramolecular cyclization of Co2(CO)6-complexed propargyl radicals was developed. The formation of requisite bis-propargyl cations A is enabled by a stabilizing metal cluster, while converging transient radicals are stereodirected by a bulky metal core and substituents at the acetylenic termini. With the flexible three- or four-carbon tethers, formation of 1,2-dialkynylcyclopentanes and 1,2-dialkynylcyclohexanes occurred with a high stereoselectivity (91–100% trans-), noticeably enhanced by a γ-TMS group that exclusively directs the cobalt-alkyne cores into diaxial positions (1,3-steric induction). The main cyclization parameters were fully preserved with an aromatic ring as a rigid tether, thus providing rapid access to trans-tetralins with terminal and internal acetylenic moieties. The observed stereoselectivity in cyclization reactions─with both flexible and rigid tethers─was interpreted in terms of the selective stereomutation of the alpha stereocenters occurring in intermediate cations or radicals. Organic ligands were separated from metal cores under oxidative conditions (Ce4+), providing access to vicinally substituted bis-alkynyl cyclopentanes, cyclohexanes, and tetralins of trans-configurations, prone to secondary transformations, and of immediate relevance to the angularly and linearly fused estrogen mimics.

Organocatalytic Assembly Based on Aromatic Donor–Acceptor Interactions for the Asymmetric Aldol Reaction on Water

AbstractAromatic donor modified prolinamide and aromatic acceptor modified thiourea were synthesized. They self‐assembled into a cooperative organocatalytic system through the aromatic donor‐acceptor interaction and catalyzed aldol reactions in high yields and stereoselectivity on water. The aromatic donor‐acceptor interaction was confirmed by mass spectrum, UV‐vis titraition and 1H‐NMR titration experiments.

H2O as the Hydrogen Donor: Stereo-Selective Synthesis of E- and Z-Alkenes by Palladium-Catalyzed Semihydrogenation of Alkynes.

It is very desirable to develop a facile controllable method for selective semihydrogenation of alkynes to alkenes with a cheap and safe hydrogen donor but remains a big challenge. H2O is one of the best choices of the transfer hydrogenation agent of the world, and the development of methods for synthesizing E- and Z-alkenes using H2O as the hydrogen source is worthwhile. In this article, a palladium-catalyzed synthesis of E- and Z-alkenes from alkynes using H2O as the hydrogenation agent was reported. The use of di-tert-butylphosphinous chloride (t-Bu2PCl) and triethanolamine/sodium acetate (TEOA/NaOAc) was essential for the stereo-selective semihydrogenation of alkynes. The general applicability of this procedure was highlighted by the synthesis of more than 48 alkenes, with good yields and high stereoselectivities.

Rapid Enantiomeric Excess Measurements of Enantioisotopomers by Molecular Rotational Resonance Spectroscopy.

Recent work in drug discovery has shown that selectively deuterated small molecules can improve the safety and efficacy for active pharmaceutical ingredients. The advantages derive from changes in metabolism resulting from the kinetic isotope effect when deuterium is substituted for a hydrogen atom at a structural position where rate limiting C-H bond breaking occurs. This application has pushed the development of precision deuteration strategies in synthetic chemistry that can install deuterium atoms with high regioselectivity and with stereocontrol. Copper-catalyzed alkene transfer hydrodeuteration chemistry has recently been shown to have high stereoselectivity for deuteration at the metabolically important benzyl C-H position. In this case, stereocontrol results in the creation of enantioisotopomers-molecules that are chiral solely by virtue of the deuterium substitution-and chiral analysis techniques are needed to assess the reaction selectivity. It was recently shown that chiral tag molecular rotational resonance (MRR) spectroscopy provides a routine way to measure the enantiomeric excess and establish the absolute configuration of enantioisotopomers. High-throughput implementations of chiral tag MRR spectroscopy are needed to support optimization of the chemical synthesis. A measurement methodology for high-throughput chiral analysis is demonstrated in this work. The high-throughput ee measurements are performed using cavity-enhanced MRR spectroscopy, which reduces measurement times and sample consumption by more than an order-of-magnitude compared to the previous enantioisotopomer analysis using a broadband MRR spectrometer. It is also shown that transitions for monitoring the enantiomers can be selected from a broadband rotational spectrum without the need for spectroscopic analysis. The general applicability of chiral tag MRR spectroscopy is illustrated by performing chiral analysis on six enantioisotopomer reaction products using a single molecule as the tag for chiral discrimination.

Innentitelbild: Strukturelle und mechanistische Studien zur Substrat‐ und Stereoselektivität der Indol‐Monooxygenase VpIndA1: Neue Wege für biokatalytische Epoxidationen und Sulfoxidationen (Angew. Chem. 17/2023)

A flavin-dependent group E monooxygenase („GEM“) has been characterized by crystal-structure analysis, computational and kinetic studies to reveal substrate binding and mechanistic details for the first time for this class of enzymes. Based on these data, Dirk Tischler, Norbert Sträter et al. (DOI: 10.1002/ange.202300657) redesigned the substrate binding pocket to synthesize chiral epoxides and sulfoxides with high stereoselectivity, a true treasure of molecular gems.

Inside Cover: Structural and Mechanistic Studies on Substrate and Stereoselectivity of the Indole Monooxygenase VpIndA1: New Avenues for Biocatalytic Epoxidations and Sulfoxidations (Angew. Chem. Int. Ed. 17/2023)

A flavin-dependent group E monooxygenase (“GEM”) has been characterized by crystal-structure analysis, computational and kinetic studies to reveal substrate binding and mechanistic details for the first time for this class of enzymes. Based on these data, Dirk Tischler, Norbert Sträter et al. (DOI: 10.1002/anie.202300657) redesigned the substrate binding pocket to synthesize chiral epoxides and sulfoxides with high stereoselectivity, a true treasure of molecular gems.

Organocatalytic Synthesis of Triflones Bearing Two Non‐Adjacent Stereogenic Centers

AbstractTrifluoromethylsulfones (triflones) are useful compounds for synthesis and beyond. Yet, methods to access chiral triflones are scarce. Here, we present a mild and efficient organocatalytic method for the stereoselective synthesis of chiral triflones using α‐aryl vinyl triflones, building blocks previously unexplored in asymmetric synthesis. The peptide‐catalyzed reaction gives rise to a broad range of γ‐triflylaldehydes with two non‐adjacent stereogenic centers in high yields and stereoselectivities. A catalyst‐controlled stereoselective protonation following a C−C bond formation is key to control over the absolute and relative configuration. Straightforward derivatization of the products into, e.g., disubstituted δ‐sultones, γ‐lactones, and pyrrolidine heterocycles highlights the synthetic versatility of the products.

Organocatalytic Synthesis of Triflones Bearing Two Non-Adjacent Stereogenic Centers.

Trifluoromethylsulfones (triflones) are useful compounds for synthesis and beyond. Yet, methods to access chiral triflones are scarce. Here, we present a mild and efficient organocatalytic method for the stereoselective synthesis of chiral triflones using α-aryl vinyl triflones, building blocks previously unexplored in asymmetric synthesis. The peptide-catalyzed reaction gives rise to a broad range of γ-triflylaldehydes with two non-adjacent stereogenic centers in high yields and stereoselectivities. A catalyst-controlled stereoselective protonation following a C-C bond formation is key to control over the absolute and relative configuration. Straightforward derivatization of the products into, e.g., disubstituted δ-sultones, γ-lactones, and pyrrolidine heterocycles highlights the synthetic versatility of the products.

Semi-Rational Design of Diaminopimelate Dehydrogenase from Symbiobacterium thermophilum Improved Its Activity toward Hydroxypyruvate for D-serine Synthesis

D-serine plays an essential role in the field of medicine and cosmetics. Diaminopimelate dehydrogenase (DAPDH) is a kind of oxidoreductase that can reduce keto acid into the corresponding D-amino acid. Because of its high stereoselectivity and lack of by-product production, DAPDH has become the preferred enzyme for the efficient one-step synthesis of D-amino acids. However, the types of DAPDH with a reductive amination function reported so far are limited. Although the DAPDH from Symbiobacterium thermophilum (StDAPDH) demonstrates reductive amination activity toward a series of macromolecular keto acids, activity toward hydroxypyruvate (HPPA) for D-serine synthesis has not been reported. In this study, we investigated the activity of the available StDAPDH/H227V toward HPPA by measuring the desired product D-serine. After homologous structure modeling and docking analysis concerning the substrate-binding pocket, four residues, D92, D122, M152, and N253, in the active pocket were predicted for catalyzing HPPA. Through single-point saturation mutation and iterative mutation, a mutant D92E/D122W/M152S was obtained with an 8.64-fold increase in enzyme activity, exhibiting a specific activity of 0.19 U/mg and kcat value of 3.96 s−1 toward HPPA. Using molecular dynamics simulation, it was speculated that the increase in enzyme activity might be related to the change in substrate pocket size and the enhancement of the interactions between the substrate and key residues.

Exploring the factors controlling the mechanism and the high stereoselectivity of the polar [3+2] cycloaddition reaction of the N,N'-cyclic azomethine imine with 3-nitro-2-phenyl-2H-chromene. A molecular electron density theory study

Exploring the factors controlling the mechanism and the high stereoselectivity of the polar [3+2] cycloaddition reaction of the N,N'-cyclic azomethine imine with 3-nitro-2-phenyl-2H-chromene. A molecular electron density theory study

Total Synthesis of Trisaccharide Repeating Unit of Staphylococcus aureus Type 8 (CP8) Capsular Polysaccharide.

Herein, we report a highly efficient total synthesis of Staphylococcus aureus type 8 trisaccharide repeating unit in a lesser number of steps and high stereoselectivity. The complex trisaccharide contains rare amino sugars, viz., d-fucosamine, l-fucosamine, and 2-acetamido d-mannuronic acid. The installation of consecutive sterically hindered 1,2-cis glycosidic linkages, especially β-mannosylation, is the key challenge in this synthesis. The total synthesis of target molecule was completed via a longest linear sequence of 18 steps in 7.1% overall yield.

Carboxylate‐Directed Palladium‐Catalyzed Regioselective Mizoroki‐Heck Alkenylation of γ,δ‐Unsaturated Carboxylic Acids

AbstractA carboxylate‐directed palladium‐catalyzed Mizoroki–Heck alkenylation of γ,δ‐unsaturated carboxylic acids with alkenyl bromides is reported. This carboxylate group is effective for chelation to a Pd center, enabling the distal alkenylation of electronically unbiased internal alkenes in the formation of conjugated 1,3‐dienes with high stereoselectivity. In addition, the conjugated 1,3‐diene products can be used for synthetic applications through reduction, epoxidation, methylation, amidation, and cycloaddition.magnified image

Highly Stereoselective Syntheses of α,α-Disubstituted (E)- and (Z)-Crotylboronates

We report herein stereoselective syntheses of α,α-disubstituted (E)- and (Z)-crotylboronates. Starting from α-boryl (E)- or (Z)-crotylboronate, base-mediated alkylation occurred exclusively at the position α to the boryl groups to give targeted boronates while retaining the geometries of the alkenes in the starting crotylboronates. Under proper conditions, the resulting α,α-disubstituted crotylboronates underwent aldehyde addition to give allylated products with high stereoselectivities.

Enantioselective Formal Total Synthesis of Damsin

AbstractEnantioselective formal total synthesis of damsin, which was isolated from Compositae Ambrosia maritima L., is described. The highly enantio- and diastereoselective catalytic Mukaiyama–Michael reaction and subsequent highly stereoselective epimerization shorten the synthetic steps. It is also reported that the enolate, which was formed by the reaction of β-keto sulfone with lithium naphthalenide, reacted with methyl cyanoformate to form a quaternary stereogenic center in high yield with high stereoselectivity. This finding extends the synthetic chemistry starting from β-keto sulfones.

A Practical and Robust Zwitterionic Cooperative Lewis Acid/Acetate/Benzimidazolium Catalyst for Direct 1,4-Additions.

A catalyst type is disclosed allowing for exceptional efficiency in direct 1,4-additions. The catalyst is a zwitterionic entity, in which acetate binds to CuII , which is formally negatively charged and serving as counterion for benzimidazolium. All 3 functionalities are involved in the catalytic activation. For maleimides productivity was increased by a factor >300 compared to literature (TONs up to 6700). High stereoselectivity and productivity was attained for a broad range of other Michael acceptors as well. The polyfunctional catalyst is accessible in only 4 steps from N-Ph-benzimidazole with an overall yield of 96 % and robust during catalysis. This allowed to reuse the same catalyst multiple times with nearly constant efficiency. Mechanistic studies, in particular by DFT, give a detailed picture how the catalyst operates. The benzimidazolium unit stabilizes the coordinated enolate nucleophile and prevents that acetate/acetic acid dissociate from the catalyst.

Asymmetric Hydrogenation of Tetrasubstituted α,β-Unsaturated Ketones: Access to Chiral 2-Substituted Cyclopentyl Aryl Ketones

Asymmetric hydrogenation of tetrasubstituted alkenes is an important but challenging research topic. Herein, we report an efficient iridium-catalyzed asymmetric hydrogenation of tetrasubstituted α,β-unsaturated ketones for the synthesis of chiral 2-substituted cyclopentyl aryl ketones, an important chiral structural motif for the preparation of chiral pharmaceuticals and bioactive molecules. The reaction proceeded very well with good functional group compatibility and delivered the hydrogenated products in high yields and stereoselectivities (up to 99% yield, >20:1 dr and 99% ee). In addition, the reaction could be carried out on a gram-scale, and all four stereoisomers of the hydrogenated products bearing two contiguous stereocenters were obtained. Furthermore, the hydrogenated product can be transformed into the ERβ agonist Erteberel, and the reaction pathway was also studied via deuterium-labelling experiments.