N-hydroxy-γ-lactams are produced through an enzymatic sequence combining a lipase-catalyzed hydroxylamidation with an oxidase/peroxidase-induced ene-type cyclization. This methodology provides a mild and scalable access to N-heterocyclic building blocks from basic γ,δ-unsaturated esters and aqueous hydroxylamine, and its utility is illustrated by the formal total synthesis of the tetracyclic alkaloid cephalotaxine.
The assembly of enzymatic cascades and multi-step reaction sequences represents an attractive alternative to traditional synthetic-organic approaches. The biocatalytic reaction mediators offer not only mild conditions and permit the use of environmentally benign reagents, but the high compatibility of different enzymes promises more streamlined reaction setups. In this study, a triple-enzymatic strategy was developed that enables the direct conversion of γ,δ-unsaturated esters to N-hydroxy-γ-lactam building blocks. Hereby, a lipase-catalyzed hydroxylaminolysis generates hydroxamic acid intermediates that are subsequently aerobically activated by horseradish peroxidase and glucose oxidase to cyclize in an intramolecular nitroso ene reaction. Utilizing the hydroxylaminolysis/ene-cyclization sequence for the preparation of an aza-spirocyclic lactam, the multi-enzymatic methodology was successfully employed in the synthesis of key intermediates en route to alkaloids of the Cephalotaxus family.
Open Access
  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.
Open Access
  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Pig Liver Esterase and Novozym® 435 showed good selectivity for the enzymatic kinetic resolution of cyclic carbonates. Several glycerol-derived carbonates were converted reaching er values of up to 99 : 1. Scalability of the reaction and recyclability of Novozym® 435 were demonstrated. Bioactive products were synthesized in good yields (81–89 %) and selectivity (90 : 10–94 : 6 er) using chiral carbonates as building blocks.
The biocatalytic kinetic resolution of cyclic carbonates derived from glycerol is reported. A selection of 26 esterases and lipases was tested for the asymmetric hydrolysis of the model substrate (epichlorohydrin carbonate) in aqueous medium. Among them, Pig Liver Esterase and Novozym® 435 showed the best selectivity with E=38 and 49, respectively. Both enzymes were employed for the conversion of 12 glycerol derivatives under optimized conditions. The resolution of halogenated carbonates afforded the unconverted enantiomer in up to >99 : 1 er. Furthermore, Novozym® 435 was successfully recycled 10 times without significant loss of activity. Upscaling and isolation of the chiral carbonate was also demonstrated. Subsequent conversion of this chiral building block allowed the direct one-pot synthesis of (S)-Guaifenesin, (S)-Mephenesin and (S)-Chlorphenesin in up to 89 % yield and 94 : 6 er.
Tetrazine-Norbornene ligation through inverse electron-demand Diels-Alder reaction has been employed as a novel strategy to immobilize a peptide-based catalyst onto different mesoporous silica supports. Functionalized silica monoliths as well as silica particles in packed bed reactors have been applied in the enantioselective flow catalysis of the addition reaction between ß-nitrostyrene and n-butanal.
Organocatalysis via the enamine mechanism developed to one of the most relevant tools in carbonyl chemistry and is widely used in asymmetric organic synthesis. In this work, a strategy is presented to conveniently immobilize a peptide-based catalyst on silica supports for use in continuous flow catalysis reactions. A set of different porous silica supports is investigated spanning from mesoporous silica particles with defined pore sizes suitable for packed bed column reactors to silica monoliths with hierarchical meso-macropore spaces. While the silica supports are functionalized with norbornene entities, the peptide-based organocatalyst is modified with a tetrazine moiety, enabling the immobilization via inverse electron-demand Diels-Alder (IEDDA) reaction. The ligation results in catalyst loadings up to 0.2 mmol g-1, without compromising the mesopore network. The catalytic activity of the materials is proven by the asymmetric C−C coupling reaction of n-butanal to ß-nitrostyrene proceeding in high yield and enantioselectivity in both batch and continuous flow setups.
Synthesis
DOI: 10.1055/a-2118-3046
The diverse applicability of diazo compounds as versatile reagents has enlarged the chemical toolbox in organic synthesis. Over the past few decades, transition-metal-catalyzed diazo compound activation has ignited the classical synthetic methodology via utilizing highly reactive metal carbenoid species. Many reviews have also appeared in the literature that show the advantages and disadvantages of metal-catalyzed activation of diazo compounds. Recently, tris(pentafluorophenyl)borane-mediated diazo activation reactions has remodeled this research area due to the potential for mild, environmentally friendly, metal-free, nontoxic reaction conditions, and the diverse reactivity patterns of boranes towards diazo compounds. In this review, we discuss the reactivity of the boron–diazo precursor adducts with compounds using catalytic and stoichiometric halogenated triarylboranes and, the mechanism of N2 release from the diazo reagent. This generates the reactive carbene species as a key intermediate which can further be exploited for O–H, N–H, S–H, and C–H insertions, azide insertion, carbonate transfer, C–C and C=C bond forming reactions, [2+2] or [2+4] cascade cyclization reactions, annulation reactions, etc.1 Introduction2 Diazo Activation Using Stoichiometric Boranes3 Diazo Activation Using Catalytic B(C6F5)3
4 B(C6F5)3-Catalyzed Diazo Activation Reactions5 Conclusions
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany
Article in Thieme eJournals:
Table of contents | Abstract | open access Full text
Hydrolysis of biorenewable muconic acids or their corresponding lactones in high temperature water allows the selective synthesis of levulinic acids. This enables direct access to novel substituted levulinic acids from biomass, creating novel opportunities for industrial applications. 3-Propyllevulinic acid was used to synthesize a novel plasticizer which performed equally well as a commercial, petrochemical phthalate-based benchmark.
Levulinic acid is a key biorenewable platform molecule. Its current chemical production from sugars is plagued by limited yields, char formation and difficult separations. An alternative and selective route starting from muconic acid via simple heating in water at high temperature (180 °C) has been developed. Muconic acid can be obtained from sugars or catechol fermentation. Chemical oxidation of catechol is another possibility which advantageously can also be applied on substituted catechols, hereby providing substituted muconic acids. When applying the disclosed hydrothermal protocol on these substrates hitherto unknown substituted levulinic acids were accessed. In particular, 3-propyllevulinic acid has been synthesized from 4-propylcatechol, prepared from pine wood. This propylated derivative has been used for the synthesis of a 3-propyllevulinate diester, i.e. butane-1,4-diyl bis(4-oxo-3-propylpentanoate), via esterification with 1,4-butanediol. The diester showed superior performance as plasticizer in comparison to the corresponding levulinate diester in both PVC (polyvinyl chloride) and PLA (polylactic acid). It plasticizes equally effective as the notorious commercial phthalate-based benchmark DEHP (di-2-ethylhexyl phthalate) in PVC.
Nature Catalysis, Published online: 31 July 2023; doi:10.1038/s41929-023-00994-5
The generation of nitrogen-centred radicals and their subsequent reaction with control of stereoselectivity is a difficult task in synthetic chemistry. Now, the photoenzymatic production of nitrogen-centred radicals and their use in challenging enantioselective intermolecular radical hydroaminations is reported.
A ketone-catalyzed reaction for the enantioselective oxidation of N atoms was developed and effectively applied to the atroposelective kinetic resolution of QUINAPOs and related compounds. Both highly enantioenriched QUINAPO N-oxide products could be readily converted into QUINAPs without loss of chiral integrity. QUINAPO N-oxide was also shown to act as a chiral Lewis base catalyst in a series of enantioselective transformations.
QUINAPs have emerged as a pivotal class of axially chiral compounds with remarkable features in the stereoinduction of diverse enantioselective transformations. However, the confined substrate range and extravagant price still pose challenges, limiting their broader utilization. Herein, we describe the first atroposelective oxidation of an N atom using a chiral ketone catalyst, allowing the kinetic resolution of QUINAPOs to give both the unreacted substrates and their corresponding N-oxides with excellent enantioselectivity. Importantly, the enantioenriched products can be readily converted into the QUINAP targets without any loss of stereochemical integrity. Mechanistic investigations indicate that a dioxirane, generated through the oxidation of the ketone with oxone, acts as the active catalytic species. Furthermore, we have successfully extended this catalytic system to the kinetic resolution of QUINOLs and the dynamic kinetic transformation of pyridine analogues of QUINAPO possessing a labile stereogenic axis. The practicality of the developed protocol is further demonstrated by the successful application of QUINAPO N-oxide as a Lewis base catalyst in a series of enantioselective transformations.
Unprecedented chemodivergent Staudinger reactions of secondary phosphine oxides (SPO) have been developed. Reagent-controlled 1- or 2-nitrogen atom exclusions from azides have been achieved. Conversion of a chiral SPO to a phosphinic amide was stereoretentive, and the potassium salt of natural product LL-D05139β was synthesized for the first time.
Unprecedented Staudinger reaction modes of secondary phosphine oxides (SPO) and organic azides are herein disclosed. By the application of various additives, selective nitrogen atom exclusion from the azide group has been achieved. Chlorotrimethylsilane mediates a stereoretentive Staudinger reaction with a 2-N exclusion which provides a valuable method for the synthesis of phosphinic amides and can be considered complementary to the stereoinvertive Atherton–Todd reaction. Alternatively, a 1-N exclusion pathway is promoted by acetic acid to provide the corresponding diazo compound. The effectiveness of this protocol has been further demonstrated by the total synthesis of the diazo-containing natural product LL-D05139β, which was prepared as a potassium salt for the first time in 6 steps and 26.5 % overall yield.
The atroposelective formation of isoquinoline heterocycles by a PIII/PV=O redox organocatalyzed Staudinger–aza-Wittig reaction is described. With N2 release and aromatization as ideal driving forces, the method permits the synthesis of a broad range of atropisomeric isoquinolines under mild conditions with enantioselectivities of up to 98 : 2 e.r. and 93 % yield.
Herein, we describe the feasibility of atroposelective PIII/PV=O redox organocatalysis by the Staudinger–aza-Wittig reaction. The formation of isoquinoline heterocycles thereby enables the synthesis of a broad range of valuable atropisomers under mild conditions with enantioselectivities of up to 98 : 2 e.r. Readily prepared azido cinnamate substrates convert in high yield with stereocontrol by a chiral phosphine catalyst, which is regenerated using a silane reductant under Brønsted acid co-catalysis. The reaction provides access to diversified aryl isoquinolines, as well as benzoisoquinoline and naphthyridine atropisomers. The products are expeditiously transformed into N-oxides, naphthol and triaryl phosphine variants of prevalent catalysts and ligands. With dinitrogen release and aromatization as ideal driving forces, it is anticipated that atroposelective redox organocatalysis provides access to a multitude of aromatic heterocycles with precise control over their configuration.
RNA shows immense biomedical potential but applications are limited by its instability and poor cellular uptake. Herein, we present a general approach of reversibly binding peptide dimers that stabilize double-stranded RNA, protect it against degradation by nucleases and enhance cellular RNA uptake. Under reducing conditions peptide dimers monomerize thereby triggering a controlled release of RNA.
Double-stranded RNAs (dsRNA) possess immense potential for biomedical applications. However, their therapeutic utility is limited by low stability and poor cellular uptake. Different strategies have been explored to enhance the stability of dsRNA, including the incorporation of modified nucleotides, and the use of diverse carrier systems. Nevertheless, these have not resulted in a broadly applicable approach thereby preventing the wide-spread application of dsRNA for therapeutic purposes. Herein, we report the design of dimeric stapled peptides based on the RNA-binding protein TAV2b. These dimers are obtained via disulfide formation and mimic the natural TAV2b assembly. They bind and stabilize dsRNA in the presence of serum, protecting it from degradation. In addition, peptide binding also promotes cellular uptake of dsRNA. Importantly, peptide dimers monomerize under reducing conditions which results in a loss of RNA binding. These findings highlight the potential of peptide-based RNA binders for the stabilization and protection of dsRNA, representing an appealing strategy towards the environment-triggered release of RNA. This can broaden the applicability of dsRNA, such as short interfering RNAs (siRNA), for therapeutic applications.
Manganese(I) complexes bearing simple, non-bifunctional bis(NHC) ligands were investigated as hydrogenation catalysts. Applying these complexes with KHBEt3 as additive, various carboxylic acid esters and, additionally, ketones, nitriles, N-heteroarenes and alkenes were successfully hydrogenated. Mechanistic investigations by control experiments and DFT calculations indicate an inner-sphere mechanism and reveal the role of BEt3 as cocatalyst.
The use of bis(NHC) manganese(I) complexes 3 as catalysts for the hydrogenation of esters was investigated. For that purpose, a series of complexes has been synthesized via an improved two step procedure utilizing bis(NHC)-BEt3 adducts. By applying complexes 3 with KHBEt3 as additive, various aromatic and aliphatic esters were hydrogenated successfully at mild temperatures and low catalyst loadings, highlighting the efficiency of the novel catalytic system. The versatility of the developed catalytic system was further demonstrated by the hydrogenation of other substrate classes like ketones, nitriles, N-heteroarenes and alkenes. Mechanistic experiments and DFT calculations indicate an inner sphere mechanism with the loss of one CO ligand and reveal the role of BEt3 as cocatalyst.
Efficient directed evolution protocols for nonribosomal peptide synthetases are needed to adapt the structures of antibiotic peptides for the fight against antimicrobial resistance. Here, an easily reproducible directed evolution protocol was used to reprogram the synthetase for the antibiotic peptide gramicidin S. A few mutations were sufficient to incorporate the non-standard building block piperazic acid instead of proline with perfect specificity.
Engineering of biosynthetic enzymes is increasingly employed to synthesize structural analogues of antibiotics. Of special interest are nonribosomal peptide synthetases (NRPSs) responsible for the production of important antimicrobial peptides. Here, directed evolution of an adenylation domain of a Pro-specific NRPS module completely switched substrate specificity to the non-standard amino acid piperazic acid (Piz) bearing a labile N−N bond. This success was achieved by UPLC-MS/MS-based screening of small, rationally designed mutant libraries and can presumably be replicated with a larger number of substrates and NRPS modules. The evolved NRPS produces a Piz-derived gramicidin S analogue. Thus, we give new impetus to the too-early dismissed idea that widely accessible low-throughput methods can switch the specificity of NRPSs in a biosynthetically useful fashion.
Oxygenation and amination reactions are widespread in synthetic chemistry to produce valuable compounds. Nowadays, the importance of sustainable strategies to introduce oxygen and amino functionalities into organic molecules is increasing. This review discusses recent examples of multi-step biocatalytic cascades involving oxy- and amino-functionalization reactions to produce value-added compounds such as pharmaceuticals and polymer precursors.
Biocatalytic cascades are a powerful tool for building complex molecules containing oxygen and nitrogen functionalities. Moreover, the combination of multiple enzymes in one pot offers the possibility to minimize downstream processing and waste production. In this review, we illustrate various recent efforts in the development of multi-step syntheses involving C−O and C−N bond-forming enzymes to produce high value-added compounds, such as pharmaceuticals and polymer precursors. Both in vitro and in vivo examples are discussed, revealing the respective advantages and drawbacks. The use of engineered enzymes to boost the cascades outcome is also addressed and current co-substrate and cofactor recycling strategies are presented, highlighting the importance of atom economy. Finally, tools to overcome current challenges for multi-enzymatic oxy- and amino-functionalization reactions are discussed, including flow systems with immobilized biocatalysts and cascades in confined nanomaterials.
Nature Chemistry, Published online: 10 August 2023; doi:10.1038/s41557-023-01295-x
Bicyclic lactones are valuable motifs for the synthesis of natural products and bioactive molecules. Now, a palladium-catalysed protocol has been developed to access unsaturated bicyclic lactones in one step from corresponding carboxylic acids. The method demonstrates reverse site selectivity for C(sp3)–H activation to form diverse bicyclic cores.
A bifunctional fusion enzyme with phosphite dehydrogenase and flavin reductase activities has been constructed and characterised. Co-expression of this single polypeptide regeneration system with tryptophan halogenases and carrier-free immobilisation in combiCLEAs facilitates preparative-scale synthesis of halotryptophan from a single cultivation. Extension of the catalytic cascade with a specific dioxygenase enables single cultivation one-pot synthesis of l-4-Cl-kynurenine on a preparative scale.
Flavin-dependent halogenases have attracted increasing interest for aryl halogenation at unactivated C−H positions because they are characterised by high regioselectivity, while requiring only FADH2, halide salts, and O2. Their use in combined crosslinked enzyme aggregates (combiCLEAs) together with an NADH-dependent flavin reductase and an NADH-regeneration system for the preparative halogenation of tryptophan and indole derivatives has been previously described. However, multiple cultivations and protein purification steps are necessary for their production. We present a bifunctional regeneration enzyme for two-step catalytic flavin regeneration using phosphite as an inexpensive sacrificial substrate. This fusion protein proved amenable to co-expression with various flavin-dependent Trp-halogenases and enables carrier-free immobilisation as combiCLEAs from a single cultivation for protein production and the preparative synthesis of halotryptophan. The scalability of this system was demonstrated by fed-batch fermentation in bench-top bioreactors on a 2.5 L scale. Furthermore, the inclusion of a 6-halotryptophan-specific dioxygenase into the co-expression strain further converts the halogenation product to the kynurenine derivative. This reaction cascade enables the one-pot synthesis of l-4-Cl-kynurenine and its brominated analogue on a preparative scale.
Minute amounts of supported Au nanoparticles on TiO2 (0.1 mol%) catalyze quantitatively at room temperature the transfer hydrogenation of azoarenes to hydrazoarenes by ammonia borane complex, in ethanol as solvent. At 1 mol% catalyst loading level and 2.5 molar equivalents of ammonia borane the process occurs instantaneously. The catalyst is recyclable and reusable. In the absence of reducing agent, hydrazoarenes undergo smooth Au/TiO2-catalyzed aerobic oxidation back to azoarenes.
Herein, we report the reaction system based on Pd(II) catalyst and Tl(OCOCF3)3 for the electrophilic C3−H alkenylation of 2,6-dialkokypyridines with alkenes. Synergistic action of the Pd/thioether ligand catalytic system and Tl(III) results in efficient C−H alkenylation of various nitrogen heteroaromatics with complete regioselectivity. Remarkably, the use of a sterically hindered thioether ligand and the Pd(II)/Tl(III) system enables mono-selective C3(5)−H alkenylation of 2,6-dialkoxypyridines, and subsequent introduction of a second, different alkene affords unsymmetrical, multi-substituted pyridine derivatives. Mechanistic studies indicate that the reaction proceeds via electrophilic thallation of heteroarenes followed by Pd-catalyzed Heck-type reaction. The utility of this method is showcased by its application to the late-stage functionalization of structurally complex bioactive molecules having 2,6-dialkoxypyridine as a core structure.
The atroposelective formation of isoquinoline heterocycles by a PIII/PV=O redox organocatalyzed Staudinger–aza-Wittig reaction is described. With N2 release and aromatization as ideal driving forces, the method permits the synthesis of a broad range of atropisomeric isoquinolines under mild conditions with enantioselectivities of up to 98 : 2 e.r. and 93 % yield.
Herein, we describe the feasibility of atroposelective PIII/PV=O redox organocatalysis by the Staudinger–aza-Wittig reaction. The formation of isoquinoline heterocycles thereby enables the synthesis of a broad range of valuable atropisomers under mild conditions with enantioselectivities of up to 98 : 2 e.r. Readily prepared azido cinnamate substrates convert in high yield with stereocontrol by a chiral phosphine catalyst, which is regenerated using a silane reductant under Brønsted acid co-catalysis. The reaction provides access to diversified aryl isoquinolines, as well as benzoisoquinoline and naphthyridine atropisomers. The products are expeditiously transformed into N-oxides, naphthol and triaryl phosphine variants of prevalent catalysts and ligands. With dinitrogen release and aromatization as ideal driving forces, it is anticipated that atroposelective redox organocatalysis provides access to a multitude of aromatic heterocycles with precise control over their configuration.
Pig Liver Esterase and Novozym® 435 showed good selectivity for the enzymatic kinetic resolution of cyclic carbonates. Several glycerol-derived carbonates were converted reaching er values of up to 99 : 1. Scalability of the reaction and recyclability of Novozym® 435 were demonstrated. Bioactive products were synthesized in good yields (81–89 %) and selectivity (90 : 10–94 : 6 er) using chiral carbonates as building blocks.
The biocatalytic kinetic resolution of cyclic carbonates derived from glycerol is reported. A selection of 26 esterases and lipases was tested for the asymmetric hydrolysis of the model substrate (epichlorohydrin carbonate) in aqueous medium. Among them, Pig Liver Esterase and Novozym® 435 showed the best selectivity with E=38 and 49, respectively. Both enzymes were employed for the conversion of 12 glycerol derivatives under optimized conditions. The resolution of halogenated carbonates afforded the unconverted enantiomer in up to >99 : 1 er. Furthermore, Novozym® 435 was successfully recycled 10 times without significant loss of activity. Upscaling and isolation of the chiral carbonate was also demonstrated. Subsequent conversion of this chiral building block allowed the direct one-pot synthesis of (S)-Guaifenesin, (S)-Mephenesin and (S)-Chlorphenesin in up to 89 % yield and 94 : 6 er.
