Catalytic Asymmetric Synthesis of Chiral Benzofuranones

Asymmetric Synthesis of Benzofuranones with a C3 Quaternary Center via an Addition/Cyclization Cascade Using Noncovalent N-Heterocyclic Carbene Catalysis

An asymmetric addition/cyclization cascade of amidoesters and iminoquinones is developed using noncovalent N-heterocyclic carbene (NHC) catalysis. The process enables access to various functionalized benzofuranones with an all-carbon quaternary stereocenter with high yields and ee values. The reaction displays a broad substrate scope. Via product modifications, enantioenriched synthesis of biologically relevant spirocyclic lactones and lactams is achieved. Substrate activation via noncovalent interaction with NHC is suggested for the process.

A concise update on the synthetic transformation of aurones via asymmetric cycloaddition, annulation, and Michael/Mannich reactions

This review provides a comprehensive overview of the significance of aurone cores in organic chemistry, highlighting their crucial role as synthetic intermediates. With their innate electrophilic reactivity and convenient accessibility, aurone cores play a vital role in catalysing the development of novel methodologies and facilitating the creation of intricate compounds. The objective of this review is to present a current and insightful compilation that summarizes the progress in aurone synthetic transformations, focusing on diverse cycloaddition ([3 + 2], [4 + 2], [4 + 3], [10 + 2]) and annulation reactions.

Transition Metal-Catalyzed Enantioselective Synthesis of Chiral Five- and Six-Membered Benzo O-heterocycles

Enantiomerically enriched five- and six-membered benzo oxygen heterocycles are privileged architectures in functional organic molecules. Over the last several years, many effective methods have been established to access these compounds. However, comprehensive documents cover updated methodologies still in highly demand. In this review, recent transition metal catalyzed transformations lead to chiral five- and six-membered benzo oxygen heterocycles are presented. The mechanism and chirality transfer or control processes are also discussed in details.

Synthesis of (E)-2-(1-(methoxyimino)ethyl)-2-phenylbenzofuran-3(2H)-ones from (E)-1-(benzofuran-2-yl)ethan-1-one O-methyl oximes and iodobenzenes via a palladium-catalyzed dearomative arylation/oxidation reaction

A new method has been successfully developed that offers a facile and reliable approach for synthesizing (E)-2-(1-(methoxyimino)ethyl)-2-phenylbenzofuran-3(2H)-one, providing 28 compounds. This optimized process enables efficient preparation of a wide range of compounds using readily available (E)-1-(benzofuran-2-yl)ethan-1-one O-methyl oxime and iodobenzene, and provides alternative ideas for the structural modification of benzofuran ketones.

Asymmetric synthesis of spirocyclic isobenzofuranones via a squaramide-catalysed sulfa-Michael desymmetrisation reaction

Enantioselective synthesis of spirocyclohexenone isobenzofuranones has been achieved through an organocatalysed sulfa-Michael desymmetrisation reaction. A cinchona-derived squaramide effectively promotes the desymmetrisation of spirocyclic 2,5-cyclohexadienone isobenzofuranones via the controlled addition of various aryl thiols to generate two vicinal stereocenters with perfect diastereoselectivities and up to very good enantioselectivities.

Cu-Catalyzed N-Arylation of Prolinamides: Access to Enantioenriched DMAP Analogues and Its Application in Black Rearrangement

A CuI-catalyzed C-N coupling reaction of 3-bromo-DMAP with l-prolinamides was conducted at 80 °C in 12-16 h, where the prolinamide's structure had an accelerating effect on the Ullmann-type reaction. This reaction was used to construct chiral 3-amino DMAP catalysts. Furthermore, enantioenriched DMAP analogue C8 was applied in an asymmetric Black rearrangement of 2-benzofuranylcarbonates, affording 3,3-disubstituted benzofuran-2-ones in up to 96% yield and 97% ee.

Pd-catalyzed Aerobic Cross-Dehydrogenative Coupling of Catechols with 2-Oxindoles and Benzofuranones: Reactivity Difference Between Monomer and Dimer

Persistent radicals, which are generated from 2-oxindole or benzofuranone dimers, are useful tools for designing the radical-based cross-coupling reaction to provide molecules containing a quaternary carbon. The persistent radical is accessible from both the dimer and monomer; however, the reactivity difference between these substrates for the oxidative cross-coupling reaction is not fully understood, most likely because of the mechanistic complexity. Here, we present details of an aerobic cross-dehydrogenative coupling (CDC) reaction using various monomers and catechols. UV-Vis analysis and mechanistic control experiments showed that the monomer is less reactive than the dimer under aerobic conditions. Our Pd(II)-BINAP-μ-hydroxo complex significantly improved the reactivity of the monomers for the aerobic CDC reaction with catechols, yielding results comparable to those of the corresponding dimer. The procedure, which enables the generation of the persistent radical in situ, is particularly useful when employing the monomer that is not readily converted to the corresponding dimer.

Visible-Light-Induced Photoredox-Catalyzed Selective 1,4-Difluoroalkylesterification of 1-Aryl-1,3-dienes

A selective three-component 1,4-difluoroalkylesterification of 1-aryl-1,3-dienes enabled by dual photoredox and copper catalysis is described. This protocol uses commercially available CF₂-reagents as radical precursors and carboxylic acids as oxygen-based nucleophiles, providing access to difluoroalkylated allylic esters. This protocol could be extended to intramolecular two-component 1,4-difluoroalkylesterification to access 3-substituted benzobutyrolactones. Preliminary mechanistic studies support a radical process.

Catalyst-Controlled Selectivity Switch in Three-Component Reaction: An NHC-Catalyzed Strategy for the Synthesis of δ-Lactone-Fused Spirobenzofuran-3-ones

An efficient, three-component reaction of aldehydes and benzofuran-3-ones was developed. This process provides a new approach for the preparation of synthetically and biologically important spirobenzofuran-3-one derivatives with moderate-to-good yields under mild conditions. A switch of intramolecular to intermolecular domino Michael-aldol-lactonization leading to differential product formation was achieved by different NHCs catalysis.

Endophytic Fungi as a Source of Antiviral Compounds - A Review

Endophytic fungi are a rich source of secondary metabolites. The interactions between endophytes and their hosts lead to the production of several bioactive substances grouped into different classes, each having a wide variety of effects against various pathogens. The metabolites obtained from these organisms include steroids, alkaloids, phenols, isocoumarins, xanthones, quinones, and terpenoids, among others. These substances are known to have antibiotic, antiparasitic, antifungal, and antiviral effects. This review summarizes secondary metabolites with antiviral effects produced by endophytic fungi and highlights the importance of research in developing novel antiviral substances. We demonstrate that endophytic fungi are a rich source of secondary metabolites that combat pathologies caused by viruses. Optimizing practical and biotechnological screening tools for the research of these metabolites will provide promising drugs to combat these infections.

Bioactive Secondary Metabolites of the Genus Diaporthe and Anamorph Phomopsis from Terrestrial and Marine Habitats and Endophytes: 2010-2019

The genus Diaporthe and its anamorph Phomopsis are distributed worldwide in many ecosystems. They are regarded as potential sources for producing diverse bioactive metabolites. Most species are attributed to plant pathogens, non-pathogenic endophytes, or saprobes in terrestrial host plants. They colonize in the early parasitic tissue of plants, provide a variety of nutrients in the cycle of parasitism and saprophytism, and participate in the basic metabolic process of plants. In the past ten years, many studies have been focused on the discovery of new species and biological secondary metabolites from this genus. In this review, we summarize a total of 335 bioactive secondary metabolites isolated from 26 known species and various unidentified species of Diaporthe and Phomopsis during 2010-2019. Overall, there are 106 bioactive compounds derived from Diaporthe and 246 from Phomopsis, while 17 compounds are found in both of them. They are classified into polyketides, terpenoids, steroids, macrolides, ten-membered lactones, alkaloids, flavonoids, and fatty acids. Polyketides constitute the main chemical population, accounting for 64%. Meanwhile, their bioactivities mainly involve cytotoxic, antifungal, antibacterial, antiviral, antioxidant, anti-inflammatory, anti-algae, phytotoxic, and enzyme inhibitory activities. Diaporthe and Phomopsis exhibit their potent talents in the discovery of small molecules for drug candidates.

Formal Aerobic Oxidative Cross-Coupling of Benzofuranones with Azo Compounds Using Pd-μ-hydroxo Complex

We have developed a catalytic aerobic oxidative dimerization reaction of benzofuranones using a Pd(II)-µ-hydroxo complex. Radical-radical cross-coupling of the resulting dimers with azo compounds enabled the one-pot synthesis of structurally congested benzofuranones having two distinct vicinal all-carbon quaternary centers.

Rhodium-Catalyzed ortho-Selective Carbene C-H Insertion of Unprotected Phenols Directed by a Transient Oxonium Ylide Intermediate

ortho-Selective carbene C-H insertion of unprotected phenols is achieved with dimethyl diazomalonate under the catalysis of [Rh(COD)Cl]₂. After the C-H insertion, subsequent cyclization and electrophilic addition of another carbene molecule affords tris-carboxylate-substituted 2-benzofuranones as the final reaction products. The enantioselective variant has been developed with the use of chiral diene ligands. Mechanistic experiments indicate that a transient oxonium ylide directing group might be responsible for the regiocontrol at the C-H activation step.

Tailoring Tryptophan Synthase TrpB for Selective Quaternary Carbon Bond Formation

We previously engineered the β-subunit of tryptophan synthase (TrpB), which catalyzes the condensation of l-serine and indole to l-tryptophan, to synthesize a range of noncanonical amino acids from l-serine and indole derivatives or other nucleophiles. Here we employ directed evolution to engineer TrpB to accept 3-substituted oxindoles and form C-C bonds leading to new quaternary stereocenters. Initially, the variants that could use 3-substituted oxindoles preferentially formed N-C bonds on N1 of the substrate. Protecting N1 encouraged evolution toward C-alkylation, which persisted when protection was removed. Six generations of directed evolution resulted in TrpB Pfquat with a 400-fold improvement in activity for alkylation of 3-substituted oxindoles and the ability to selectively form a new, all-carbon quaternary stereocenter at the γ-position of the amino acid products. The enzyme can also alkylate and form all-carbon quaternary stereocenters on structurally similar lactones and ketones, where it exhibits excellent regioselectivity for the tertiary carbon. The configurations of the γ-stereocenters of two of the products were determined via microcrystal electron diffraction (MicroED), and we report the MicroED structure of a small molecule obtained using the Falcon III direct electron detector. Highly thermostable and expressed at >500 mg/L E. coli culture, TrpB Pfquat offers an efficient, sustainable, and selective platform for the construction of diverse noncanonical amino acids bearing all-carbon quaternary stereocenters.

Catalytic enantioselective Michael addition of 2-substituted benzofuran-3-ones to 2-enoyl pyridines

An organocatalytic diastereo- and enantioselective synthesis of 2,2'-disubstituted benzofuran-3-ones bearing adjacent quaternary and tertiary stereocenters has been achieved through Michael addition of 2-substituted benzofuran-3-ones to 2-enoyl pyridines. Both the enantiomeric forms of the major diastereomer were obtained using l-proline derived squaramide and quinine derived bis squaramide with excellent yield (up to 98%) and stereoselectivities (up to 97 : 3 dr and 98% ee). The control experiment revealed that the presence and position of nitrogen atoms in the 2-enoylpyridine have played a crucial role in the stereochemical outcome of the product.

3-Hydroxy-3-((3-methyl-4-nitroisoxazol-5-yl)methyl)indolin-2-one as a versatile intermediate for retro-Henry and Friedel–Crafts alkylation reactions in aqueous medium

The first example of a retro-Henry type reaction is reported using 3-hydroxy-3-((3-methyl-4-nitroisoxazol-5-yl)methyl)indolin-2-ones which are prepared via catalyst-free Henry reaction of 3,5-dimethyl-4-nitroisoxazole and isatin.

Highly efficient enantioselective synthesis of bispiro[benzofuran-oxindole-pyrrolidine]s through organocatalytic cycloaddition

The first organocatalyzed [3 + 2] cyclisation of N-2,2,2-trifluoroethylisatin ketimines with aurones has been developed.

Asymmetric Allylic C-H Alkylation via Palladium(II)/ cis-ArSOX Catalysis

We report the development of Pd(II)/ cis-aryl sulfoxide-oxazoline ( cis-ArSOX) catalysts for asymmetric C-H alkylation of terminal olefins with a variety of synthetically versatile nucleophiles. The modular, tunable, and oxidatively stable ArSOX scaffold is key to the unprecedented broad scope and high enantioselectivity (37 examples, avg. > 90% ee). Pd(II)/ cis-ArSOX is unique in its ability to effect high reactivity and catalyst-controlled diastereoselectivity on the alkylation of aliphatic olefins. We anticipate that this new chiral ligand class will find use in other transition metal catalyzed processes that operate under oxidative conditions.

Stereodivergent Synthesis of 3-Aminooxindole Derivatives Containing Vicinal Tetrasubstituted Stereocenters via the Mannich Reaction

A highly enantioselective stereodivergent synthesis of 3-aminooxindole derivatives was accomplished via asymmetric Mannich reaction between 2-substituted benzofuran-3-one and isatin-derived ketimines. Both anti and syn-selective chiral 3,3-disubstituted amino oxindoles bearing two adjacent tetrasubstituted stereogenic centers with high yield and excellent enantioselectivities were obtained using readily available cinchona-alkaloid derived organocatalysts. The control experiment revealed that oxygen atom present in the benzofuran ring played an important role in switching diastereodivergence. The obtained Mannich product was further transformed into a biologically important 2,3-dihydrobenzofuran derivative having three contiguous stereocenters with no loss of enantioselectivity.

Anionic Annulation of 3-Cyanophthalides with Allene Carboxylates: A Carbon-Conserved Synthesis of Naphtho[ b]furanones

The reaction of 3-cyanophthalides with allene carboxylates in the presence of ^tBuOLi results in a tandem annulation furnishing naphtho[ b]furanones in good yields with no loss of carbon. The carbon economy is explained by a tandem process, in which the initially expelled cyanide induces the second annulation.

Metal-Organic Frameworks for Heterogeneous Basic Catalysis

Great attention has been given to metal-organic frameworks (MOFs)-derived solid bases because of their attractive structure and catalytic performance in various organic reactions. The extraordinary skeleton structure of MOFs provides many possibilities for incorporation of diverse basic functionalities, which is unachievable for conventional solid bases. The past decade has witnessed remarkable advances in this vibrant research area; however, MOFs for heterogeneous basic catalysis have never been reviewed until now. Therefore, a review summarizing MOFs-derived base catalysts is highly expected. In this review, we present an overview of the recent progress in MOFs-derived solid bases covering preparation, characterization, and catalytic applications. In the preparation section, the solid bases are divided into two categories, namely, MOFs with intrinsic basicity and MOFs with modified basicity. The basicity can originate from either metal sites or organic ligands. Different approaches used for generation of basic sites are included, and each approach is described with representative examples. The fundamental principles for the design and fabrication of MOFs with basic functionalities are featured. In the characterization section, experimental techniques and theoretical calculations employed for characterization of basic MOFs are summarized. Some representive experimental techniques, such as temperature-programmed desorption of CO₂ (CO₂-TPD) and infrared (IR) spectra of different probing molecules, are covered. Following preparation and characterization, the catalytic applications of MOFs-derived solid bases are dealt with. These solid bases have potential to catalyze some well-known "base-catalyzed reactions" like Knoevenagel condensation, aldol condensation, and Michael addition. Meanwhile, in contrast to conventional solid bases, MOFs show some different catalytic properties due to their special structural and surface properties. Remarkably, characteristic features of MOFs-derived solid bases are described by comparing with conventional inorganic counterparts, keeping in mind the current opportunities and challenges in this field.

An enantioselective conjugate addition reaction of 3-substituted benzothiophen-2-ones and 2-phthalimidoacrylates

A highly enantioselective conjugate addition reaction of 3-substituted benzothiophen-2-ones to 2-phthalimidoacrylates has been developed using a bifunctional tertiary-amine thiourea catalyst.

A Serendipitous Synthesis of Bis-Heterocyclic Spiro 3(2H)-Furanones

(Z) Enol triflates 6, 11b-d, (E) enol triflate 11e, and phenol triflate 11a, derived from β-keto esters or 2-carboalkoxy phenols, respectively, react with N-Boc 2-lithiopyrrolidine (5a), N-Boc N-methylaminomethyllithium (5b), or 2-lithio-1,3-dithiane (14) to afford 3(2H)-furanones in modest to good yields (38-81%). Product and carbanion reagent studies suggest that the 3(2H)-furanone is formed in a cascade of reactions involving nucleophilic acyl substitution, enolate formation, trifluoromethyl transfer, iminium or sulfenium ion formation, and subsequent ring closure to form the 3(2H)-furanone. The use of 2-lithio-1,3-dithiane affords a cyclic α-keto-S,S,O-orthoester in which the functionality can be selectively manipulated for synthetic applications.

Organocatalytic asymmetric formal arylation of benzofuran-2(3H)-ones with cooperative visible light photocatalysis

An asymmetric arylation of benzofuran-2(3H)-ones with 1-naphthols has been developed through a cooperative visible light photocatalysis with asymmetric hydrogen-bond catalysis.

Highly enantioselective Michael addition reactions of 2-substituted benzofuran-3(2H)-ones to nitroolefins

A highly enantioselective Michael addition reaction of 2-substituted benzofuran-3(2H)-ones to nitroolefins was promoted by a bifunctional squaramide catalyst. As a result, a number of chiral 2,2'-substituted benzofuran-3-one derivatives, bearing adjacent quaternary-tertiary stereocenters, were efficiently synthesized with excellent enantioselectivities.