Total Syntheses of Drimane-Type Sesquiterpenoids Enabled by a Gold-Catalyzed Tandem Reaction

The chemical constituents of endophytic fungus Nigrospora chinensis of Gannan navel orange

A new drimane sesquiterpene 11-methoxyl-danilol (1) was obtained from endophytic fungus Nigrospora chinensis of Gannan navel orange pulp. Its structure was established to possess a natural rarely-occurring tricyclic acetal fused ring system by means of spectroscopic data analyses. Meanwhile, five known compounds danilol (2), redoxcitrinin (3), euphorbol (4), ergosta-7,24(24')-dien-3β-ol (5), and ergosta-4,6,8(14),22-tetraen-3-one (6) were also co-isolated in this fungus. The results of antibacterial and cytotoxic activity screenings showed that compound 5 displayed antibacterial activities against Staphylococcus aureus and MRSA (methicillin-resistant S. aureus) with MIC value of 50 μg/mL. [Figure: see text].

Anti-Oxidant, Anti-Mutagenic Activity and Safety Evaluation of Antrocin

Antrocin is a novel compound isolated from Antrodia cinnamomea, and is classified as a sesquiterpene lactone. The therapeutic efficacy of antrocin has been studied, and it has shown an antiproliferative effect on various cancers. The aim of this study was to evaluate the anti-oxidant activity, potential genotoxicity, and oral toxicity of antrocin. Ames tests with five different strains of Salmonella typhimurium, chromosomal aberration tests in CHO-K1 cells, and micronucleus tests in ICR mice were conducted. The results of anti-oxidant capacity assays showed that antrocin has great anti-oxidant activity and is a moderately strong antimutagenic agent. In the results of the genotoxicity assays, antrocin did not show any mutagenic potential. In the 28-day oral toxicity test, Sprague Dawley rats were gavaged with 7.5 or 37.5 mg/kg of antrocin for 28 consecutive days. In addition, 7.5 mg/kg sorafenib, an anti-cancer drug, was used as a positive control for toxicity comparison. At the end of the study, antrocin did not produce any toxic effects according to hematology, serum chemistry, urine analysis, or histopathological examinations. According to the results of the genotoxicity and 28-day oral toxicity study, antrocin, at a dose of 37.5 mg/kg, did not cause adverse effects and can be a reference dose for therapeutic agents in humans.

The Biosynthetic Gene Cluster of Mushroom-Derived Antrocin Encodes Two Dual-Functional Haloacid Dehalogenase-like Terpene Cyclases

(-)-Antrocin (1), produced by the medicinal mushroom Antrodia cinnamomea, is a potent antiproliferative compound. The biosynthetic gene cluster of 1 was identified, and the pathway was characterized by heterologous expression. We characterized a haloacid dehalogenase-like terpene cyclase AncC that biosynthesizes the drimane-type sesquiterpene (+)-albicanol (2) from farnesyl pyrophosphate (FPP). Biochemical characterization of AncC, including kinetic studies and mutagenesis, demonstrated the functions of two domains: a terpene cyclase (TC) and a pyrophosphatase (PPase). The TC domain first cyclizes FPP to albicanyl pyrophosphate, and the PPase domain then removes the pyrophosphate to form 2. Intriguingly, AncA (94 % sequence identity to AncC), in the same gene cluster, converts FPP into (R)-trans-γ-monocyclofarnesol instead of 2. Notably, Y283/F375 in the TC domain of AncA serve as a gatekeeper in controlling the formation of a cyclofarnesoid rather than a drimane-type scaffold.

Transcriptomic discovery of a theranostic signature (SERPINE1/MMP3/COL1A1/SPP1) for head and neck squamous cell carcinomas and identification of antrocinol as a candidate drug

Head and neck squamous cell carcinomas (HNSCC) are prevalent malignancies with a disappointing prognosis, necessitating the search for theranostic biomarkers for better management. Based on a meta-analysis of transcriptomic data containing ten clinical datasets of HNSCC and matched nonmalignant samples, we identified SERPINE1/MMP3/COL1A1/SPP1 as essential hub genes as the potential theranostic biomarkers. Our analysis suggests these hub genes are associated with the extracellular matrix, peptidoglycans, cell migration, wound-healing processes, complement and coagulation cascades, and the AGE-RAGE signaling pathway within the tumor microenvironment. Also, these hub genes were associated with tumor-immune infiltrating cells and immunosuppressive phenotypes of HNSCC. Further investigation of The Cancer Genome Atlas (TCGA) cohorts revealed that these hub genes were associated with staging, metastasis, and poor survival in HNSCC patients. Molecular docking simulations were performed to evaluate binding activities between the hub genes and antrocinol, a novel small-molecule derivative of an anticancer phytochemical antrocin previously discovered by our group. Antrocinol showed high affinities to MMP3 and COL1A1. Notably, antrocinol presented satisfactory drug-like and ADMET properties for therapeutic applications. These results hinted at the potential of antrocinol as an anti-HNSCC candidate via targeting MMP3 and COL1A1. In conclusion, we identified hub genes: SERPINE1/MMP3/COL1A1/SPP1 as potential diagnostic biomarkers and antrocinol as a potential new drug for HNSCC.

Phosphine-catalyzed Divergent Domino Processes between γ-Substituted Allenoates and Carbonyl-Activated Alkenes

Highly enantioselective and chemodivergent domino reactions between γ-substituted allenoates and activated alkenes have been developed. In the presence of NUSIOC-Phos, triketone enone substrates smoothly reacted with γ-substituted allenoates to form bicyclic furofurans in good yields with high stereoselectivities. Alternatively, the reaction between diester-activated enone substrates and γ-substituted allenoates formed chiral conjugated 1,3-dienes in good yields with excellent enantioselectivities. Notably, by employing substrates with subtle structural difference, under virtually identical reaction conditions, we were able to access two types of chiral products, which are of biological relevance and synthetic importance.

Highly enantioselective and chemodivergent domino reactions between γ-substituted allenoates and activated alkenes have been developed.

Synthesis of Phenol-Derived cis-Vinyl Ethers Using Ethynyl Benziodoxolone

The stereoselective synthesis of cis-β-phenoxyvinyl benziodoxolones (cis-β-phenol-VBXs) from an ethynyl benziodoxolone-acetonitrile complex (EBX-MeCN) and various phenols is reported herein. The reaction tolerates different phenol derivatives, including complex natural products, and can be conducted under mild conditions. The synthesis was performed in an aqueous solvent in the absence and presence of a catalytic amount of a base. Selectively mono- and di-deuterated cis-β-phenol-VBXs were also prepared. cis-β-Phenol-VBXs were stereospecifically derivatized to cis-alkynylvinyl ethers and cis-iodovinyl ethers without loss of stereoselectivity or reduction in the deuterium/hydrogen ratio.

Catalytic Hydroxycyclopropanol Ring-Opening Carbonylative Lactonization to Fused Bicyclic Lactones

A novel palladium-catalyzed ring opening carbonylative lactonization of readily available hydroxycyclopropanols was developed to efficiently synthesize tetrahydrofuran (THF) or tetrahydropyran (THP)-fused bicyclic γ-lactones, two privileged scaffolds often found in natural products. The reaction features mild reaction conditions, good functional group tolerability, and scalability. Its application was demonstrated in a short total synthesis of (±)-paeonilide. The fused bicyclic γ-lactone products can be easily diversified to other medicinally important scaffolds, which further broadens the application of this new carbonylation method.

Terpenoids from the medicinal mushroom Antrodia camphorata: chemistry and medicinal potential

Covering: up to February 2020Antrodia camphorata is a medicinal mushroom endemic to Taiwan for the treatment of intoxication, liver injury, cancer, and inflammation. Owing to its rare occurrence and potent pharmacological activities, efforts have been devoted to identify its bioactive constituents, especially terpenoids. Since 1995, a total of 162 terpenoids including triterpenoids, meroterpenoids, sesquiterpenoids, diterpenoids, and steroids have been characterized. The ergostane-type triterpenoids (antcins) and meroterpenoids (antroquinonols) are characteristic constituents of A. camphorata. The terpenoids show anti-cancer, hepatoprotective, anti-inflammatory, anti-diabetic, and neuroprotective activities. This review summarizes the research progress on terpenoids in A. camphorata during 1995-2020, including structural diversity, resources, biosynthesis, pharmacological activities, metabolism, and toxicity. The medicinal potential of the terpenoids is also discussed.

Synthesis of Natural (-)-Antrocin and its Enantiomer via Stereoselective Aldol Reaction

The total synthesis of (-)-antrocin and its enantiomer are presented. Antrocin (-)-1 is an important natural product which acts as an antiproliferative agent in a metastatic breast cancer cell line (IC₅₀: 0.6 μM). The key features of this synthesis are: (a) selective anti-addition of trimethylsilyl cyanide (TMSCN) to α,β-unsaturated ketone; (b) resolution of (±)-7 using chiral auxiliary L-dimethyl tartrate through formation of cyclic ketal diastereomers followed by simple column chromatography separation and acid hydrolysis; (c) substrate-controlled stereoselective aldol condensation of (+)-12 with monomeric formaldehyde and pyridinium chlorochromate (PCC) oxidation for synthesis of essential lactone core in (-)-14; and (d) non-basic Lombardo olefination of the carbonyl at the final step to yield (-)-antrocin. In addition, (+)-9 cyclic ketal diastereomer was converted to (+)-antrocin with similar reaction sequences.

Gold-Catalyzed Hydroalkoxylation/Povarov Reaction Cascade of Alkynols with N-Aryl Imines: Synthesis of Tetrahydroquinolines

A one-pot gold-catalyzed hydroalkoxylation/Povarov reaction cascade of alkynols with N-aryl imines or in situ generated iminium has been developed. The protocol provides a facile access to a series of fused tricyclic tetrahydroquinolines with a broad substrate scope using readily available materials under mild conditions. The unique mechanistic feature is the dual function of the gold catalyst, which first catalyzed the intramolecular hydroalkoxylation of alkynols, and upon the formation of dihydrofuran species, promoted the following Povarov reaction with high stereoselectivity.

Synthesis, Characterization, and Reactivity of an Ethynyl Benziodoxolone (EBX)-Acetonitrile Complex

The synthesis of a crystalline ethynyl-1,2-benziodoxol-3(1 H)-one (EBX)-acetonitrile complex is described. EBX has been widely used as an active species for a variety of reactions; however, its high instability has so far prevented its isolation. The EBX-acetonitrile is self-assembled into a double-layered honeycomb structure through weak hypervalent iodine secondary interactions and hydrogen bonding. The N-ethynylation of a variety of sulfonamides using the EBX-acetonitrile complex as a substrate under mild conditions is also described.

Sequential Michael Addition/Electrophilic Alkynylation: Synthesis of α-Alkynyl-β-Substituted Ketones and Chromanones

We describe the synthesis of α-alkynyl-β-substituted cyclic ketones and analogue chromanones via one-pot Michael addition/hypervalent iodine-based α-alkynylation. Cu(I)-catalyzed Michael addition using either alkyl-aluminum or Grignard reagents, followed by diastereoselective electrophilic alkynylation of the resulting enolate by 1-ethynyl-1λ³,2-benziodoxol-3(1H)-one (EBX) resulted in the α-alkynyl-β-substituted cyclic ketones or chromanones within 34-89% yield (16 examples). The reaction was successfully upscaled to the 5 mmol scale, and further functionalization of a model alkynylated ketone was demonstrated.

Light-Driven Intermolecular Charge Transfer Induced Reactivity of Ethynylbenziodoxol(on)e and Phenols

Ethynylbenziodoxol(on)es (EBXs) have been widely used in organic synthesis as electrophilic alkyne-transfer reagents involving carbon- and heteroatom-based nucleophiles. However, potential reactions of EBXs with phenols remain uninvestigated. Here, we present the formation of ( Z)-2-iodovinyl phenyl ethers with excellent regio- and stereoselectivity through the reactivity between EBXs and phenols driven by visible light. We propose that this light-activated transformation proceeds through electron donor-acceptor complexes to enable new reactivity beyond existing mechanisms for alkynylation of carbon- and heteroatom-based nucleophiles. This operationally robust process was employed for the synthesis of diverse ( Z)-2-iodovinyl phenyl ethers through irradiating a solution containing a phenyl-EBX, a phenol, and the base Cs₂CO₃ with a commercially available blue LED at room temperature. The ( Z)-2-iodovinyl phenyl ether products can be further stereospecifically functionalized to form trisubstituted alkenes, demonstrating the potential of these products en route to chemical complexity.

A chiral pool approach for asymmetric syntheses of (-)-antrocin, (+)-asperolide C, and (-)-trans-ozic acid

Ozonolysis of aromatic abietane (+)-carnosic acid (4) is used to create an important intermediate in an enantiomerically pure form, resulting in a simple, concise, readily scalable, and asymmetric synthesis of (-)-antrocin (1). This strategy not only provides an efficient approach to (-)-antrocin (1) synthesis but can also be readily adopted for the syntheses of optically pure (+)-asperolide C (2) and (-)-trans-ozic acid (3) from the naturally abundant aromatic abietanes (+)-podocarpic acid (5) and (+)-dehydroabietic acid (6). The strategy presented here is an example of the use of naturally occurring aromatic abietanes as a chiral pool and offers an account of the asymmetric synthesis of terpenoids.

Gold Catalysis for Heterocyclic Chemistry: A Representative Case Study on Pyrone Natural Products

2-Pyrones and 4-pyrones are common structural motifs in bioactive natural products. However, traditional methods for their synthesis, which try to emulate the biosynthetic pathway of cyclization of a 1,3,5-tricarbonyl precursor, are often harsh and, therefore, not particularly suitable for applications to polyfunctionalized and/or sensitive target compounds. π-Acid catalysis, in contrast, has proved to be better for a systematic exploration of the pyrone estate. To this end, alkynes are used as stable ketone surrogates, which can be activated under exceedingly mild conditions due to the pronounced carbophilicity of [LAu]⁺ fragments (L=two electron donor ligand); attack of a tethered ester carbonyl group onto the transient alkyne-gold complex then forges the pyrone ring in a fully regiocontrolled manner.

Total Syntheses of the Reported Structures of Curcusones I and J through Tandem Gold Catalysis

Total syntheses of the reported structures of the rhamnofolane diterpene natural products curcusones I and J in racemic form were achieved. The synthetic strategy features a novel tandem gold-catalyzed furan formation and furan-allene [4+3] cycloaddition to build the 5,7-fused ring system with an oxa-bridge in one step, and a stereoselective exo-Diels-Alder reaction to form the 6-membered ring. The newly developed tandem gold catalysis is quite general and can be scaled up. Our syntheses suggest that structural revisions of curcusones I and J are needed.

Total Synthesis of (+)-Antrocin and Its Diastereomer and Clarification of the Absolute Stereochemistry of (-)-Antrocin

Using 2,2-dimethyl cyclohexanone as the starting compound, (+)-antrocin and its diastereomer have been synthesized. The absolute stereochemistry of (-)-antrocin, a natural sesqui-terpenoid and an antagonist in some types of cancer cells, was clarified using the character data of (+)-antrocin. The synthetic procedure involved two key steps: (1) the reaction of vinyl magnesium bromide with 2,2-dimethyl-6-t-butyl-dimethyl-silyoxy-methyl-1-cyclo-hexanone to give a vinyl cyclohexanol derivative and (2) a highly stereoselective intramolecular Diels-Alder (IMDA) reaction of the camphanate-containing triene intermediate. The relative energy levels of the possible transition states of the IMDA reaction of the camphanate-containing triene were obtained from density functional theory calculations, proving the stereospecific formation of the target molecule.

Synthesis of Natural Products and the Development of Synthetic Methodology: The Case Study of (–)-Atrop-abyssomicin C

During our attempt to follow the planned synthetic route to the naturally occurring antibiotic (–)- atrop-abyssomicin C, we encountered two shortcomings, which forced us to reconsider our tactics and find new methods to overcome the problems. These methods turned out to be of general applicability, as demonstrated later in total syntheses of two other natural products: (+)-allokainic acid and (-)-gabosine H. The paper provides a brief account of these endeavors.

Recent advances on the total synthesis of alkaloids in mainland China

Alkaloids are a large family of natural products that mostly contain basic nitrogen atoms. Because of their intriguing structures and important functions, they have long been popular targets for synthetic organic chemists. China's chemists have made significant progress in the area of alkaloid synthesis over past decades. In this article, selected total syntheses of alkaloids from research groups in mainland China during the period 2011–16 are highlighted.

Total synthesis of orientalol F via gold-catalyzed cycloisomerization of alkynediol

The total synthesis of orientalol F was accomplished in 13 steps using gold-catalyzed tandem cycloisomerization of alkynediol as a key step.

Metal-free benzannulation of 1,7-diynes towards unexpected 1-aroyl-2-naphthaldehydes and their application in fused aza-heterocyclic synthesis

A novel I₂-mediated benzannulation of 1,7-diyne-involved 1,4-oxo-migration was established, providing unexpected 1-aroyl-2-naphthaldehydes. The resulting 1-aroyl-2-naphthaldehydes were successfully applied in the synthesis of polycyclic isoindoles.

Natural product analogues: towards a blueprint for analogue-focused synthesis

A review of approaches to natural product analogues leads to the suggestion of new methods for the generation of biologically active natural product-like scaffolds.

Gold-catalysed reactions of diynes

In this critical review the reactivity patterns observed with different types of diyne substrates in gold catalysis are discussed. Apart from the many examples from homogeneous catalysis, the few examples from heterogeneous gold catalysis are also included. With a proper arrangement of the two alkynes unique and exciting reactivity patterns like 1,3-carbonyl transpositions, carbene transfer reactions, cascade annulations, macrocyclisations or the formation of gold vinylidene intermediates are observed. These reactions are of interest for organic synthesis, for pharmaceutical and medicinal chemistry and for material science.

Domino Strategy for the Stereoselective Construction of Angularly Fused Tricyclic Ethers

A stereoselective synthesis of decahydrofuro[3,2-d]isochromene derivatives has been achieved by the condensation of 2-cyclohexenylbutane-1,4-diol with aldehydes in the presence of a stochiometric amount of BF3·OEt2 in dichloromethane at -78 °C. Similarly, the condensation of 2-cyclopentenylbutan-1,4-diol with aldehydes provides the corresponding octahydro-2H-cyclopenta[c]furo[2,3-d]pyran derivatives in good yields with high diastereoselectivity. It is an elegant strategy for the quick construction of tricyclic architectures with four contiguous stereogenic centers in a single step. These tricyclic frameworks are the integral part of numerous natural products.

Towards a general diastereoselective route to oxabicyclo[3.2.1]octanes via a gold-catalysed cascade reaction

The development of an efficient diastereoselective synthesis of the oxabicyclo[3.2.1]octane ring system bearing two oxygenated quaternary chiral centres represents a significant challenge. This motif can be found in a wide range of natural products with significant biological activities. Here we report the synthesis of such kind of scaffold using a cyclohexane-trans-1,4-diol with an alkyne side chain in the presence of Au(I) catalyst. This is a domino process in which two C–H, two C–O and one C–C bond is assembled through a sequence of cyclization/semi-pinacol rearrangements. This strategy has been successfully applied to the asymmetric formal total synthesis of (+)-cortistatins.

Oxygenated bicyclic cores are common to many natural and bioactive compounds, but their efficient synthesis can be difficult. Here, the authors report a gold-catalysed diastereoselective process for the synthesis of oxabicyclo[3.2.1]octanes via a domino process.

Gold-Catalyzed Cycloisomerization and Diels-Alder Reaction of 1,6-Diyne Esters with Alkenes and Diazenes to Hydronaphthalenes and -cinnolines

A method for the efficient preparation of hydronaphthalene and -cinnoline derivatives by Au(I)-catalyzed cycloisomerzation of 1,6-diyne esters followed by a Diels-Alder reaction with alkenes or diazenes under mild conditions at room temperature with catalyst loadings as low as 1 mol % is described.