A Facile Phenol-Driven Intramolecular Diastereoselective Thermal/Base-Catalyzed Dipolar [2 + 2] Annulation Reactions: An Easy Access to Complex Bioactive Natural and Unnatural Benzopyran Congeners

Biomimetic Total Synthesis of the Rubiginosin Meroterpenoids

Total synthesis of the Rhododendron meroterpenoids rubiginosins A and G, which both contain unusual 6-6-6-4 ring systems, has been achieved using a bioinspired cascade approach. Stepwise synthesis of these natural products, and the related 6-6-5-4 meroterpenoids fastinoid B and rhodonoid B, from naturally occurring chromene precursors is also reported.

Regioselective condensation of hydroxyaromatic compounds with 2,5-dimethoxytetrahydrofuran: facile one-pot synthesis of new substituted diaryl-fused 2,8-dioxabicyclo[3.3.2]nonanes comprising central ketal moieties

Regioselective synthesis of a series of novel diaryl- and naphthyl-fused 2,8-dioxabicyclo[3.3.2]nonanes was accomplished by a one-pot reaction of p-substituted phenols and 2-naphthol with 2,5-dimethoxytetrahydrofuran.

ortho-Quinone Methide Cyclizations Inspired by the Busseihydroquinone Family of Natural Products

A series of cascade reactions of o-quinone methides have been developed based on the proposed biosynthesis of busseihydroquinone and parvinaphthol meroterpenoid natural products. The polycyclic framework of the most complex family members, busseihydroquinone E and parvinaphthol C, was assembled by an intramolecular [4 + 2] cycloaddition of an electron-rich chromene substrate. The resultant cyclic enol ether underwent rearrangements under acidic or oxidative conditions, which led to a new total synthesis of rhodonoid D.

Recent advances in the synthesis of gem-dimethylcyclobutane natural products

Covering: January 2000 to July 2018 gem-Dimethylcyclobutanes are a common motif found in a multitude of natural products, and thus these structures have captivated synthetic chemists for years. However, until the turn of the century, most synthetic efforts relied upon the use of widely available terpenes, such as pinene or caryophyllene, that already contain the gem-dimethylcyclobutane motif. This approach limits the scope of molecules that can be accessed readily. This review highlights recent syntheses in which the gem-dimethylcyclobutane is assembled via de novo approaches. An outlook on the future of this research area is also provided.

The winding road of the uvaretin class of natural products: from total synthesis to bioactive agent discovery

Herein, we disclose the development of a synthetic route to gain access to the uvaretin class of chalcone natural products. In this, the construction of a small library was achieved, and the collection was evaluated for cytotoxicity and other biological properties. Uvaretin (1) was accessed via a seven-step route in an overall yield of 15.1%. Within this route, the unsaturated enone variant of uvaretin (2), also a natural product, was accessed in a 16.7% yield over six steps. This route provides a nearly three-fold increase in yields of 1 and 2 in comparison to the previous synthetic route accessing them in 5.8% and 3.0% overall yields, respectively. Evaluation of 1 and 2 revealed IC₅₀ values between 2.0 and 5.1 μM in the cancerous cell lines HeLa, U937, A549, and MIA PaCa-2. Screening of the whole chalcone library set led to the discovery of over 30 compounds, within six cancerous cell lines, possessing single digit μM IC₅₀ activity as sole agents. Furthermore, multiple library members were found to possess promising potentiating properties with known chemotherapeutic agents.

Synthesis of (-)-Hebelophyllene E: An Entry to Geminal Dimethyl-Cyclobutanes by [2+2] Cycloaddition of Alkenes and Allenoates

The first synthesis of hebelophyllene E is presented, along with assignment of its previously unknown relative configuration through synthesis of epi-ent-hebelophyllene E. Development of a catalytic enantioselective [2+2] cycloaddition of alkenes and allenoates provides access to the required chiral geminal dimethylcyclobutanes. Key to its success is the identification of a novel oxazaborolidine catalyst which promotes the cycloaddition in high enantioselectivities with good functional-group tolerance (9 examples, up to 97:3 e.r.). Thus, a late-stage cycloaddition using a fully functionalized alkene, followed by a diastereoselective reduction allows a concise entry to this class of natural products.

Anti-HIV and cytotoxic biphenyls, benzophenones and xanthones from stems, leaves and twigs of Garcinia speciosa

Eleven previously undescribed compounds, including four benzophenones (garciosones A-D), four xanthones (garciosones E-H) and three biphenyls (garciosines A-C), along with eighteen known compounds were isolated from the stems, leaves and twigs of Garcinia speciosa Wall. (Clusiaceae). Their structures were established by extensive spectroscopic analysis. For garciosines A-C, the structures were confirmed by single crystal X-ray diffraction analysis. Most of the isolated compounds were evaluated for their cytotoxic activity and anti-HIV-1 activity using the syncytium inhibition assay and HIV-1 reverse transcriptase (RT) assay. The known compounds, 4,6,3',4'-tetrahydroxy-2-methoxybenzophenone and macluraxanthone, displayed significant cytotoxic activity with the ED₅₀ in the range of 1.85-11.76 μM. 1,5-Dihydroxyxanthone exhibited the most potent anti-HIV activity against syncytium formation with EC₅₀ < 17.13 μM (SI > 25.28) and 2-(3,3-dimethylallyl)-1,3,7-trihydroxyxanthone was the most active compound in the HIV-1 reverse transcriptase assay with IC₅₀ value of 58.24 μM. Structure-activity relationship of some isolated compounds were also discussed.

Total Syntheses of (±)-Rhodonoids A and B and C12-epi-Rhodonoid B

Total syntheses of (±)-rhodonoids A and B and C12-epi-rhodonoid B are described here. A unified strategy employed in these syntheses is an intramolecular oxa-[3 + 3] annulation for accessing the chromene unit. A Fe(OTf)₃-promoted diastereoselective cationic [2 + 2] cycloaddition and a photochemical [2 + 2] cycloaddition were featured to construct the cyclobutane core of (±)-rhodonoids A and B and C12-epi-rhodonoid B, respectively. Fe(OTf)₃ also leads to an interesting bridged tetracycle, which was unambiguously confirmed by single crystal X-ray analysis.

Insecticidal and Enzyme Inhibitory Activities of Sparassol and Its Analogues against Drosophila suzukii

Drosophila suzukii is an economically important pest in America and Europe as well as in Asia. Sparassol and methyl orsellinate are naturally produced by the cultivating mushrooms Sparassis cripta and Sparassis latifolia. Fumigant and contact toxicities of synthetic sparassol and its analogues, methyl orsellinate and methyl 2,4-dimethoxy-6-methylbenzoate (DMB), were investigated. Negligible fumigant activity was observed from the tested compounds. However, DMB showed the strongest contact toxicity, followed by sparassol and methyl orsellinate. The possible modes of action of the compounds were assessed for their acetylcholinesterase (AChE)- and glutathione S-transferase (GST)-inhibiting activities. AChE activity was weakly inhibited by methyl orsellinate and DMB, but GST was inhibited by sparassol, methyl orsellinate, and DMB. Thus, DMB could be a promising alternative to common insecticides as it can be easily synthesized from sparassol, which is the natural product of Sparassis species. Sparassis species could be an industrial resource of DMB.

Sequential Ketene Generation from Dioxane-4,6-dione-keto-dioxinones for the Synthesis of Terpenoid Resorcylates

Trapping of the ketene generated from the thermolysis of 2-methyl-2-phenyl-1,3-dioxane-4,6-dione-keto-dioxinone at 50 °C with primary, secondary, or tertiary alcohols gave the corresponding dioxinone β-keto-esters in good yield under neutral conditions. These intermediates were converted by palladium(0)-catalyzed decarboxylative allyl migration and aromatization into the corresponding β-resorcylates. These transformations were applied to the syntheses of the natural products (±)-cannabiorcichromenic and (±)-daurichromenic acid.

Construction of chiral bridged tricyclic benzopyrans: enantioselective catalytic Diels-Alder reaction and a one-pot reduction/acid-catalyzed stereoselective cyclization

An asymmetric two-step approach to chiral bridged tricyclic benzopyrans, core structures featured in various natural products, is described. In the synthesis, an unprecedented enantioselective catalytic decarboxylative Diels-Alder reaction is developed using readily available coumarin-3-carboxylic acids and aldehydes as reactants under mild reaction conditions. Notably, the decarboxylation-assisted release of the catalyst enables the process to proceed efficiently with high enantio- and diastereoselectivity. Furthermore, a one-pot procedure for either a LiAlH4 - or NaBH4 -mediated reduction with subsequent acid-catalyzed intramolecular cyclization of the Diels-Alder adducts was identified for the efficient formation of the chiral bridged tricyclic benzopyrans.

Catalyst-controlled regio- and stereoselective synthesis of diverse 12H-6,12-methanodibenzo[d,g][1,3]dioxocines

Regio- and stereoselective synthesis of 12H-6,12-methanodibenzo[d,g][1,3]dioxocines has been accomplished by the EDDA and PTSA-catalyzed cascade reactions of resorcinols and 2-hydroxychalcones.

1-(5-Hy-droxy-2,2,8,8-tetra-methyl-2H,8H-pyrano[2,3-f]chromen-6-yl)ethanone

In the title compound, C₁₈H₂₀O₄,the pyran ring of the chromene unit adopts a half-chair conformation. An intra­molecular O—H⋯O hydrogen bond occurs. In the crystal, mol­ecules are linked along the b axis by C—H⋯O hydrogen bonds.

The chemistry of the polycyclic polyprenylated acylphloroglucinols

With their fascinating biological profiles and stunningly complex molecular architectures, the polycyclic polyprenylated acylphloroglucinols (PPAPs) have long provided a fertile playing field for synthetic organic chemists. In particular, the recent advent of innovative synthetic methods and strategies together with C-C bond-forming reactions and asymmetric catalysis have revitalized this field tremendously. Consequently, PPAP targets which once seemed beyond reach have now been synthesized. This Review aims to highlight the recent achievements in the total synthesis of PPAPs, as well as notable methods developed for the construction of the bicyclo[3.3.1] core of these chemically and biologically intriguing molecules.

Total synthesis of (+)-nankakurines A and B and (±)-5-epi-nankakurine A

The first total syntheses of the Lycopodium alkaloids (+)-nankakurine A (2), (+)-nankakurine B (3), and the originally purported structure 1 of nankakurine A were accomplished. The syntheses of 2 and 3 feature a demanding intramolecular azomethine imine cycloaddition as the key step for generating the octahydro-3,5-ethanoquinoline moiety and installing the correct relative configuration at the spiropiperidine ring juncture. The cyclization precursor was prepared from octahydronaphthalene ketone 50, which was assembled from enone (+)-9 and diene 48 by a cationic Diels-Alder reaction. The Diels-Alder reactants were synthesized from 5-hexyn-1-ol (16) and (+)-pulegone (49), respectively. The tetracyclic ring system of 1 was generated using an unprecedented nitrogen-terminated aza-Prins cyclization cascade. The enantioselective total syntheses of (+)-nankakurine A (2) and (+)-nankakurine B (3) establish the relative and absolute configuration of these alkaloids and are sufficiently concise that substantial quantities of 2 and 3 were prepared for biological studies. (+)-Nankakurine A and (+)-nankakurine B showed no effect on neurite outgrowth in rat hippocampal H-19 cells over a concentration range of 0.3-10 μM.

Constituents of Asarum sieboldii with inhibitory activity on lipopolysaccharide (LPS)-induced NO production in BV-2 microglial cells

Bioassay-guided fractionation of the root extract of Asarum sieboldii led to the isolation of the four active compounds (-)-sesamin (1), (2E,4E,8Z,10E)-N-(2-methylpropyl)dodeca-2,4,8,10-tetraenamide (2), kakuol (3), and '3,4,5-trimethoxytoluene' (=1,2,3-trimethoxy-5-methylbenzene; 4), in terms of inhibition of lipopolysaccharide (LPS)-induced nitric oxide (NO) production. Compounds 1-4 showed potent inhibition of NO production, with IC(50) values in the low nanomolar-to-micromolar range. Also isolated were the known compounds methylkakuol (5), '3,5-dimethoxytoluene', safrole, asaricin, methyleugenol, and (-)-asarinin, which were found to be inactive in the above assay. Among the ten known isolates, compounds 1, 2, and 5 were found for the first time in this plant.

Total synthesis of atrochamins F, H, I, and J through cascade reactions

A concise and efficient cascade-based total synthesis of artochamins F, H, I, and J is described. The potential biogenetic connection between artochamin F, or a derivative thereof, and artochamins H, I, and J, through an unusual formal [2+2] cycloaddition process, was shown to be feasible. An alternative mechanism for this transformation is also proposed.