Toward an Asymmetric Synthesis of the Dimeric Pyranonaphthoquinone Antibiotic Crisamicin A

Annulations with Butenolides and Phthalides: New Entries to Isocoumarins, 3,4-Dihydroisocoumarins, and Benzofurans

The reactions of the anions of butenolides and substituted phthalides with sorbate esters and mono-epoxy sorbate esters furnish isocoumarins, 3,4-dihydroisocoumarins, and benzofurans. The yields range from 41% to 71%.

N-Heterocyclic carbene based catalytic platform for Hauser-Kraus annulations

The venerable Hauser–Kraus annulation is an effective and convergent method for generating oxygenated polycyclic aromatic compounds. Despite its application in complex molecule synthesis, the harsh and strongly basic conditions can limit its utility in more functionalized molecular settings. We have developed the first catalytic Hauser–Kraus annulation based on N-heterocyclic carbene catalysis that proceeds under milder conditions. We demonstrate the scope of the transformation in the presence of several functional groups. We also propose a concerted mechanism for the annulation that proceeds through a non-canonical Breslow intermediate.

The venerable Hauser–Kraus annulation is an effective and convergent method for generating oxygenated polycyclic aromatic compounds. A catalytic Hauser–Kraus annulation has been developed based on N-heterocyclic carbene catalysis.

Stereoselective Total Synthesis of the Dimeric Naphthoquinonopyrano-γ-lactone (-)-Crisamicin A: Introducing the Dimerization Site by a Late-Stage Hartwig Borylation

The first stereoselective total synthesis of the dimeric naphthoquinonopyrano-γ-lactone (-)-crisamicin A was realized (13 steps, 5% overall yield). 1,4,5-Trimethoxynaphthalene, reached in five known steps, was brominated at C-3 to install a but-3-enoic ester by an ensuing Heck coupling. An asymmetric Sharpless dihydroxylation followed and gave a β-hydroxy-γ-lactone with >99.9% ee. Its OH substituent and acetaldehyde established the dihydropyran ring in a completely diastereoselective oxa-Pictet-Spengler cyclization. The 2,3-fused anisole moiety allowed the C⁵-H bond under Hartwig's conditions to be borylated. This set the stage for engaging the resulting C⁵-B bond in an oxidative dimerization, which led to a binaphthohydroquinon-5-yl. The latter was advanced to synthetic crisamicin A by a double CAN oxidation (→ a binaphthoquinon-5-yl) and a double demethylation.

The genus Micromonospora as a model microorganism for bioactive natural product discovery

This review covers the development of the genus Micromonospora as a model for natural product research and the timeline of discovery progress from the classical bioassay-guided approaches through the application of genome mining and genetic engineering techniques that target specific products. It focuses on the reported chemical structures along with their biological activities and the synthetic and biosynthetic studies they have inspired. This survey summarizes the extraordinary biosynthetic diversity that can emerge from a widely distributed actinomycete genus and supports future efforts to explore under-explored species in the search for novel natural products.

We explore the genus Micromonospora as a model for natural product research and the discovery progress from the classical bioassay-guided approaches through to the application of genome mining and genetic engineering techniques that target specific products.

Unified synthesis and cytotoxic activity of 8-O-methylfusarubin and its analogues

A simple and unified synthesis of four related pyranonaphthoquinone natural products, e.g. 8-O-methylfusarubin, 8-O-methylanhydrofusarubin, fusarubin and anhydrofusarubin, is reported. The key synthetic features include the precedented Diels-Alder cycloaddition to assemble the naphthalene skeleton, selective formylation and acetonylation and intramolecular acetalization to construct the pyran ring. Manipulation of the oxidation state of the naphthoquinone core was performed to construct the two analogues, fusarubin and anhydrofusarubin. This work also highlights an unprecedented directing effect of the hydroxymethylene group in the selective hypervalent iodine-mediated quinone oxidation. The four synthetic compounds were evaluated for their in vitro cytotoxic activities against six human cancer cells. 8-O-Methylfusarubin was the most potent analogue and displayed excellent cytotoxic activity against MCF-7 breast cancer cells with an IC₅₀ value of 1.01 μM with no cytotoxic effect on noncancerous Vero cells, which could potentially be a promising lead compound for anti-breast cancer drug discovery.

Total Synthesis of Tetarimycin A, (±)-Naphthacemycin A9, and (±)-Fasamycin A: Structure-Activity Relationship Studies against Drug-Resistant Bacteria

Making use of a reductive olefin coupling reaction and Michael-Dieckmann condensation as two key operations, we have completed a concise total synthesis of tetarimycin A, (±)-naphthacemycin A₉, and (±)-fasamycin A in a highly convergent and practical protocol. Synthetic procedures thus developed have also been applied to provide related analogues for structure-activity relationship studies, thereby coming to the conclusion that the free hydroxyl group at C-10 is essential for exerting inhibitory activities against a panel of Gram-positive bacteria, including drug-resistant strains VRE and MRSA.

Pyranonaphthoquinones - isolation, biology and synthesis: an update

Covering: 2008 to 2015. A review on the isolation, biological activity and synthesis of pyranonaphthoquinone natural products from 2008-2015 is providedThis review discusses the isolation, biological activity and synthesis of pyranonaphthoquinone natural products, covering the years 2008-2015. The pyranonaphthoquinones are a group of metabolites sharing a common naphtho[2,3-c]pyran-5,10-dione ring system that have been isolated from a wide range of microorganisms, plants and insects. In addition to their synthetically challenging molecular structures, pyranonaphthoquinones exhibit a wide array of biological activity, including anti-bacterial, anti-fungal and anti-cancer properties. The therapeutic potential of these compounds has led to a dynamic interplay between total synthesis and biological evaluation.

Regiodefined synthesis of brominated hydroxyanthraquinones related to proisocrinins

Dibromobenzoisofuranone 12, synthesized in six steps, was regiospecifically annulated with 5-substituted cyclohexenones 13/36 in the presence of LiOt-Bu to give brominated anthraquinones 14/38 in good yields. Darzens condensation of 30 was shown to give chain-elongated anthraquinone 32. Alkaline hydrolysis of 38 furnished 39 representing desulfoproisocrinin F.

Enantioselective synthesis of 4H-pyranonaphthoquinones via sequential squaramide and silver catalysis

An enantioselective Michael addition/hydroalkoxylation reaction between 2-hydroxy-1,4-naphthoquinones and alkyne-tethered nitroalkenes catalysed by a cinchona-derived squaramide/silver(i) salt has been developed.

Studies on Antibiotics Active against Resistant Bacteria. Total Synthesis of MRSA-Active Tetarimycin A and Its Analogues

Making use of the Hauser-Kraus annulation as a key step, the first total synthesis of tetarimycin A has been accomplished in a highly convergent and operationally simple manner. Preliminary SAR not only validated that tetarimycin A exhibited potent activity against MRSA and VRE at a low MIC value but also identified that the hydroxyl group at C-10 was essential for antibacterial activities.

Asymmetric Synthesis of Dihydronaphthoquinones Containing Adjacent Stereocenters via a Sulfa-Michael Addition Triggered Ring-Expansion Approach

A novel asymmetric synthetic approach for the construction of enantioenriched functionalized dihydroquinones incorporating adjacent quaternary and tertiary stereocenters has been reported, in which enantioenriched 3-allylic phthalides engaged in an unprecedented sulfa-Michael addition-triggered stereoselective ring-expansion reaction, and furnished the desired sulfur-incoporated dihydronaphthoquinones with high stereoselectivity.

Design, synthesis, and anticancer activities of new compounds bearing the quinone–pyran–lactone tricyclic pharmacophore

A simple and effective synthesis of the tricyclic quinone–pyran–lactone skeleton was developed and subsequent structural modification was conducted.