A divergent and selective synthesis of ortho- and para-quinones from phenols

Total Synthesis of (S)-Cularine via Nucleophilic Substitution on a Catechol

Catechols are part of many essential chemicals and are valuable, typically nucleophilic intermediates used in synthesis. Here we describe an unexpected transformation in which they play the role of the electrophile in a formal nucleophilic aromatic substitution. We made this discovery while studying a seven-membered dioxepin ring formation during a synthesis of the benzyltetrahydroisoquinoline (S)-cularine. We suggest a chain mechanism for this new transformation that is triggered by molecular oxygen and that propagates an electrophilic ortho-quinone.

Dearomatization of Electron-Deficient Phenols to ortho-Quinones: Bidentate Nitrogen-Ligated Iodine(V) Reagents

Despite their broad utility, the synthesis of ortho-quinones remains a significant challenge, in particular, access to electron-deficient derivatives remains an unsolved problem. Reported here is the first general method for the synthesis of electron-deficient ortho-quinones by direct oxidation of phenols. The reaction is enabled by a novel bidentate nitrogen-ligated iodine(V) reagent, a previously unexplored class of compounds which we have termed Bi(N)-HVIs. The reaction is extremely general and proceeds with excellent regioselectivity for the ortho over para isomer. Functionalization of the ortho-quinone products was examined, resulting in a facile one-pot synthesis of catechols, as well as the incorporation of a variety of heteroatom nucleophiles. This method represents the first synthetic application of Bi(N)-HVIs and demonstrates their potential as a platform for the further development of highly reactive, but also highly tunable, I(V) reagents.

Genome mining and biosynthesis of a polyketide from a biofertilizer fungus that can facilitate reductive iron assimilation in plant

Fungi have the potential to produce a large repertoire of bioactive molecules, many of which can affect the growth and development of plants. Genomic survey of sequenced biofertilizer fungi showed many secondary metabolite gene clusters are anchored by iterative polyketide synthases (IPKSs), which are multidomain enzymes noted for generating diverse small molecules. Focusing on the biofertilizer Trichoderma harzianum t-22, we identified and characterized a cryptic IPKS-containing cluster that synthesizes tricholignan A, a redox-active ortho-hydroquinone. Tricholignan A is shown to reduce Fe(III) and may play a role in promoting plant growth under iron-deficient conditions. The construction of tricholignan by a pair of collaborating IPKSs was investigated using heterologous reconstitution and biochemical studies. A regioselective methylation step is shown to be a key step in formation of the ortho-hydroquinone. The responsible methyltransferase (MT) is fused with an N-terminal pseudo-acyl carrier protein (ψACP), in which the apo state of the ACP is essential for methylation of the growing polyketide chain. The ψACP is proposed to bind to the IPKS and enable the trans MT to access the growing polyketide. Our studies show that a genome-driven approach to discovering bioactive natural products from biofertilizer fungi can lead to unique compounds and biosynthetic knowledge.

A Catalyst-Controlled Aerobic Coupling of ortho-Quinones and Phenols Applied to the Synthesis of Aryl Ethers

ortho-Quinones are underutilized six-carbon-atom building blocks. We herein describe an approach for controlling their reactivity with copper that gives rise to a catalytic aerobic cross-coupling with phenols. The resulting aryl ethers are generated in high yield across a broad substrate scope under mild conditions. This method represents a unique example where the covalent modification of an ortho-quinone is catalyzed by a transition metal, creating new opportunities for their utilization in synthesis.

A bio-inspired synthesis of oxindoles by catalytic aerobic dual C-H functionalization of phenols

We report a bio-inspired approach to the synthesis of oxindoles, which couples the energetic requirements of dehydrogenative C–N bond formation to the reduction of oxygen.

Nitrogen-containing heterocycles are fundamentally important to the function of pharmaceuticals, agrochemicals and materials. Herein, we report a bio-inspired approach to the synthesis of oxindoles, which couples the energetic requirements of dehydrogenative C–N bond formation to the reduction of molecular oxygen (O₂). Our method is inspired by the biosynthesis of melanin pigments (melanogenesis), but diverges from the biosynthetic polymerization. Mechanistic analysis reveals the involvement of Cu^II-semiquinone radical intermediates, which enable dehydrogenative carbon–heteroatom bond formation that avoids a catechol/quinone redox couple. This mitagates the deleterious polarity reversal that results from phenolic dearomatization, and enables a high-yielding phenolic C–H functionalization under catalytic aerobic conditions. Our work highlights the broad synthetic utility and efficiency of forming C–N bonds via a catalytic aerobic dearomatization of phenols, which is currently an underdeveloped transformation.