Recent Advances in the Chemistry of Masked Ortho -Benzoquinones and Their Applications in Organic Synthesis

Asymmetric Total Synthesis of (+)-Chuanxiongnolide L1 via a Stereoselective Oxidative Dearomatization/Diels-Alder Strategy

The first asymmetric total synthesis of chuanxiongnolide L1 was achieved in 16 steps and 1.9% overall yield by employing a bioinspired chiral auxiliary strategy. The key steps involving asymmetric oxidative dearomatization of chiral amino ether and subsequent asymmetric Diels-Alder reaction of the resulting masked chiral ortho-benzoquinone were adopted.

Aniline Derivatives from Lignin under Mild Conditions Enabled by Electrochemistry

The development of environmentally friendly methods for the valorization of important phenolic platform chemicals originating directly from lignin-first depolymerization into value-added N-chemicals, such as aniline derivatives, is of high industrial interest. In this work, we tackle this challenging transformation by the judicious combination of electrochemical conversion and chemical functionalization steps. In the first step, lignin-derived para-substituted guaiacols and syringols undergo an atom-efficient, room-temperature anodic oxidation using methanol both as solvent and reagent towards the formation of the corresponding cyclohexadienone derivatives, which are subsequently converted to synthetically challenging ortho-methoxy substituted anilines by reaction with ethyl glycinate hydrochloride under mild conditions. The developed method was applied to crude lignin depolymerization bio-oils, derived from reductive catalytic fractionation (RCF) mediated either by copper-doped porous metal oxide (Cu20 PMO) or Ru/C, allowing the selective production of 4-propanol-2-methoxyaniline (1Gb) and 4-propyl-2-methoxyaniline (2Gb), respectively, from pine lignocellulose. Finally, the application of 2Gb was further studied in the synthesis of carbazole 2Gc, a lignin-derived analogue of biologically active alkaloid murrayafoline A.

Molecular Reconstruction with Stereochemical Relay: An Investigation into the Rearrangement of Spiro[4.5]decadienone to Benzoxepane

Herein, we report the unusual skeletal rearrangement of spiro[4.5]decadienone to benzoxepane. In particular, Lewis acid-promoted epoxide-opening ipso-cyclization of aryl epoxides afforded spiro[4.5]decadienone intermediates. Subsequent thermal activation assembled a benzoxepane core via rearomative molecular reorganization. The sequence was high-yielding and highly diastereoselective but sensitive to the aromatic substitution pattern and the epoxide side chain. Mechanistic studies suggested that the rearrangement proceeded via an uncommon intramolecular enolate attack onto the electrophilic O of p-quinone oxonium zwitterion. DFT calculations helped rationalize the product distribution and the origin of diastereoselectivity. Initial investigation into the application of this chemical transformation is also presented.

Influence of Ring Strain on the Formation of Rearrangement vs Cyclization Isotwistane Products in the Acyl Radical Reaction of Bicyclo[2.2.2]octanone

An acyl radical reaction of bicyclo[2.2.2]octenone to yield either rearranged or cyclized isotwistane products is described. The influence of ring strain on the reaction was demonstrated by alternating the sizes of the fused ring in the starting material. DFT calculations showed that the reaction is under thermodynamic control and proceeds via a 5-exo-trig cyclization intermediate, which undergoes either hydrogen-atom transfer (HAT) to give a cyclized product or rearrangement via a twistane intermediate to give a rearranged product.

Switchable Oxidative Reactions of N-allyl-2-Aminophenols: Palladium-Catalyzed Alkoxyacyloxylation vs an Intramolecular Diels-Alder Reaction

The Pd(II)-catalyzed reaction of N-allyl-2-aminophenols in the presence of PhI(OCOR)₂ as the oxidant resulted in an alkoxyacyloxylation process, with the formation of functionalized dihydro-1,4-benzoxazines. The reaction performed in the absence of palladium catalyst switched to an intramolecular Diels–Alder reaction (IMDA) pathway, which was the result of an oxidative dearomatization of the 2-aminophenol, nucleophilic addition, and Diels–Alder reaction cascade, highlighting the role of the oxidant as both a nucleophilic donor and an oxidizing agent.

Dearomatization of a 3-Hydroxypyridine Through an Unexpected Oxidative Deformylation Process: An Entry to Azacyclohexadienones

Phenols are well-known precursors of cyclohexadienones, which have widespread applications in organic synthesis. In contrast, their hydroxypyridine counterparts have not been explored yet. An unprecedented oxidative dearomatization of a 3-hydroxypyridine involving an unexpected deformylation step is reported. The chemical reactivity of the resulting unreported azacyclohexadiene-type compound was also explored.

A gold-triggered dearomative spirocarbocyclization/Diels–Alder reaction cascade towards diverse bridged N-heterocycles

An efficient chemo- and diastereoselective gold-triggered post-Ugi non-oxidativeortho-dearomative spirocarbocyclization/Diels–Alder reaction cascade sequence has been developed to deliver diverse bridged polycyclic N-heterocycles.

Facile construction of diverse polyheterocyclic scaffolds via gold-catalysed dearomative spirocyclization/1,6-addition cascade

A post-Ugi gold-catalysed chemo- and diastereo-selective cascade dearomative spirocyclization/1,6-addition sequence is disclosed for the facile synthesis of diverse fused polyheterocyclic scaffolds bearing indole, pyrrole, benzothiophene, furan or electron-rich arene cores.

Biomimetic Syntheses of (±)-Isopalhinine A, (±)-Palhinine A, and (±)-Palhinine D

The first total synthesis of isopalhinine A, as well as unified syntheses of palhinine A and palhinine D, were successfully accomplished by means of a biomimetic strategy that proceeds through a bioinspired 5/6/6/9 tetracyclic intermediate, which mimics the amino ketone form of palhinine D. An early-stage direct S_N 2 cyclization to construct the nine-membered azonane ring minimized the transannular strain that would otherwise be increased by the twisted nature of the isotwistane skeleton. Then, a diastereoselective Diels-Alder reaction of a masked ortho-benzoquinone using the nine-membered ring as a steric shielding group furnished a functionalized 6/6/9 tricyclic skeleton and established the desired stereochemistry at the C3, C7, C12, and C15 positions in one step. A thiol-mediated acyl radical cyclization gave the bioinspired intermediate bearing three differentiated oxygen-containing functional groups, from which all three total syntheses could be completed in either two or three additional steps.

Direct Regioselective [3 + 2] Cycloaddition Reactions of Masked Difluorodiazoethane with Electron-Deficient Alkynes and Alkenes: Synthesis of Difluoromethyl-Substituted Pyrazoles

Phenylsulfone difluorodiazoethane (PhSO₂CF₂CHN₂), an easy-to-prepare and bench-stable masked CF₂-building block, has been developed. The synthetic utility of this reagent is demonstrated by a direct regioselective [3 + 2] cycloaddition with electron-deficient alkynes and alkenes. This protocol enables facile construction of an array of difluoromethyl-substituted pyrazoles in good to high yields under mild reaction conditions.

Diastereoselective construction of 4-indole substituted chromans bearing a ketal motif through a three-component Friedel–Crafts alkylation/ketalization sequence

The first TfOH-catalyzed three-component Friedel–Crafts alkylation/ketalization sequence of indoles, alcohols and ortho-hydroxychalcones was developed to afford a wide range of 4-indole substituted chromans bearing a ketal motif in 77–99% yields. Notably, only a simple filtration was needed to purify them. By altering methanol to CHCl₃, 2,4-bisindole substituted chroman with the same indole substituent at the C2 and C4 positions was afforded. Moreover, 2,4-bisindole substituted chromans with different indole substituents could be obtained by treatment of 4-indole monosubstituted chromans with another indole molecule.

The first TfOH-catalyzed three-component Friedel–Crafts alkylation/ketalization sequence of indoles, alcohols and ortho-hydroxychalcones was developed to afford a wide range of 4-indole substituted chromans bearing a ketal motif in 77–99% yields.