A Strategy To Obtain o-Naphthoquinone Methides: Ag(I)-Catalyzed Cyclization of Enynones for the Synthesis of Benzo[ h]chromanes and Naphthopyryliums

Metal-free radical cascade cyclization/haloazidation of enynones to access functionalized 1-indanones

A novel three-component radical cyclization/haloazidation of enynones, employing TMSN3 as the azide source and NIS (NBS or NCS) as the halogen source, has been developed under metal-free conditions for the efficient synthesis of various 1-indanones with moderate yields and acceptable Z/E ratio. The reaction progresses through a sequence involving radical addition, 5-exo-dig cyclization, and radical coupling, ultimately resulting in the formation of three new chemical bonds and a new ring in a single step. The synthetic benefits of this method have been proven by large-scale experiments and late-stage modification.

Cooperative Catalysis-Enabled (4 + 3) Cycloaddition of 2-Indolylmethanols with In Situ-Generated ortho-Naphthoquinone Methides

A cooperative catalysis-enabled (4 + 3) cycloaddition of 2-indolylmethanols with ortho-naphthoquinone methides (o-NQMs), which were in situ-generated from enynones, has been established in the presence of silver/Brønsted acid cocatalysts. In the reaction pathway, the key o-NQM intermediates were formed through Ag(I)-catalyzed cyclization of enynones, while the indole-based carbocation intermediates were generated via Brønsted acid-catalyzed dehydration of 2-indolylmethanols. By this approach, a wide range of seven-membered cyclohepta[b]indoles were synthesized in good yields with high efficiency under mild reaction conditions, which serves as a useful strategy toward constructing indole-fused seven-membered rings. Moreover, the catalytic asymmetric version of this (4 + 3) cycloaddition has been realized under the cooperative catalysis of Ag(I) with chiral phosphoric acid, which offered chiral cyclohepta[b]indole with a good enantioselectivity (75% ee). This work not only represents the first cooperative catalysis-enabled (4 + 3) cycloaddition of 2-indolylmethanols but also provides a good example for o-NQM-involved cycloadditions, which will contribute to the chemistry of 2-indolylmethanols and enrich the research contents of cooperative catalysis.

Catalytic Radical-Triggered Annulation/Iododifluoromethylation of Enynones for the Stereospecific Synthesis of 1-Indenones

A new Pd(II)-catalyzed annulation/iododifluoromethylation of enynones has been developed for the synthesis of versatile 1-indanones with moderate to good yields (26 examples). The present strategy enabled the concomitant incorporation of two important difluoroalkyl and iodo functionalities into 1-indenone skeletons with (E)-stereoselectivity. The mechanistic pathway was proposed, consisting of the difluoroalkyl radical-triggered α,β-conjugated addition/5-exo-dig cyclization/metal radical cross-coupling/reductive elimination cascade.

Catalytic Benzannulation Reactions of Enynones for Accessing Heterocycle-Incorporating Diarylmethanes

Two catalytic benzannulation/1,4-addition cascades of available enynones with indoles or tetronic acid-derived enamino lactones as C-nucleophiles are reported, producing 53 examples of heterocycle-incorporating diarylmethanes with moderate to excellent yields. The combination of AgOTf and Sc(OTf)3 permits bimetallic synergistic catalysis of the reaction of enynones with indoles to access 35 triarylmethanes in generally good yields with ortho-naphthoquinone methides, generated in situ, as the key intermediates. Exchanging indoles for tetronic acid-derived enamino lactones resulted in 18 diarylmethanes by the Ag-catalyzed benzannulation/1,4-addition cascade. Both reactions are advantageous for their high atom utilization and mild reaction conditions, providing intriguing complementary approaches for the direct syntheses of new heterocycle-containing diarylmethanes.

Synthesis of Diversified Pyrazolo[3,4-b]pyridine Frameworks from 5-Aminopyrazoles and Alkynyl Aldehydes via Switchable C≡C Bond Activation Approaches

A cascade 6-endo-dig cyclization reaction was developed for the switchable synthesis of halogen and non-halogen-functionalized pyrazolo[3,4-b]pyridines from 5-aminopyrazoles and alkynyl aldehydes via C≡C bond activation with silver, iodine, or NBS. In addition to its wide substrate scope, the reaction showed good functional group tolerance as well as excellent regional selectivity. This new protocol manipulated three natural products, and the arylation, alkynylation, alkenylation, and selenization of iodine-functionalized products. These reactions demonstrated the potential applications of this new method.

Asymmetric Synthesis of Chromans Through Bifunctional Enamine-Metal Lewis Acid Catalysis

Cooperative enamine-metal Lewis acid catalysis has emerged as a powerful tool to construct carbon-carbon and carbon-heteroatom bond forming reactions. A concise synthetic method for asymmetric synthesis of chromans from cyclohexanones and salicylaldehydes has been developed to afford tricyclic chromans containing three consecutive stereogenic centers in good yields (up to 87 %) and stereoselectivity (up to 99 % ee and 11 : 1 : 1 dr). This difficult organic transformation was achieved through bifunctional enamine-metal Lewis acid catalysis. It is believed that the strong activation of the salicylaldehydes through chelating to the metal Lewis acid and the bifunctional nature of the catalyst accounts for the high yields and enantioselectivity of the reaction. The absolute configurations of the chroman products were established through X-ray crystallography. DFT calculations were conducted to understand the mechanism and stereoselectivity of this reaction.

CuI-mediated benzannulation of (ortho-arylethynyl)phenylenaminones to assemble α-aminonaphthalene derivatives

A copper-mediated annulation protocol for new (ortho-arylethynyl)phenyl enaminones bearing a N,N-dimethylamine moiety was developed to facilely install a series of α-aminonaphthalene derivatives.

Construction of Partially Protected Nonsymmetrical Biaryldiols via Semipinacol Rearrangement of o-NQM Derived from Enynones

The construction of partially protected nonsymmetrical biaryldiols catalyzed by AgBF₄ has been achieved. The approach facilitates the formation of two new aryl rings and the introduction of two hydroxyl groups (one free and one TBS-protected) via the o-NQM generation/semipinacol rearrangement cascade, featuring high atom- and step-economy to afford a diverse array of partially protected nonsymmetrical biaryldiols under mild conditions.

Allylsilane Reagent-Controlled Divergent Asymmetric Catalytic Reactions of 2-Naphthoquinone-1-methide

Herein, the first allylsilane reagent-controlled divergent asymmetric Hosomi-Sakurai conjugate allylation and hetero-Diels-Alder (HDA) with 2-naphthoquinone-1-methide (2-Nap-Q-1-M) under the catalysis of Sc^III /Feng ligand complex is reported. With these methods, a variety of uniquely substituted chiral allyl-functionalized diaryl compounds and naphthopyran products were obtained in a straightforward and highly stereoselective (up to 96.5:3.5 e.r.) manner under mild conditions. Moreover, it is demonstrated that 2-Nap-Q-1-M can serve as an efficient diene for a side asymmetric Diels-Alder (D-A) reaction. This principle can provide a straightforward access to hydrophenalene in an optically active form, which represents a structural core of various natural products and bioactive molecules.

1,4-Addition of o-naphthoquinone methides induced by silver-catalyzed cyclization of enynones: an approach to unsymmetrical triarylmethanes and benzo[f]chromenes

The 1,4-addition reaction of ortho-naphthoquinone methide (o-NQM) intermediates induced by silver-catalyzed ring formation with electron-rich aromatic compounds and α-methylene ketones has been developed.