Stereoselective synthesis of 6/7/6-fused heterocycles with multiple stereocenters via an internal redox reaction/inverse electron-demand hetero-Diels-Alder reaction sequence

A highly stereoselective synthesis of fused heterocycles with multiple stereocenters via an internal redox reaction/inverse electron-demand hetero-Diels-Alder (IEDHDA) reaction sequence is described. The present reaction sequence has three interesting features: (1) complete control of two potentially competitive processes, i.e., hetero-Diels-Alder reaction and [1,5]-hydride shift; (2) one-shot construction of the complicated 6/7/6-fused heterocyclic structure having multiple stereocenters; and (3) high control of its stereoselectivity. When alkenylidene barbiturates with an allyl benzyl ether moiety were treated with a catalytic amount of Sc(OTf)3 and 2,2'-bipyridine, the internal redox reaction/IEDHDA reaction proceeded successively to afford 6/7/6-fused heterocycles in good chemical yields with good to excellent diastereoselectivities.

Divergent synthesis of multi-substituted phenanthrenes via an internal redox reaction/ring expansion sequence

We report an effective synthetic route to multi-substituted phenanthrenes via an internal redox reaction/ring expansion sequence. The interesting feature of the present system is that it allows for the divergent synthesis of the target skeleton depending on the selected Lewis acid catalyst. When benzylidene malonates with a cyclic structure at the ortho-position were treated with BF3·OEt2, three sequential processes (internal redox reaction/elimination of the alkoxy group/ring expansion) proceeded to give phenanthrene derivatives in which the alkoxycarbonyl (CO2R) group and the alkyl (R) group were in close proximity to each other, in good chemical yields. In sharp contrast, treatment with Bi(OTf)3 exclusively led to the formation of another type of phenanthrene, whose R group was positioned distal to the CO2R group.

Synthesis of Polysubstituted Naphthalenes by a Hydride Shift Mediated C-H Bond Functionalization/Aromatization Sequence

A synthetic strategy for forming multisubstituted naphthalenes based on hydride shift mediated C(sp3)-H bond functionalization was developed. This strategy consists of three successive transformations: (1) an intramolecular hydride shift mediated C(sp3)-H bond functionalization; (2) a decarboxylative fragmentation; and (3) an oxidation reaction. When benzylidene malonates having a 2-alkoxyethyl group at the ortho position were treated with a catalytic amount of Al(OTf)3, the hydride shift/cyclization reaction proceeded smoothly to afford tetralin derivatives in good chemical yields. The resulting tetralins were easily converted into naphthalenes by exposing them to modified Krapcho decarboxylation reaction conditions (LiCl, DMSO, and heating under an O2 atmosphere). The one-pot operation of these two reactions was also realized.

Dearomatization of 3-Aminophenols for Synthesis of Spiro[chromane-3,1'-cyclohexane]-2',4'-dien-6'-ones via Hydride Transfer Strategy-Enabled [5+1] Annulations

The Sc(OTf)3-catalyzed dearomative [5+1] annulations between readily available 3-aminophenols and O-alkyl ortho-oxybenzaldehydes were developed for synthesis of spiro[chromane-3,1'-cyclohexane]-2',4'-dien-6'-ones. The "two-birds-with-one-stone" strategy was disclosed by the dearomatization of phenols and direct α-C(sp3)-H bond functionalization of oxygen through cascade condensation/[1,5]-hydride transfer/dearomative-cyclization process. In addition, the antifungal activity assay and derivatizations of products were conducted to further enrich the utility of the structure.

Lewis acid-catalyzed formal 1,3-aminomethyl migration

Here we report a Lewis acid-catalyzed 1,3-aminomethyl migration rection. When δ-amino acid derivatives were treated with a catalytic amount of Sc(OTf)3, 1,3-migration of the aminomethyl group proceeded smoothly to afford β-amino acid derivatives in moderate to good chemical yields. Detailed investigation suggested that this migration reaction proceeded through the fragmentation/recombination pathway.

Aromatization-driven cascade [1,5]-hydride transfer/cyclization for synthesis of spirochromanes

An aromatization-driven hydride transfer-involved α-C(sp3)–H bond functionalization of the oxygen atom was developed. Easily prepared p-quinone methides were applied to initiate [1,5]-hydride transfer/cyclization for generating spirochromanes.

Microwave-accelerated and benzoyl peroxide (BPO)-initiated cyclization of 1,5-enynes having cyano groups with cyclic alkanes under metal-free conditions

A novel and efficient method for preparing exocyclic indan derivatives, with this method involving benzoyl peroxide (BPO)-initiated cyclization of 1,5-enynes having cyano groups with simple cyclic alkanes under microwave irradiation, has been developed. The presented approach showed advantages of simple conditions, an environmentally friendly protocol, good functional-group tolerance, and high yields of products.

Controllable construction of pharmaceutically significant scaffolds of 2,3-dihydroquinolin-4-one and benzoazepine-5-one via redox-neutral cascade hydride transfer/cyclization

The scaffolds of 2,3-dihydroquinolin-4-one and benzoazepine-5-one were controllably constructed relying on cascade condensation/redox-neutral [1,6]/[1,7]-hydride transfer/cyclization from 2-aminoacetophenone and various aldehydes as well as isatins.

Redox-triggered dearomative [5 + 1] annulation of indoles with O-alkyl ortho-oxybenzaldehydes for the synthesis of spirochromanes

The dearomative [5 + 1] annulation of 2-methylindoles with new five-membered synthon was developed through cascade [1,5]-hydride transfer/dearomative cyclization in HFIP for the synthesis of spirochromanes bearing the 2-methylindolenine skeleton.

HFIP-mediated three-component imidization of electron-rich arenes with in situ formed spiroindolenines for facile construction of 2-arylspiroindolenines

The three-component reaction of o-aminobenzaldehydes with 5-hydroxyindole and electron-rich arenes has been achieved through HFIP-mediated cascade hydride transfer/dearomative cyclization/CDC-type imidization at room temperature under air.

Diastereoselective construction of structurally diverse 2,3-dihydroquinolin-4-one scaffolds via redox neutral cascade [1,7]-hydride transfer/cyclization

The 2,3-dihydroquinolin-4-one scaffolds were constructed diastereoselectively via cascade Knoevengel condensation/[1,7]-HT/cyclization/transesterification featuring novel structures and diastereoselective construction of all-carbon quaternary centers.

Gold-Catalyzed Hydroarylation Reactions: A Comprehensive Overview

The hydroarylation of alkynes, alkene, and, allene is a cost-effective and efficient way to incorporate unsaturated moieties into aromatic substrates. This review focuses on gold-catalyzed hydroarylation, which produces aromatic alkenes,...

Divergent Access to Seven/Five-Membered Rings Based on [1,6]-Hydride Shift/Cyclization Process

We have achieved a divergent access to seven/five-membered rings based on a [1,6]-hydride shift/cyclization process from benzylidenemalonate with an o-alkoxymethyl group. Whereas Yb(OTf)₃ afforded benzoxepines (with a seven-membered ring) selectively, indanes (with a five-membered ring) were the main products when Sc(OTf)₃ was employed.

Access to Polycyclic Indole-3,4-Fused Nine-Membered Ring via Cascade 1,6-Hydride Transfer/Cyclization

A cascade aldimine condensation/1,6-hydride transfer/Mannich-type cyclization of indole-derived phenylenediamine with aldehydes was developed for one-step construction of a polycyclic indole-3,4-fused skeleton. Aldehyde serves as a key to start the whole process, including 1,6-hydride transfer enabled δ-C(sp³)-H activation of the secondary amine. The challenges of construction of medium-sized rings are addressed via hydride transfer chemistry.

1,5-Hydride-Shift-Triggered Cyclization for the Synthesis of Unsymmetric Julolidines

Directly accessible 8-substituted tetrahydroquinolines undergo 1,5-hydride-shift-triggered cyclization to provide difficult to access julolidine derivatives in yields of 21–98% under scandium(III) triflate catalysis. Additionally, the scope of the reaction, several follow-up transformations and a remarkable side process discovered during optimization of the conditions are highlighted.

Imidazol-5-ones as a substrate for [1,5]-hydride shift triggered cyclization

(2-Aminobenzylidene)-imidazolones were used as substrates for [1,5]-hydride shift triggered cyclization under promotion by TiCl₄ at room temperature.

Recent advances in hydride transfer-involved C(sp3)–H activation reactions

This review summarizes the recent progresses (2016–2020) in the hydride transfer-enabled C(sp³)–H activation according to the reaction types, categorized into the intramolecular/intermolecular C(sp³)–H functionalization, and hydride reduction.

Rapid access to 3-indolyl-1-trifluoromethyl-isobenzofurans by hybrid use of Lewis/Brønsted acid catalysts

We report herein a rapid access to 3-indolyl-1-trifluoromethyl-isobenzofurans via a [1,4]-hydride shift/cyclizatin/intermolecular nucleophilic addition reaction sequence. In this process, a Lewis acid promoted internal redox reaction ([1,4]-hydride shift/cyclization) followed by a Brønsted acid promoted intermolecular reaction (generation of cyclic oxonium cation/intermolecular Friedel-Crafts reaction) occurred to give various 3-indolyl-1-trifluoromethyl-isobenzofurans in good chemical yields.

Hydrogen-bonding-assisted redox-neutral construction of tetrahydroquinolines via hydride transfer

The hydrogen-bonding-assisted construction of tetrahydroquinolines decorated with structurally diverse 3,3'-difunctional groups has been realized via a hydride transfer-involved three-step cascade reaction in the presence of morpholine. This protocol solves the limitation of acyclic 1,3-dicarbonyl compounds by one-pot synthesis of tetrahydroquinolines, featuring operational simplicity, broadly applicable substrates, and metal- and acid-free conditions with EtOH as a hydrogen-bonding donor.

Expeditious Synthesis of Multisubstituted Quinolinone Derivatives Based on Ring Recombination Strategy

We achieved a concise construction of 3-substituted quinolinone derivatives based on a ring recombination strategy. In this process, seven transformations involving two types of cyclization proceeded in one pot to afford various quinolinone derivatives in good to excellent chemical yields (up to 98%).