Construction of Diverse Fused Chromene Frameworks via Isocyanide-Based Three-Component Reaction

A convenient synthetic protocol for diverse fused chromenes was successfully developed by a three-component reaction of alkyl isocyanides, dialkyl but-2-ynedioates, and various cyclic 1,3-dipolarophiles containing o-hydroxyphenyl group. In the absence of any catalyst, the three-component reaction of alkyl isocyanides, dialkyl but-2-ynedioates, and 3-(o-hydroxyarylidene)indolin-2-ones in tetrahydrofuran at 60 °C resulted in unique functionalized spiro[cyclobuta[c]chromene-1,3'-indolines] in good yields and with high diastereoselectivity. However, the similar three-component reaction with 2-(5-halo-2-hydroxyarylidene)indolin-2-ones afforded unexpected chain products in satisfactory yields. In addition, the three-component reaction of alkyl isocyanides, dialkyl but-2-ynedioates, and 2-(o-hydroxyarylidene)-1,3-indanediones in tetrahydrofuran at 60 °C resulted in complex indeno[2',1':5,6]pyrano[3,4-c]chromene derivatives in high yields and with high diastereoselectivity.

Gold catalysis in quinoline synthesis

Quinolines are biologically and pharmaceutically important N-heterocyclic aromatic compounds, which have broad applications in medicinal chemistry. Thus, their efficient synthesis has attracted extensive attention, and a broad range of synthetic strategies have been established. Of note, gold-catalyzed methodologies for the synthesis of quinolines have greatly advanced this field. Various gold-catalyzed intermolecular annulation reactions, such as annulations of aniline derivatives with carbonyl compounds or alkynes, annulations of anthranils with alkynes, and annulations based on A3-coupling reactions, as well as intramolecular cyclization reactions of azide-tethered alkynes, 1,2-diphenylethynes, and 2-ethynyl N-aryl indoles, have been developed. This review provides an overview of this exciting research area. Typical achievements in reaction methodologies and plausible reaction mechanisms are summarized.

Nucleophile-Controlled Trapping of Gold Carbene by Nitriles and Water: Synthesis of 5H-Pyrimido[5,4-b]indoles and 2-Benzylidene-3-indolinones

A gold-catalyzed, nucleophile-controlled cascade reaction of N-(2-azidophenyl-ynyl)methanesulfonamides with nitriles and water is described that provides structurally diverse 5H-pyrimido[5,4-b]indoles and 2-benzylidene-3-indolinones in good to excellent yields. Mechanistic studies indicate that the β-sulfonamido-α-imino gold carbene is the key intermediate which is generated through the gold-catalyzed cyclization of N-(2-azidophenyl-ynyl)methanesulfonamides and undergoes formal [4 + 2] cascade annulation with nitriles and intramolecular SN2' type reaction with water, respectively.

Copper-Catalyzed Tandem Cyclization/Direct C(sp2)-H Annulation of Azide-Ynamides via α-Imino Copper Carbenes: Access to Azepino[2,3-b:4,5-b']diindoles

A novel copper-catalyzed tandem cyclization/direct C(sp²)-H annulation of phenyl azide-ynamides via α-imino copper carbenes has been developed, which provides a concise and flexible approach for the construction of a range of valuable azepino[2,3-b:4,5-b']diindoles in mostly good to excellent yields with high chemoselectivities. This tandem reaction also exhibits a broad substrate scope, excellent functional group tolerance, simple operation, and mild reaction conditions.

Construction of the quinobenzoxazine core via gold-catalyzed dual annulation of azide-tethered alkynones with anthranils

A new catalytic method for the construction of the quinobenzoxazine core has been developed employing the gold-catalyzed cyclization of o-azidoacetylenic ketones in the presence of anthranils. The overall process comprises of a gold-catalyzed 6-endo-dig cyclisation of o-azidoacetylenic ketone leading to a α-imino gold carbene and subsequent carbene transfer to anthranil leading to the 3-aryl-imino-quinoline-4-one intermediate, which undergoes 6π-electrocyclization and aromatization to form the central quinobenzoxazine core. This transformation provides a new approach to a diverse array of quinobenzoxazine structures, in addition to being scalable and having mild reaction conditions.

Rhodium-catalyzed coupling-cyclization of o-alkynyl/propargyl arylazides or o-azidoaryl acetylenic ketones with arylisocyanides: synthesis of 6H-indolo[2,3-b]quinolines, dibenzonaphthyridones and dihydrodibenzo[b,g] [1,8]-naphthyridines

The developed rhodium-catalyzed coupling-cyclization provides a new strategy for the assembly of 6H-indolo[2,3-b]quinolines, dibenzonaphthyridones and dihydrodibenzo[b,g] [1,8]-naphthyridines.

Access to 3-Sulfonamidoquinolines by Gold-Catalyzed Cyclization of 1-(2'-Azidoaryl)propargylsulfonamides through 1,2-N Migration

We describe a gold-catalyzed cyclization of 1-(2'-azidoaryl)propargylsulfonamides for the synthesis of 3-sulfonamidoquinolines, featuring a rare and highly selective 1,2-N migration. The key α-imino gold carbene intermediate is generated through an intramolecular nucleophilic attack of the azide group to the Au-activated triple bonds in a 6-endo-dig manner.

TfOH-Promoted Decyanative Cyclization toward the Synthesis of 2,1-Benzisoxazoles

A novel solvent-free, TfOH-promoted decyanative cyclization approach for the synthesis of 2,1-benzisoxazoles has been developed. The reactions are complete instantly at room temperature and result in the formation of the desired 2,1-benzisoxazoles in a 34-97% isolated yield.

Multicomponent cascade reactions of HKAs: synthesis of highly functionalized 5H-chromeno[4,3-d]pyrimidines

A novel protocol was developed for the construction of functionalized 5H-chromeno[4,3-d]pyrimidines from 3-formylchromones, heterocyclic ketene aminals, and amidine hydrochlorides via a novel cascade reaction involving ring-opening and 1,3-H shift reactions.

Gold(i)-catalyzed redox transformation of o-nitroalkynes with indoles for the synthesis of 2,3′-biindole derivatives

A gold(i)-catalyzed cascade reaction of o-nitroalkynes with indoles has been reported for the rapid assembly of 2-indolyl indolone N-oxides, which exhibit high anticancer potency against SCLC cells.

Copper-Catalyzed Azide-Ynamide Cyclization to Generate α-Imino Copper Carbenes: Divergent and Enantioselective Access to Polycyclic N-Heterocycles

Here an efficient copper-catalyzed cascade cyclization of azide-ynamides via α-imino copper carbene intermediates is reported, representing the first generation of α-imino copper carbenes from alkynes. This protocol enables the practical and divergent synthesis of an array of polycyclic N-heterocycles in generally good to excellent yields with broad substrate scope and excellent diastereoselectivities. Moreover, an asymmetric azide-ynamide cyclization has been achieved with high enantioselectivities (up to 98:2 e.r.) by employing BOX-Cu complexes as chiral catalysts. Thus, this protocol constitutes the first example of an asymmetric azide-alkyne cyclization. The proposed mechanistic rationale for this cascade cyclization is further supported by theoretical calculations.

Nitrene Transfer and Carbene Transfer in Gold Catalysis

Catalytic transformations involving metal carbenes are considered one of the most important aspects of homogeneous transition metal catalysis. Recently, gold-catalyzed generation of gold carbenes from readily available alkynes represents a significant advance in metal carbene chemistry. This Review summarizes the advances in the gold-catalyzed nitrene-transfer reactions of alkynes with nitrogen-transfer reagents, such as azides, nitrogen ylides, isoxazoles, and anthranils, and gold-catalyzed carbene-transfer reactions, involving oxygen atom-transfer reactions of alkynes with nitro compounds, nitrones, sulfoxides, and pyridine N-oxides, through the presumable α-imino gold carbene and α-oxo gold carbene intermediates, respectively. Gold-catalyzed processes are reviewed by highlighting their product diversity, selectivity, and applicability, and the mechanistic rationale is presented where possible.

Gold(iii)-catalyzed azide-yne cyclization/O-H insertion cascade reaction for the expeditious construction of 3-alkoxy-4-quinolinone frameworks

A gold-catalyzed 6-endo-dig azide-yne cyclization/O-H insertion cascade reaction of azide-tethered alkynes with alcohols has been developed, and it provides an expeditious access to 3-alkoxy-4-quinoline derivatives in good to high yields under mild and neutral reaction conditions with broad substrate generality. The utility of this method is emphasized by a scalable experiment and concise total synthesis of a bioactive natural product Leiokinine A, and other bioactive quinoline analogs.

Silver-catalyzed cascade reactions of 3-cyanochromone with 1,1-enediamines: synthesis of highly functionalized 2-(pyridin-3-yl)-chromeno[2,3-d]pyrimidines

A novel protocol for the construction of 2-(pyridin-3-yl)-chromeno[2,3-d]pyrimidines from 3-cyanochromone with 1,1-enediamines via an unprecedented cascade reaction has been developed by simply refluxing the reactants under the catalysis of silver carbonate.

Synthesis of 3-(2-quinolyl) chromones from ynones and quinoline N-oxides via tandem reactions under transition metal- and additive-free conditions

The synthesis of 3-(2-quinolyl) chromones from ynones and quinoline N-oxides via a sequential [3+2] cycloaddition/ring-opening/O-arylation reaction under transition metal- and additive-free conditions is reported.