The Synthesis of Aromatic Heterocycles from Propargylic Compounds

Cationic indium catalysis as a powerful tool for generating α-alkyl propargyl cations for SN1 reactions

Dehydration is an abundant and promising process in chemical, biochemical, and industrial fields. Dehydration methods can contribute to building a modern and sustainable society with minimal environmental impact. Breakthrough advances in the dehydrative SN1 reaction can be achieved through the discovery of new cationic indium catalysts. Here we show that the breakthrough advances in the dehydrative SN1 reaction can be achieved using the cationic indium catalysts. The dehydrative carbon-carbon bond formation of α-alkyl propargyl alcohols afforded a wide variety of α-aryl- and heteroaryl-propargyl compounds. Mechanistic investigations into this process revealed that the InCl3/AgClO4/Bu4NPF6/1,1'-binaphthol catalytic system generated a powerful cationic indium catalyst that could promote the dehydration of alcohols. Labile α-alkyl propargyl cations were found to self-condense, and the catalyst system efficiently regenerated propargyl cations for reaction with nucleophiles. This propargylation reaction directly proceeded from the corresponding alcohols under mild and open-air conditions and tolerated a broad scope of functional groups. Furthermore, a wide variety of nucleophiles, including aromatic and heteroaromatic compounds, phenols, alcohols, and sulfonamides, reacted with the corresponding cations to afford the propargyl compounds in good to high yields. Finally, the synthetic utility of this reaction was demonstrated by the synthesis of colchicine and allocolchicine analogues. The dehydration process could help create new compounds that were previously impossible to synthesize and is more eco-friendly and efficient than conventional methods.

Recent approaches for the synthesis of pyridines and (iso)quinolines using propargylic Alcohols

Propargylic alcohols are one of the readily available and highly explored building blocks in organic synthesis. They show distinct reactivities compared to simple alcohols and/or alkynes, and hence provide diverse possibilities to develop novel synthetic strategies for the construction of polycyclic systems, including heterocycles. The six-membered heterocycles, pyridines, quinolines, and isoquinolines, are very important privileged structures in medicinal chemistry and drug discovery due to their broad spectrum of biological activities. They are also part of vitamins, nucleic acids, pharmaceuticals, antibiotics, dyes, and agrochemicals. Many synthetic strategies have been developed for the rapid and efficient generation of these cyclic systems. One such strategy is employing the propargylic alcohols as reactants in the form of either a 3-carbon component or 2-carbon unit. Thus, in this review article, we aimed to summarize various approaches to pyridines, quinolines, and isoquinolines from propargylic alcohols. To the best of our knowledge, so far, no focused reviews have appeared on this topic in the literature. Due to the many reports available, we also restricted ourselves to the developments during the past 17 years, i.e., 2005-2021. We strongly believe that this review article provides comprehensive coverage of research articles on the title topic, and will be of great value for the organic synthetic community for further developments in this area of research.

Indium-Catalysed Transfer Hydrogenation for the Reductive Cyclisation of 2-Alkynyl Enones towards Trisubstituted Furans

Indium tribromide catalysed the transfer hydrogenation from dihydroaromatic compounds, such as the commercially available γ-terpinene, to enones, which resulted in the cyclisation to trisubstituted furan derivatives. The reaction was initiated by a Michael addition of a hydride nucleophile to the enone subunit followed by a Lewis-acid-assisted cyclisation and the formation of a furan-indium intermediate and a Wheland intermediate derived from the dihydroaromatic starting material. The product was formed by protonation from the Wheland complex and replaced the indium tribromide substituent. In addition, a site-specific deuterium labelling of the dihydroaromatic HD surrogates resulted in site specific labelling of the products and gave useful insights into the reaction mechanism by H-D scrambling.

Recent advances in metal catalyzed or mediated cyclization/functionalization of alkynes to construct isoxazoles

This review summarized the recent developments in metal catalyzed or mediated cyclization/functionalization of alkynes to construct isoxazoles.

Nucleophile-assisted cyclization of β-propargylamino acrylic compounds catalyzed by gold(i): a rapid construction of multisubstituted tetrahydropyridines and their fused derivatives

A variety of medicinally important multisubstituted tetrahydropyridines, 4-aryl piperidines, isoquinolines and fused pyridines were prepared in 1–3 step sequences via nucleophile-assisted Au(i)-catalyzed cyclizations of easily available azaenynes.

Copper-catalyzed, silver-mediated formal [3+2] cycloaddition of simple alkynes with β-ketoesters through propargylic C(sp3)–H functionalization

The propargylic C(sp³)–H functionalization strategy has been successfully employed in the copper-catalyzed [3+2] cycloaddition of simple alkynes with β-ketoesters. Under optimal conditions, the reaction proceeds smoothly to give a variety of highly functionalized furans in moderate to high yields.

Reactions of secondary propargylamines with heteroallenes for the synthesis of diverse heterocycles

This focused review aims to summarize recent developments in the processes involving additions of secondary propargylamines to various heteroallenes and subsequent transition metal-catalyzed or electrophile-mediated cyclizations.

A new microwave-assisted thionation-heterocyclization process leading to benzo[c]thiophene-1(3H)-thione and 1H-isothiochromene-1-thione derivatives

The first example of a MW-assisted tandem thionation/S-cyclization process, leading to benzothiophenethione and isothiochromenethione derivatives, is reported.

Iodine-mediated regioselective guanylation-amination of propargylamines towards the synthesis of diversely substituted 2-aminoimidazoles

A diversity-oriented approach for the synthesis of 2-aminoimidazoles is presented.

Tridentate P,N,N-ligand promoted copper-catalyzed [3 + 2] cycloaddition of propargylic esters with β-enamino esters: synthesis of highly functionalized pyrroles

By employment of a newly developed tridentate P,N,N-ligand, a copper-catalyzed [3 + 2] cycloaddition of propargylic esters with β-enamino esters for the construction of highly functionalized pyrroles has been developed.

Copper-mediated synthesis of pyrazolo[1,5-a]pyridines through oxidative linkage of C–C/N–N bonds

Copper-mediated synthesis of pyrazolo[1,5-a]pyridine-3-carboxylates through oxidative linkage of C–C and N–N bonds under mild reaction conditions with broad substrate scope is described.

Recent advances in the synthesis of thiophene derivatives by cyclization of functionalized alkynes

This review is intended to highlight some recent and particularly interesting examples of the synthesis of thiophene derivatives by heterocyclization of readily available S-containing alkyne substrates.

Silver(i)-catalyzed annulation for the regioselective synthesis of N-imino-γ-carbolinium ylides from hydrazones of indole-3-carbonyl derivatives and propargylic alcohols

A regioselective efficient synthetic approach to N-imino-γ-carbolinium ylides via AgOTf-catalyzed iminoannulation from indole derivatives and propargylic alcohols has been developed.