Transition metal catalysed cascade C-C and C-O bond forming events of alkynes

The past few decades have witnessed the emergence of domino reactions as a powerful tool for the multi-functionalization of alkynes for the rapid and smooth construction of complex molecular architectures. In this context, employing transition metal catalysis, vicinal/geminal cascade functionalization of alkynes involving C-C and C-O bond-formation reactions, has become a preferred strategy for the synthesis of oxygenated motifs. Despite this significant progress, reviews documenting such strategies are either metal/functional group-centric or target-oriented, thus hampering further developments. Therefore, in this review, different conceptual approaches based on C-C and C-O bond-forming events of alkynes such as carboxygenation (C-C and CO bond formation), carboalkoxylation (C-C and C-OR bond formation), and carboacetoxylation (C-C and C-OAc bond formations) are discussed, and examples from the literature from the last two decades are presented. Further, we have presented detailed insights into the mechanism of different transformations.

Emergence of 2-Pyrone and Its Derivatives, from Synthesis to Biological Perspective: An Overview and Current Status

Pyrone moieties are present in natural products and can be synthesized by a diverse range of synthetic methods, resulting in the formation of various derivatives through chemical modifications. Many pyrone-based derivatives are commercially available and are biocompatible. They are building blocks of various intermediates in organic synthesis. They possess remarkable biological properties including antimicrobial, antiviral, cytotoxic, and antitumor activity. These characteristics have made them valuable for the development of drugs. We have summarized recent developments in the synthesis of 2-pyrone and its derivatives and their potential applications. With regard to synthetic approaches, the focus has been on metal-free and transition metal-catalyzed reactions.

Palladium-Catalyzed Aminocyclization-Coupling Cascades: Preparation of Dehydrotryptophan Derivatives and Computational Study

Dehydrotryptophan derivatives have been prepared by palladium-catalyzed aminocyclization-Heck-type coupling cascades starting from o-alkynylaniline derivatives and methyl α-aminoacrylate. Aryl, alkyl (primary, secondary, and tertiary), and alkenyl substituents have been introduced at the indole C-2 position. Further variations at the indole benzene ring, as well as the C-2-unsubstituted case, have all been demonstrated. In the case of C-2 aryl substitution, the preparation of the o-alkynylaniline substrate by Sonogashira coupling and the subsequent cyclization-coupling cascade have been performed in a one-pot protocol with a single catalyst. DFT calculations have revealed significant differences in the reaction profiles of these reactions relative to those involving methyl acrylate or methacrylate, and between the reactions of the free anilines and their corresponding carbamates. Those calculations suggest that the nature of the alkene and of the acid HX released in the HX/alkene exchange step that precedes C-C bond formation could be responsible for the experimentally observed differences in reaction efficiencies.

Synthesis of Highly Conjugated Functionalized 2-Pyridones by Palladium-Catalyzed Aerobic Oxidative Dicarbonation Reactions of N-(Furan-2-ylmethyl) Alkyne Amides and Alkenes as Coupling Partners

A mild, step-economical method for the synthesis of highly conjugated functionalized 2-pyridones from N-(furan-2-ylmethyl) alkyne amides is reported. This method involves Pd-catalyzed aerobic oxidative dicarbonation reactions of alkynes with carbon nucleophiles of a furan ring and an acrylate or styrene as coupling partners. The UV-vis absorption spectra of some of the 2-pyridones indicated that they absorbed shortwave radiation, suggesting their potential utility for filtration of such radiation.

De Novo Protocol for the Construction of Benzo[a]fluorenes via Nitrile/Alkene Activation

Unprecedented chemo- and regioselective synthesis of benzo[a]fluorenes and naphthamide-substituted benzo[a]fluorenes were constructed from the reaction of (E)-2-aroyl-3-(2-(arylalkynes/alkenes)aryl)acrylonitrile scaffolds under metal-free conditions via the activation of nitriles and alkenes, respectively. A tentative reaction mechanism was proposed for this homofunctionalization of nitriles. Control experiments showed that the reaction proceeds via selective nitrile or alkene protonation, depending upon the substrates. Additionally, we demonstrated an alternative expeditious route for the synthesis of disubstituted benzo[a]fluorenes in the presence of TfOH alone.

Synthesis of organoselenyl isoquinolinium imides via iron(III) chloride-mediated tandem cyclization/selenation of N'-(2-alkynylbenzylidene)hydrazides and diselenides

This report describes the synthesis of organoselenyl isoquinolinium imides through a tandem cyclization between N'-(2-alkynylbenzylidene)hydrazides and diselenides. The reaction was carried out at room temperature under an ambient atmosphere using cheap iron(iii) chloride as the metallic source. The strategy shows good tolerance to a broad range of N'-(2-alkynylbenzylidene)hydrazides and diselenides, and forms C-N and C-Se bonds in one step. The obtained product is further transformed into a bioactive H-pyrazolo[5,1-a]isoquinoline skeleton easily via a silver catalyzed [3 + 2] cycloaddition.

Nickel(II)-Catalyzed [5 + 1] Annulation of 2-Carbonyl-1-propargylindoles with Hydroxylamine To Synthesize Pyrazino[1,2-a]indole-2-oxides in Water

An atom-economical and practical method for the efficient synthesis of various pyrazino[1,2-a]indole-2-oxides was developed through a nickel(II)-catalyzed [5 + 1] annulation of 2-carbonyl-1-propargylindoles with hydroxylamine in water without using an organic solvent. The reaction involved an initial condensation of 2-carbonyl-1-propargylindoles with hydroxylamine to afford oxime intermediates, which then underwent a nickel(II)-catalyzed 6-exo-dig cyclization. Preliminary studies showed that (n-Bu)₄NI served as a phase transfer catalyst and promoted the formation of active nickel(II) species. More importantly, the nickel(II) salt and phase transfer catalyst-in-water could be recycled seven times, and a gram scalable product was easily obtained in good yields through a filtration and washing protocol.

Palladium-Catalyzed Cyclization of N-Acyl- o-alkynylanilines with Isocyanides Involving a 1,3-Acyl Migration: Rapid Access to Functionalized 2-Aminoquinolines

A simple and expedient approach for the synthesis of functionalized 2-aminoquinolines via palladium-catalyzed annulation of N-acyl- o-alkynylanilines with isocyanides has been developed with high atom economy, in which an unconventional 6- endo-dig cyclization process is observed. Further investigations of the mechanism demonstrated that an intramolecular acyl migration of the N-protecting groups was involved in this transformation.

Gold(I)-Catalyzed Cyclization/Carbonylation Cascade Reaction of 1,6-Diynes: An Access to β,γ-Unsaturated Ketones

A gold-catalyzed cyclization/carbonylation cascade reaction of 1,6-diynes is reported. The reaction goes through 6-exo-dig and 6-endo-dig cyclizations in sequence, followed by hydration to provide the β,γ-unsaturated ketones with moderate to high yields under mild reaction conditions. This is the first example of intercepting the postulated 1,3-oxazine vinylgold intermediate with another pendant alkyne, which not only verifies the proposed mechanism but also provides the ketone products with cyclized 1,2,3,6-tetrahydropyridine or 3,6-dihydro-2H-pyran frameworks from corresponding diynes.

KBr/K2S2O8-mediated dibromohydration of N-(2-alkynylaryl)acetamide

A neighbouring acetamino group-participated regioselective dibromohydration of N-(2-alkynylaryl)acetamide is described here for the synthesis of N-(2-(2,2-dibromo-2-arylacetyl)aryl)acetamide.

Cascade Pd(II)-catalyzed Wacker lactonization-Heck reaction: rapid assembly of spiranoid lactones

An unprecedented Pd-catalyzed cascade Wacker-Heck lactonization-cyclization sequence is reported. The process provides a facile approach to bi- and tricyclic spiranoid lactones in good yields. The reaction shows general substrate scope and a broad functional group tolerability. In addition, a rare 4-exo trig Heck-type cyclization is demonstrated.

Reactivity of alkynylindole-2-carboxamides in [Pd]-catalysed C–H activation and phase transfer catalysis: formation of pyrrolo-diindolones vs. β-carbolinones

[Pd]-Catalysed reaction of 3-alkynylindole-2-carboxamidesviaC–H activation leads to pyrrolodiindolones; in contrast, under phase-transfer catalysis, the same precursors afford β-carbolinones.

Synthesis of benzo[1,3]oxazines via copper(i)-catalyzed cascade annulation of nitriles, aldehydes and diaryliodonium salts

A copper(i)-catalyzed one-pot [2 + 2 + 2] cascade annulation reaction of diaryliodoniums, nitriles, and aldehydes has been developed for the efficient synthesis of 2,4-substituted benzoxazine derivatives.