Synthesis of Indoles by Reductive Cyclization of Nitro Compounds Using Formate Esters as CO Surrogates

Pd-Catalyzed cascade Heck cyclization/carbonylation of indoles with aryl formates: enantioselective construction of indolo[2,1-a]isoquinolines

An efficient palladium-catalyzed cascade cyclization/carbonylation of indoles with aryl formates to access ester-functionalized indolo[2,1-a]isoquinoline scaffolds has been developed. In addition, an asymmetric variant is also achieved using a chiral phosphine ligand, affording the indolo[2,1-a]isoquinoline products in good yields and enantioselectivities.

Transition Metal-Catalyzed Transformations of Chalcones

Chalcones are a class of naturally occurring flavonoid compounds associated to a variety of biological and pharmacological properties. Several reviews have been published describing the synthesis and biological properties of a vast array of analogues. However, overviews on the reactivity of chalcones has only been explored in a few accounts. To fill this gap, a systematic survey on the most recent developments in the transition metal-catalyzed transformation of chalcones was performed. The chemistry of copper, palladium, zinc, iron, manganese, nickel, ruthenium, cobalt, rhodium, iridium, silver, indium, gold, titanium, platinum, among others, as versatile catalysts will be highlighted, covering the literature from year 2000 to 2023, in more than 380 publications.

Applying green chemistry principles to iron catalysis: mild and selective domino synthesis of pyrroles from nitroarenes

An efficient and general cascade synthesis of pyrroles from nitroarenes using an acid-tolerant homogeneous iron catalyst is presented. Initial (transfer) hydrogenation using the commercially available iron-Tetraphos catalyst is followed by acid catalysed Paal-Knorr condensation. Both formic acid and molecular hydrogen can be used as green reductants in this process. Particularly, under transfer hydrogenation conditions, the homogeneous catalyst shows remarkable reactivity at low temperatures, high functional group tolerance and excellent chemoselectivity transforming a wide variety of substrates. Compared to classical heterogeneous catalysts, this system presents complementing reactivity, showing none of the typical side reactions such as dehalogenation, debenzylation, arene or olefin hydrogenation. It thereby enhances the chemical toolbox in terms of orthogonal reactivity. The methodology was successfully applied to the late-stage modification of multi-functional drug(-like) molecules as well as to the one-pot synthesis of the bioactive agent BM-635.

Effective Synthesis of 4-Quinolones by Reductive Cyclization of 2'-Nitrochalcones Using Formic Acid as a CO Surrogate

4-Quinolones are the structural elements of many pharmaceutically active compounds. Although several approaches are known for their synthesis, the introduction of an aryl ring in position 2 is problematic with most of them. The reductive cyclization of o-nitrochalcones by pressurized CO, catalyzed by ruthenium or palladium complexes, has been previously reported to be a viable synthetic strategy for this aim, but the need for pressurized CO lines and autoclaves has prevented its widespread use. In this paper, we describe the use of the formic acid/acetic anhydride mixture as a CO surrogate, which allows us to perform the reaction in a cheap and commercially available thick-walled glass tube without adding any gaseous reagent. The obtained yields are often high and compare favorably with those previously reported by the use of pressurized CO. The procedure was applied to a three-step synthesis from commercially available and cheap reagents of the alkaloid Graveoline.

Formic Acid as Carbon Monoxide Source in the Palladium-Catalyzed N-Heterocyclization of o-Nitrostyrenes to Indoles

The reductive cyclization reaction of o-nitrostyrenes to generate indoles has been investigated for three decades using CO as a cheap reducing agent, but it remains an interesting area of research and improvements. However, using toxic CO gas has several drawbacks. As a result, it is highly preferable to use safe and efficient surrogates for in situ generation of CO from nontoxic and affordable sources. Among several CO sources that have been previously explored for the generation of gaseous CO, here we report the use of cheap and readily available formic acid as an effective reductant for the reductive cyclization of o-nitrostyrenes. The reaction is air and water tolerant and provides the desired indoles in yields up to 99%, at a low catalyst loading (0.5 mol %) and without generating toxic or difficult to separate byproducts. A cheap glass thick-walled "pressure tube" can be used instead of less available autoclaves, even on a 2 g scale, thus widening the applicability of our protocol.

Synthesis of Multifused Pyrrolo[1,2-a]quinoline Systems by Tandem Aza-Michael-Aldol Reactions and Their Application to Molecular Sensing Studies

Herein, we have presented a weak acid-promoted tandem aza-Michael-aldol strategy for the synthesis of diversely fused pyrrolo[1,2-a]quinoline (tricyclic to pentacyclic scaffolds) by the construction of both pyrrole and quinoline ring in one pot. The described protocol fabricated two C-N bonds and one C-C bond in the pyrrole-quinoline rings which have been sequentially formed under transition-metal-free conditions by the extrusion of eco-friendly water molecules. A ketorolac drug analogue has been synthesized following the current protocol, and one of the synthesized tricyclic pyrrolo[1,2-a]quinoline fluorophores has been used to detect highly toxic picric acid via the fluorescence quenching effect.

Phenyl Formate as a CO Surrogate for the Reductive Cyclization of Organic Nitro Compounds to Yield Different N-Heterocycles: No Need for Autoclaves and Pressurized Carbon Monoxide

The reductive cyclization of different organic nitro compounds by carbon monoxide, catalyzed by transition metal complexes, is a very efficient and clean strategy for the synthesis of many N-heterocycles. However, its use requires the use of autoclaves and pressurized CO lines. In this perspective, the authors will present the results obtained in their laboratories on the use of phenyl formate as a convenient CO surrogate, able to liberate carbon monoxide under the reaction conditions and allowing the use of a cheap glass pressure tube as a reaction vessel. In most cases, yields were better than those previously reported by the use of pressurized CO, proving that the use of CO surrogates can be a viable alternative to the gaseous reagent.

Synthesis of 3-Acyloxyindolenines by TiCl3-Mediated Reductive Cyclization of 2-(ortho-Nitroaryl)-Substituted Enol Esters

In the presence of TiCl₃, the reductive cyclization of tetrasubstituted enol esters bearing a 2-(ortho-nitroaryl) substituent affords 3-acyloxy-2,3-disubstituted indolenines in good yields. A domino process involving the partial reduction of nitro to a nitroso group followed by 5-center-6π-electrocyclization, 1,2-acyloxy migration, and the further reduction of the resulting nitrone intermediate accounts for the reaction outcome. The so-obtained indolenines are converted smoothly to 2,2-disubstituted oxindoles via a sequence of saponification and semipinacol rearrangement.

Annulation strategies for diverse heterocycles via the reductive transformation of 2-nitrostyrenes

Reduction of the stable nitro group is a fundamental and widely used transformation for the construction of complex and functionalized heterocyclic architectures. The unfolding of the reactivity of the nitro group in the 2-nitrostyrene moiety not only triggers the formation of carbon-nitrogen bonds, but also offers the opportunity for annulation and heteroannulation, thereby providing a cascade process for the synthesis of highly conjugated natural and unnatural molecules. In this review, we comprehensively discuss the use of 2-nitrostyrene motifs in the synthesis of various N-heterocycles. We offer readers an overview of the synthetic achievements achieved to date, highlighting their important features, reactivities, and mechanisms.

One-pot synthesis of 2-arylindole derivatives under transition-metal-free conditions

A new and simple method to prepare 2-arylindole derivatives under transition-metal-free conditions has been developed. When N-acetyl-2-methyl-3-nitroaniline was treated with 2-fluorobenzaldehydes in the presence of Cs2CO3 in DMF at 60 ⁰C, the desired indoles were typically obtained in moderate to good yields (up to 83%). Other aniline substrates were also employed, but only the Knoevenagel condensation occurred to give the corresponding diarylethenes in moderate to excellent yields.

Synthesis of Indoles by Palladium-Catalyzed Reductive Cyclization of β-Nitrostyrenes with Phenyl Formate as a CO Surrogate

The reductive cyclization of suitably substituted organic nitro compounds by carbon monoxide is a very appealing technique for the synthesis of heterocycles because of its atom efficiency and easiness of separation of the only stoichiometric byproduct CO2, but the need for pressurized CO has hampered its diffusion. We have recently reported on the synthesis of indoles by reductive cyclization of o-nitrostyrenes using phenyl formate as a CO surrogate, using a palladium/1,10-phenanthroline complex as catalyst. However, depending on the desired substituents on the structure, the use of β-nitrostyrenes as alternative reagents may be advantageous. We report here the results of our study on the possibility to use phenyl formate as a CO surrogate in the synthesis of indoles by reductive cyclization of β-nitrostyrenes, using PdCl2(CH3CN)2 + phenanthroline as the catalyst. It turned out that good results can be obtained when the starting nitrostyrene bears an aryl substituent in the alpha position. However, when no such substituent is present, only fair yield of indole can be obtained because the base required to decompose the formate also catalyzes an oligo-polymerization of the starting styrene. The reaction can be performed in a single glass pressure tube, a cheap and easily available piece of equipment.