Lewis Acid Mediated Vinylogous Additions of Enol Nucleophiles into an α,β-Unsaturated Platinum Carbene

TfOH-promoted synthesis of indoles and benzofurans involving cyclative transposition of vinyl ketone

A metal-free approach to construct indole ring from vinylogous amides derived from o-alkynylanilines involving cyclization, retro-aza-Michael and amine trapping cascade is reported here. This atom-economical transformation has been extended to...

Recent advances in the synthesis of pyrrolo[1,2-a]indoles and their derivatives

The pyrrolo[1,2-a]indole unit is a privileged heterocycle found in numerous natural products and has been shown to exhibit diverse pharmacological properties. Thus, recent years have witnessed immense interest from the synthesis community on the synthesis of this scaffold. In light of the ever-increasing demand for pyrrolo[1,2-a]indoles in drug discovery, this review provides an overview of recent synthesis methods for the preparation of pyrrolo[1,2-a]indoles and their derivatives. The mechanistic pathway and stereo-electronic factors affecting the yield and selectivity of the product are briefly explained. Furthermore, we have attempted to demonstrate the utility of the developed methods in the synthesis of bioactive molecules and natural products, wherever offered.

Last Decade of Unconventional Methodologies for the Synthesis of Substituted Benzofurans

This review describes the progress of the last decade on the synthesis of substituted benzofurans, which are useful scaffolds for the synthesis of numerous natural products and pharmaceuticals. In particular, new intramolecular and intermolecular C-C and/or C-O bond-forming processes, with transition-metal catalysis or metal-free are summarized. (1) Introduction. (2) Ring generation via intramolecular cyclization. (2.1) C7a-O bond formation: (route a). (2.2) O-C2 bond formation: (route b). (2.3) C2-C3 bond formation: (route c). (2.4) C3-C3a bond formation: (route d). (3) Ring generation via intermolecular cyclization. (3.1) C7a-O and C3-C3a bond formation (route a + d). (3.2) O-C2 and C2-C3 bond formation: (route b + c). (3.3) O-C2 and C3-C3a bond formation: (route b + d). (4) Benzannulation. (5) Conclusion.

Benzofuran: A Key Heterocycle - Ring Closure And Beyond

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The benzofuranyl motif present in compounds exhibits various medicinal properties and non-drug applications. These derivatives are naturally occurring compounds or synthetic materials, which cover a broad spectrum of pharmacological activities like anti-inflammatory, anti-diabetic, anti- depressant, anti-HIV, anti-microbial, anti-proliferative, anti-convulsant, cytotoxic, analgesic, etc. Few of the commercially interesting compounds from this class are, ailanthoidol (anti-inflammatory), amiodarone, dronedarone, celivarone (anti-arrhythmic), bufuralol (muscular airways relaxant), morphine, 5-(2-aminopropyl)benzofuran; 5-APB, 6-(2-aminopropyl)benzofuran; 6-APB (CNS), rifampicin (antibiotic), etc., whereas, some of the non-drug applications are in perfumery industry (bergapten) and as tannin activators in sunscreen preparations (psoralen, 8-methoxypsoralen, and angelicin). Considering these interesting biological activities and commercial utilities, a review on the synthetic aspects of this privileged scaffold was attempted. For the benefit of natural product-based drug discovery, available sources of these derivatives, extraction process and reported biological activities have also been outlined in this review.

Recent advances in the synthesis of indoles from alkynes and nitrogen sources

This review highlights ten years of success in the synthesis of indoles using alkynes and nitrogen sources as substrates.

Regioselectivity Influences in Platinum-Catalyzed Intramolecular Alkyne O-H and N-H Additions

The steric and electronic drivers of regioselectivity in platinum-catalyzed intramolecular hydroalkoxylation are elucidated. A branch point is found that divides the process between 5-exo and 6-endo selective processes, and enol ethers can be accessed in good yields for both oxygen heterocycles. The main influence arises from an electronic effect, where the alkyne substituent induces a polarization of the alkyne that leads to preferential heteroatom attack at the more electron-deficient carbon. The electronic effects are studied in other contexts, including hydroacyloxylation and hydroamination, and similar trends in directionality are predominant although not uniformly observed.

Copper-catalyzed tandem annulation/enol nucleophilic addition to access multisubstituted indoles

A method to access various multisubstituted indoles from propargylic alcohols and readily available enol nucleophiles by copper-catalyzed tandem annulation/enol nucleophilic addition has been developed. Compared to the expensive metal catalysts such as platinum, gold, silver, and palladium used previously, the most economical copper(i) catalyst could achieve this reaction efficiently. The fused heterocyclic compounds, pyrrolo[1,2-a] indoles, could be afforded by further transformation of the products. The allyl cation intermediate may be involved in the mechanism.

Copper-catalyzed oxidative benzylic C-H cyclization via iminyl radical from intermolecular anion-radical redox relay

Base-promoted C-H cleavage without transition metals opens a practical alternative for the one based on noble metals or radical initiators. The resulting carbanion can pass through radical addition to unsaturated bonds like C-N or C-C triple bonds, in which stoichiometric oxidants are needed. When in situ C-H cleavage meets catalytic carbanion-radical relay, it turns to be challenging but has not been accomplished yet. Here we report the combination of base-promoted benzylic C-H cleavage and copper-catalyzed carbanion-radical redox relay. Catalytic amount of naturally abundant and inexpensive copper salt, such as copper(II) sulfate, is used for anion-radical redox relay without any external oxidant. By avoiding using N-O/N-N homolysis or radical initiators to generate iminyl radicals, this strategy realizes modular synthesis of N-H indoles and analogs from abundant feedstocks, such as toluene and nitrile derivatives, and also enables rapid synthesis of large scale pharmaceuticals.

The synthesis of high value heterocycles from cheap starting materials remains challenging. Here, the authors develop a new strategy to realize practical and modular synthesis of N-H indoles and analogs from toluene and nitrile derivatives using naturally abundant copper salt as catalyst.

Recent Progress in the Transition Metal Catalyzed Synthesis of Indoles

Indole is the most frequently found heterocyclic core structures in pharmaceuticals, natural products, agrochemicals, dyes and fragrances. For about 150 years, chemists were absorbed in finding new and easier synthetic strategies to build this nucleus. Many books and reviews have been written, but the number of new syntheses that appear in the literature, make necessary continuous updates. This reviews aims to give a comprehensive overview on indole synthesis catalyzed by transition metals appeared in the literature in the years 2016 and 2017.

Computational study of Ru-catalyzed cycloisomerization of 2-alkynylanilides

The reaction mechanism of Ru-catalyzed cycloisomerization of 2-alkynylanilides to 3-substituted indole or 2-substituted indole was analyzed at the B3LYP level of density functional theory. The solvation effect of the system was also considered by SMD model. The calculation results show that the reaction system first forms a ruthenium π-alkyne complex. On the one hand, the cyclization reaction of the amino group and the alkyne carbon in the reaction precursor complex directly forms the 2-substituted indole product. On the other hand, the benzene ring in the reaction complex undergoes 1,2-carbon migration on the C≡C triple bond to form the vinylidene complex. After cyclization and hydrogen transfer, the catalyst is regenerated to obtain 3-substituted indole product. The latter reaction process has relatively lower activation free energy and is also the main reaction channel for this reaction, which is consistent with published experimental results. This study provides a reasonable reaction mechanism and effective experimental supplement.

Ruthenium-Catalyzed Cycloisomerization of 2-Alkynylanilides: Synthesis of 3-Substituted Indoles by 1,2-Carbon Migration

We developed ruthenium-catalyzed cycloisomerization of alkynylanilides that gave 3-substituted indoles in high yields. The reaction proceeded via the disubstituted vinylidene ruthenium complex that was formed by the 1,2-carbon migration.

An efficient approach to generate aryl carbenes: gold-catalyzed sequential activation of 1,6-diynes

An efficient approach to generate aryl gold carbenes has been developed.

Platinum-Catalyzed α,β-Unsaturated Carbene Formation in the Formal Syntheses of Frondosin B and Liphagal

Formal syntheses of tetracyclic terpenoids frondosin B and liphagal are described. Both synthetic routes rely on the use of platinum-catalyzed α,β-unsaturated carbene formation for the key C-C bond forming transformations. The successful route toward frondosin B utilizes a formal (4 + 3) cycloaddition, while the liphagal synthesis features the vinylogous addition of an enol nucleophile as a key step. Both synthetic routes are discussed, revealing insights into structural requirements in the catalytic α,β-unsaturated carbene reaction manifold.

C-C Bond Migration in the Cycloisomerization of 1,6-Enynes

A full account of our investigation of C-C bond migration in the cycloisomerization of oxygen-tethered 1,6-enynes is described. Under Pt(II) and/or Ir(I) catalysis, cyclic and acylic alkyl groups were found to undergo 1,2-shifts into metal carbenoid intermediates. Interestingly, this process does not appear to be driven by the release of ring strain, and thus provides access to large carbocyclic frameworks. The beneficial effect of CO on the Pt(II) and Ir(I) catalytic systems is also evaluated.