Synthetic Strategies in the Preparation of Phenanthridinones

Palladium nanoparticles for the synthesis of phenanthridinones and benzo[c]chromenes via C-H activation reaction

In the present work, derivatives of phenanthridine-6(5H)-ones and benzo[c]chromenes were efficiently prepared through an intramolecular C-H bond functionalization reaction catalyzed by photochemically synthesized Pd-PVP nanoparticles. The heterocycles were obtained via intramolecular arylation of the corresponding N-methyl-N-aryl-2-halobenzamide or aryl-(2-halo)benzyl ethers using K2CO3 as base in a mixture of H2O : DMA as solvent without additives or ligands. High yields of the heterocyclic compounds were achieved (up to 95%) using a moderately low catalyst loading (1-5 mol%) under an air atmosphere at 100 °C. The reaction exhibited very good tolerance to diverse functional groups (OMe, Me, t Bu, Ph, OCF3, CF3, F, Cl, -CN, Naph), and both bromine and iodine substrates showed great reactivity. Finally, the in vitro antiproliferative activity of phenanthridine-6(5H)-ones and benzo[c]chromenes was evaluated against six human solid tumor cell lines. The more active compounds exhibit activity in the low micromolar range. 1-Isopropyl-4-methyl-6H-benzo[c]chromene was identified as the best compound with promising values of activity (GI50 range 3.9-8.6 μM). Thus, the benzochromene core was highlighted as a novel organic building block to prepare potential antitumor agents.

Pd-Catalyzed Decarbonylative sp2 C-H Arylation: Construction of Five- and Six-Membered (Hetero)Cyclic Compounds

The cyclic compounds have wide applications in the design and synthesis of drugs and materials; thus, their efficient construction attracts much attention from the synthetic community. In this letter, we report an efficient method for preparing cyclic compounds starting from the readily available carboxylic acids. This reaction takes place through intramolecular decarbonylative sp2 C-H arylation, enabling efficient synthesis of a wide range of five- and six-membered cyclic compounds. Both carbo- and heterocycles can be produced under the reaction conditions. Moreover, this reaction features a wide substrate scope with high functional group tolerance. The scale-up experiments also show its practicality in organic synthesis. Those experimental results indicate that this reaction would find wide applications in the synthetic community.

Easy Access to Phenanthridinones via Metal-Free Cascade Benzannulation and C-N Bond Formation

A concise route for the synthesis of dihydrobenzo[j]phenanthridinones has been disclosed through an aryne annulation strategy under metal-free reaction conditions. The reaction involves multiple C-C and C-N bond cleavages/formations via Diels-Alder reaction, aromatization-driven C-N bond cleavage, and amide formation.

Selective access to dihydrophenanthridines and phenanthridinones via cyclisation of aryl amines onto N-tethered arynes

5,6-Dihydrophenanthridines are prepared from aryl amines via intramolecular addition to N-tethered arynes under mild conditions. A new o-silylaryl triflate precursor was developed to increase reactivity and enable electron-rich and electron-poor aryl amines to undergo cyclisation. A complete switch in product selectivity occurs when the reaction is conducted in air, affording the corresponding phenanthridin-6(5H)-one as the sole product under otherwise identical reaction conditions.

Approach to Access Benzo[j]phenanthridinones from 1,7-Enynes and Aryldiazonium Salts via a Domino Radical Relay Process Enabled by a P/N-Heteroleptic Cu(I)-Photosensitizer

A mild approach for the synthesis of benzo[j]phenanthridin-6(5H)-one derivatives from 1,7-enynes and aryldiazonium salts has been successfully developed involving a domino radical relay process enabled by a heteroleptic Cu(I)-photosensitizer under visible-light-driven photocatalytic conditions. Mechanistic studies disclosed that the oxidative quenching of the excited state of PS 4 with aryldiazonium salts via an SET process generated aryl radicals, which could play a radical initiator-terminator dual role within the whole radical relay process, namely, at the initial step acting as a radical donor to trigger the radical addition to the olefin moieties of 1,7-enynes while at the final stage serving as a radical acceptor to complete the cyclization.

A Visible-Light-Induced α-Aminoalkyl-Radical-Mediated Halogen-Atom Transfer Process: Modular Synthesis of Phenanthridinone Alkaloids

A halogen-atom transfer (XAT) strategy utilizing α-aminoalkyl radicals allows the generation of aryl radicals at room temperature, which is applied for intramolecular cyclization reactions en route to biologically relevant alkaloids. Starting from simple halogen-substituted benzamides under visible light irradiation in the presence of an organophotocatalyst (4CzIPN) and nBu₃N allows the modular construction of the phenanthridinone core, which gives facile access to drug analogs and alkaloids, e.g., from the Amaryllidaceae family. The reaction pathway most likely involves a quantum mechanical tunneling enabled transfer event to achieve aromatization-halogen-atom transfer.

Synthesis, In Silico and In Vivo Toxicity Assessment of Functionalized Pyridophenanthridinones via Sequential MW-Assisted Intramolecular Friedel-Crafts Alkylation and Direct C-H Arylation

A rapid, efficient, and original synthesis of novel pyrido[3,2,1-de]phenanthridin-6-ones is reported. First, the key cinnamamide intermediates 8a-f were easily prepared from commercial substituted anilines, cinnamic acid, and 2-bromobenzylbromide in a tandem amidation and N-alkylation protocol. Then, these N-aryl-N-(2-bromobenzyl) cinnamamides 8a-f were subjected to a TFA-mediated intramolecular Friedel-Crafts alkylation followed by a Pd-catalyzed direct C-H arylation to obtain a series of potentially bioactive 4-phenyl-4,5-dihydro-6H,8H-pyrido[3,2,1-de]phenanthridin-6-one derivatives 4a-f in good yields. Finally, the toxicological profile of the prepared final compounds, including their corresponding intermediates, was explored through in silico computational methods, while the acute toxicity toward zebrafish embryos (96 hpf-LC₅₀, 50% lethal concentration) was also determined in the present study.

DMF as CO Surrogate in Carbonylation Reactions: Principles and Application to the Synthesis of Heterocycles

Transition metal-catalyzed carbonylation reactions have emerged as one of the most relevant synthetic approaches for the preparation of carbonyl-containing molecules. The most commonly used protocol for the insertion of a carbonyl moiety is the use of carbon monoxide (CO) but, due to its toxic and explosive nature, this process is not suitable at an industrial scale. More recently, the chemistry of CO surrogates has received large attention as a way to use less expensive and more environmentally friendly methods. Among the various CO surrogates, N,N-dimethylformamide (DMF) has been paid greater attention due to its low cost and easy availability. This mini-review gives appealing insights into the application of DMF as a CO surrogate in metal-catalyzed carbonylations; in particular, in the first part we will give a general state of the art of these reactions for the preparation of carbonyl-containing molecules; then, we will take into account all the various synthetic approaches for the metal-catalyzed carbonylative synthesis of heterocycles using DMF as a CO surrogate. Each protocol has been discussed critically in order to screen the best synthetic method and to offer perspective on trends and future directions in this field.