From 3-Acyl-2-methylindoles to γ-Carbolines: Li-Promoted Cycloaddition Reaction and Its Quantum Chemical Study

DDQ/Fe(NO3)3-Catalyzed Aerobic Synthesis of 3-Acyl Indoles and an In Silico Study for the Binding Affinity of N-Tosyl-3-acyl Indoles toward RdRp against SARS-CoV-2

In the present study, we herein report a DDQ-catalyzed new protocol for the synthesis of substituted 3-acylindoles. Being a potential system for virtual hydrogen storage, introduction of catalytic DDQ in combination with Fe(NO₃)₃·9H₂O and molecular oxygen as co-catalysts offers a regioselective oxo-functionalization of C-3 alkyl-/aryllidine indolines even with scale-up investigations. Intermediate isolation, their spectroscopic characterization, and the density functional theory calculations indicate that the method involves dehydrogenative allylic hydroxylation and 1,3-functional group isomerization/aromatization followed by terminal oxidation to afford 3-acylindoles quantitatively with very high regioselectivity. This method is very general for a large number of substrates with varieties of functional groups tolerance emerging high-yield outcome. Moreover, molecular docking studies were performed for some selected ligands with an RNA-dependent RNA polymerase complex (RdRp complex) of SARS-CoV-2 to illustrate the binding potential of those ligands. The docking results revealed that few of the ligands possess the potential to inhibit the RdRp of SARS-Cov-2 with binding energies (-6.7 to -8.19 kcal/mol), which are comparably higher with respect to the reported binding energies of the conventional re-purposed drugs such as Remdesivir, Ribavirin, and so forth (-4 to -7 kcal/mol).

Lithium-Promoted Cycloaddition of Indole-2,3-dienolates and Carbon Disulfide as a One-Pot Route to Thiopyrano[4,3-b]indole-3(5H)-thiones

A new approach for the annulation of a thiopyrane ring to an indole core under mild conditions was developed. Treating 2-methyl-3-acylindoles with lithium diisopropyl amide leads to the elimination of a proton from the 2-methyl group. The lithium indole-2,3-dienolates obtained were found to react with CS₂ to give the corresponding thiopyrano[4,3-b]indole-3(5H)-thiones. The mechanism represents a stepwise addition through ion-pair formation, according to PCM/B3LYP/6-311++G**, PBE1PBE/6-311++G**, and MP2//HF/6-311++G** quantum chemical calculations. AIM calculations revealed the essential role of the Li atom at all stages of the process.

Pd(0)-Catalyzed Intramolecular Heck reaction of 2/3-Aryl(amino)methyl-3/2-bromoindoles: Syntheses of 3,4-Benzo[c]-β-carbolines, Benzo[4,5]isothiazolo[2,3-a]indole 5,5-Dioxides, and 1,2-Benzo[a]-γ-carbolines

One-pot synthesis of 3,4-benzo[c]-β-carbolines was achieved from 2-aryl(tosylamino)methyl-3-bromoindoles via 10 mol % Pd(OAc)₂/PPh₃-mediated intramolecular Heck coupling using K₂CO₃ as a base in DMF at 110 °C with concomitant aromatization through an elimination of tosylsulfinic acid. Under identical conditions, the isomeric 3-aryl(tosylamino)methyl-2-bromoindoles upon intramolecular Heck reaction furnished benzo[4,5]isothiazolo[2,3-a]indole 5,5-dioxides instead of the expected γ-carbolines. However, synthesis of the expected γ-carboline framework, 3-tosyl-6,9-dihydro-1,2-benzo[a]-γ-carbolines, could be achieved from 3-aryl(tosylamino)methyl-2-bromoindoles containing a mesitylene sulfonyl unit as a protecting group on the indole nitrogen.

Benzannulation strategies for the synthesis of carbazoles, indolocarbazoles, benzocarbazoles, and carbolines

This review presents a critical and authoritative analysis of several exciting benzannulation approaches developed in the past decade for the construction of carbazoles, indolocarbazoles, benzocarbazoles, and carbolines.

Facile Synthesis of Polysubstituted Indolizines via One-Pot Reaction of 1-Acetylaryl 2-Formylpyrroles and Enals

An efficient method for the synthesis of polysubstituted indolizines has been developed based on formal [4+2] annulation of 1-acetylaryl 2-formylpyrroles with enals, followed by oxidative aromatization. Pyridine-type six-membered rings were constructed in this transformation. This transition metal-free reaction features mild reaction conditions, a broad substrate scope, and excellent functional group tolerance. Notably, the formyl group is well tolerated under reaction conditions.