Brønsted acid-promoted synthesis of highly functionalized tetrahydrocarbazoles from diethyl (E)-5-diazo-4-oxohex-2-enedioate

Brønsted acid-promoted synthesis of highly functionalized tetrahydrocarbazoles from indoles and easily accessible diethyl (E)-5-diazo-4-oxohex-2-enedioate has been achieved. Diethyl (E)-5-diazo-4-oxohex-2-enedioate acts as a 1,4-diacceptor and undergoes tandem Michael addition followed by nucleophilic substitution at the diazo carbon with indole in the presence of a strong Brønsted acid.

An update on the progress of cycloaddition reactions of 3-methyleneindolinones in the past decade: versatile approaches to spirooxindoles

Cycloaddition reactions are of great interest due to their potential and rapid construction of optically enriched spiro-cyclic products. 3-Methyleneindolinones have been proven to be a valuable precursor in cycloaddition reactions for the construction of diverse 3,3'-spirocyclic oxindoles. Their versatile reactivity has provided a new forum for the development of a variety of building blocks and synthetic compounds, including bioactive molecules. Herein, significant accomplishments in the cycloaddition reactions of 3-methyleneindolinones for the synthesis of spirooxindoles have been summarised and elaborated. The review is outlined according to the type of cycloaddition such as [2 + 1], [2 + 2], [3 + 2], [4 + 2] and [5 + 2] cycloaddition reactions.

Recent approaches to the synthesis of tetrahydrocarbazoles

The tetrahydrocarbazole (THC) motif is ubiquitous in natural products and biologically active compounds.

Base-mediated 1,3-dipolar cycloaddition of pyridinium bromides with bromoallyl sulfones: a facile access to indolizine scaffolds

An expedient and transition-metal-free synthetic strategy has been developed for the construction of substituted indolizines from a unique combination of pyridinium salts and 2-bromoallyl sulfones. This approach does not compromise with the diverse substitutions on both the pyridinium salts and 2-bromoallyl sulfones. Wide substrate scope, operational simplicity, milder reaction conditions and good to moderate yields are the merits associated with the current approach. Moreover, this method provides two products which are amenable for the generation of a library of key indolizine building blocks.

Ring-opening cyclization of activated spiro-aziridine oxindoles with heteroarenes: a facile synthetic approach to spiro-oxindole-fused pyrroloindolines

Herein, we report a facile tandem approach for the synthesis of both spiro-oxindole-fused pyrroloindolines and benzofurano-pyrrolidines via a Lewis acid-catalyzed domino ring-opening with concomitant ring annulation using activated spiro-aziridines and heteroarenes. This method offers a new class of novel spiro-fused polycyclic pyrrolidines in a one-pot and sustainable manner with good yields and high diastereoselectivity. In addition, the structure of 3d was confirmed by single X-ray crystallography analysis.

The microwave-assisted syntheses and applications of non-fused single-nitrogen-containing heterocycles

Microwave technology has emerged as a great tool for the efficient synthesis of organic compounds and it provides opportunities for chemists to achieve chemical transformations that tend to be challenging using classical approaches. Additionally, N-heterocycles are well-known for their medicinal/biological significance, along with their applications as excellent building blocks in chemical synthesis. The dominance of N-heterocycles in drug molecules and other pharmacological agents makes them attractive scaffolds, which encourages chemists to develop a wide range of strategies towards the greener synthesis and functionalization of these heterocycles. In this regard, we have collated and discussed literature relating to the microwave-assisted synthesis and the modification of non-(benzo)fused single-nitrogen-containing N-heterocycles from the past decade. The role of the microwave technique and its benefits over the conventional approach have also been emphasized in terms of overall reaction efficiency, reaction time, yield, reduced side-product generation, neat and clean reactions, chemo-/regio-/enantio-selectivity, and the use of mild reagents/reaction conditions to achieve the objectives of green and sustainable chemistry.

Iodine-promoted one-pot synthesis of 1,3,4-oxadiazole scaffolds via sp3 C–H functionalization of azaarenes

An efficient metal-free one-pot protocol has been developed for the construction of 1,3,4-oxadiazole derivatives via iodine-mediated oxidative amination of benzylic C–H bonds of azaarenes.

Carbene-Catalyzed Construction of Carbazoles from Enals and 2-Methyl-3-oxoacetate Indoles

Direct and rapid construction of carbazoles has been successfully developed via carbene-catalyzed oxidative formal [4 + 2] annulation of enals with 2-methyl-3-oxoacetate indoles. This metal-free reaction features a broad substrate scope, features good functional-group tolerance, proceeds under mild conditions, and can be easily scaled up.

Formal [1 + 2 + 3] Annulation: Domino Access to Carbazoles and Indolocarbazole Alkaloids

A new formal [1 + 2 + 3] annulation of o-alkenyl arylisocyanides with α, β-unsaturated ketones under metal-, base-, and acid-free conditions is disclosed. This domino reaction provides a general protocol for the efficient and practical synthesis of a wide range of carbazole derivatives from readily available starting materials in a single operation. Furthermore, this methodology was used as the key step in a protecting-group-free synthesis of indolocarbazole alkaloids arcyriaflavin A and racemosin B.

Iodine promoted dual oxidative C(sp3)-H amination of 2-methyl-3-arylquinazolin-4(3H)-ones: a facile route to 1,4-diarylimidazo[1,5-a]quinazolin-5(4H)-ones

An iodine promoted tandem oxidative condensation of benzylamines and 2-methylquinazolin-4-(3H)-ones was developed to yield imidazo[1,5-a]quinazolin-5(4H)-ones via dual C(sp³)-H amination under metal free conditions in a greener way using molecular oxygen as a terminal oxidant. This tandem transformation provides an efficient approach to construct various functionalized imidazo[1,5-a]quinazolin-5(4H)-ones in a straightforward manner via a sequential amination-oxidation-annulation-aromatisation.

Temperature-modulated diastereoselective transformations of 2-vinylindoles to tetrahydrocarbazoles and tetrahydrocycloheptadiindoles

The Fe(OTf)₃-catalyzed dimerization of 2-vinylindolesviadiastereoselective [4 + 2] or [4 + 3] cyclization gives rise to tetrahydrocarbazoles at low temperature or tetrahydrocycloheptadiindoles at high temperature.

N-Bromosuccinimide (NBS)-Catalyzed C-H Bond Functionalization: An Annulation of Alkynes with Electron Withdrawing Group (EWG)-Substituted Acetyl Indoles for the Synthesis of Carbazoles

An N-bromosuccinimide-catalyzed intermolecular annulation of acetyl indoles with alkynes was developed, allowing for regioselective formation of valuable carbazoles through direct C-H bond functionalization. The readily available catalyst, wide substrate scope, gram scale synthesis, and mild conditions make this method practical. Mechanistic investigations indicate that the bromination of acetyl indole takes place to generate a bromide intermediate, followed by coupling with an alkyne and intramolecular cycloaromatization to furnish carbazole products.

Synthesis of carbazoles based on gold-copper tandem catalysis

An efficient synthetic method for carbazoles has been developed employing diazo anilinoalkynes as substrates. Sequential activation of the orthogonal functional groups embedded in diazo anilinoalkyne substrates by tandem gold-copper catalysis leads to the formation of highly substituted carbazoles. Substrate scope reveals a broad tolerability toward the substitution on aryl groups.

A dual removable activating group enabled the Povarov reaction of N-arylalanine esters: synthesis of quinoline-4-carboxylate esters

A dual removable activating group enabled Povarov reaction of N-arylalanine esters was reported.

A facile I2-catalyzed synthesis of imidazo[1,2-a]pyridines via sp3 C–H functionalization of azaarenes and evaluation of anticancer activity

A three component synthetic protocol has been developed for the synthesis of medicinally important imidazo[1,2-a]pyridines via the iodine-catalyzed oxidative amination of benzylic C–H bonds of azaarenes. The synthesized compounds were evaluated for their anti-cancer activity.

Silver(i)-catalysed domino alkyne-annulation/Diels–Alder reaction: a mild synthetic approach to tetrahydrospiro[carbazole-4,3′-indoline] scaffolds

A mild and efficient AgOTf catalyzed alkyne-annulation/Diels–Alder cascade for the construction of tetrahydrospiro[carbazole-4,3′-indoline] derivatives has been established.