Substrate-controlled Rh(III)-catalyzed regiodivergent annulation towards fused and spiro benzimidazoles

Cascade [4 + 1] Annulation through Activation of the C(sp2)-H Bond Enabling Benzothiadiazinoisoindolcarboxylate, Benzothiadiazinoisoindole, and Benzothiadiazinoisoindolepyrrolidinedione as Hybrid Spiro-Heterocyclic Frameworks†

Two structurally distinct and biologically privileged succinimide and isoindole heteroarenes bearing benzothiadiazinedioxide motif-centered hybrid conjugates are proficiently achieved through Rh(III)-catalyzed sequential C(sp2)-H bond activation, ortho-alkenylation and finally cascade intramolecular cyclization. The significant feature of this developed protocol is that the resulting diversely decorated heterocycles contain a quaternary carbon center and this has been coursed through atypical [4 + 1] annulation ignoring the prevalent [4 + 2]-cyclization pathway and interestingly the applied coupling partners (e.g., maleimide, maleate, and styrene) to materialize the protocol functioned only as C1 synthon. Furthermore, the selective reduction strategy enables to modify the hybrid conjugate of succinimide and benzothiazine dioxide to benzothiazine dioxide-based spirocyclic isoindolopyrrolidinedione skeleton following preferential reduction of one carbonyl group of imide functionality. Overall this methodology emerges to be easily handled, versatile, time-efficient, and manifests relatively unfamiliar spiro-cyclization and good functional group tolerance so easy to grab a library of the entirely new variant of decorated hybrid spiro-heterocyclic scaffolds.

Recent advances in spirocyclization of maleimides via transition-metal catalyzed C-H activation

In recent years, the maleimide scaffold has received a great deal of attention in C-H activation. Several types of products can be constructed using maleimides as a coupling partner. Alkylation, alkenylation, annulation, dehydrogenative annulation and spirocyclization are various reactions shown by maleimides in C-H activation. Thus, the maleimide scaffold has been extensively studied in the last few years in C-H activation owing to its unique reactivity. Among the diverse class of reactions of maleimides, spirocyclization is a less explored reaction. The spirocycles, in particular the spirosuccinimides are interesting candidates in drug discovery and materials chemistry. Therefore the method of spirocyclization of maleimides via C-H activation becomes an important strategy for the synthesis of a diverse array of spirosuccinimides. This review summarizes the reports available in this field from 2015-2023 and also highlights the scopes and prospects of this method.

Ru(II)-Catalyzed [4 + 2]-Annulation of 2-Alkenyl/Arylimidazoles with N-Substituted Maleimides and 1,4-Naphthoquinones: Access to Imidazo-Fused Polyheterocycles

Synthesis of imidazo-fused polyheterocyclic molecular frameworks, viz. imidazo[1,2-a]pyrrolo[3,4-e]pyridines, imidazo[2,1-a]pyrrolo[3,4-c]isoquinolines, and benzo[g]imidazo[1,2-a]quinoline-6,11-diones, has been achieved by the ruthenium(II)-catalyzed [4 + 2] C-H/N-H annulation of 2-alkenyl/2-arylimidazoles with N-substituted maleimides and 1,4-naphthoquinones. The developed protocol is operationally simple, exhibits broad substrate scope with excellent functional group tolerance, and provides the desired products in moderate to good yields. The mechanistic studies suggest that the reaction involves the formation of a C-C bond through Ru-catalyzed C(sp2)-H bond activation followed by intramolecular cyclization.

Parallel Synthesis of Pyrazolone-Fused Cinnolines by the Palladium-Catalyzed [4 + 2] Annulation of Pyrazol-3-ones with Substituted Allenoates

The synthesis of pyrazolone-fused cinnolines from pyrazol-3-ones and α,γ-substituted allenoates via a palladium-catalyzed C-H activation/annulation cascade was developed. Mechanistic studies revealed the course of the reaction. Initially, N-acyl-valine ligand-assisted ortho-C-H activation gives ortho-alkenylated intermediate. Subsequent cyclopalladation and migratory insertion of allenoate give a seven-membered palladacycle. Reductive elimination finally furnishes pyrazolone-fused cinnolines.

Rapid Synthesis of Benzimidazole-Fused Isoindoles by Rh(III)/Ru(II)-Catalyzed [4 + 1] Cascade C-H/N-H Annulation of 2-Arylbenzimidazoles with Alkynoates and Alkynamide

A Rh(III)-catalyzed [4 + 1] cyclization of 2-arylbenzimidazoles with alkynoates through C-H activation/ortho-alkenylation/intramolecular annulation cascade to obtain benzimidazole-fused isoindoles is reported. The reaction of the Rh catalyst and internal alkyne ester provides benzo[4,5]imidazo[2,1-a]isoindole acetate exclusively. Conversely, internal alkyne amide participates in the annulation process in the presence of a Ru catalyst to provide benzo[4,5]imidazo[2,1-a]isoindole acetamide. The alkyne acts as a C1 synthon and undergoes [4 + 1] cyclization rather than traditional [4 + 2] annulation.