Organocatalytic Strategies for the Synthesis of Cyclopenta-Fused Arenes and Heteroarenes

Molecular-iodine catalyzed selective construction of cyclopenta[b]indoles from indoles and acetone: a green gateway to indole-fused cycles

Molecular-iodine catalyzed access to an important class of bio-relevant indole derivatives, cyclopenta[b]indoles, has been achieved via a cascade addition/intramolecular cyclization reaction of indoles and acetone. Explorations of diverse substitution patterns revealed an essential substrate-control in the reaction. The high-density electronic core of indole is pivotal in favouring the formation of indolyl-cyclopenta[b]indole derivatives; in contrast, the electron deficiency of the core hindered the cyclization process, directing the formation of bis(indolyl)propanes. Investigations on the mechanistic pathway revealed that bis(indolyl)alkanes were the intermediates for the addition-cyclization process. This simple experimental method provides sustainable synthetic access to cyclopentannulated indoles.

Conceptual advances in nucleophilic organophosphine-promoted transformations

Catalysis by trivalent nucleophilic organophosphines has emerged as an essential tool in organic synthesis. Several new organic transformations promoted by phosphines substantiate and complement the existing synthetic chemistry tools. Mere design of the substrate and reagent combinations has introduced new modes of reactivity patterns, which are otherwise difficult to achieve. These design considerations have led to the rapid build-up of complex molecular entities and laid a solid foundation to synthesise bioactive natural products and pharmaceuticals. This article presents an overview of some of the conceptual advances, including our contributions to nucleophilic organophosphine chemistry. The scope, limitations, mechanistic insights, and applications of these metal-free transformations are discussed elaborately.

Phosphine-catalysed denitrative rearomatising (3 + 2) annulation of α,β-ynones and 3-nitroindoles

We describe a metal-free strategy to access various α-arylidene cyclopenta[b]indoles via phosphine-catalysed (3 + 2) annulation of α,β-ynones and 3-nitroindoles. For the first time, the rearomatisation of the indole nucleus was observed in such an annulative transformation. The method was extended to the synthesis of an antimalarial natural product, bruceolline E.

Pd-Catalyzed Nazarov-Type Cyclization: Application in the Total Synthesis of β-Diasarone and Other Complex Cyclopentanoids

We describe the palladium-catalyzed Nazarov-type cyclization of easily accessible (hetero)arylallyl acetates to pentannulated (hetero)arenes. This method provides ready access to various types of bi-, tri-, tetra-, and pentacyclic cyclopentanoids under neutral conditions. The synthetic utility is further demonstrated in the first total synthesis of β-diasarone and several other complex cyclopentanoids relevant to medicinal chemistry and materials science.

Phosphine-Catalyzed Intramolecular Vinylogous Aldol Reaction of α-Substituted Enones

We demonstrate the first phosphine-catalyzed intramolecular vinylogous aldol reaction (IVAR) of α-substituted enones. This strategy provides access to various pentannulated (hetero)arenes and dibenzocycloheptanones incorporated with two contiguous stereocenters, one of which is an all-carbon quaternary center. The scope of this work is further broadened through elaborations of the IVAR adducts to (i) benzannulated nine-membered carbocyclic systems, (ii) interesting classes of 1,3-dienes, 1,3,5-trienes, and 1-yn-3,5-dienes, and (iii) the analogs of echinolactone D and russujaponol F.

Palladium-catalysed 5-endo-trig allylic (hetero)arylation

A palladium-catalysed intramolecular allylic (hetero)arylation strategy for the synthesis of fused cyclopentenes incorporated with all-carbon quaternary and spiro centres is described. The method is straightforward, shows broad scope, proceeds in synthetically useful yields, and provides a rare means to construct complex cyclopentanoids. The reaction is believed to involve a kinetically unfavourable 5-endo-trig carbocyclisation of the tethered (π-allyl)palladium system. Further, this method was successfully applied as the key step in the total synthesis of diterpene natural products taiwaniaquinone H and dichroanone.

Ring-Opening/Recyclization Cascades of Monoactivated Cyclopropanes

A variety of cyclopropyl aryl ketones undergo uncatalyzed cascade ring-opening/recyclization reactions to generate indenones and fluorenones. In addition, a new strategy to access 3-hydroxyindanones possessing two contiguous stereogenic centers, one of them being an all-carbon quaternary center, was also established. During the course of the investigation, pronounced solvent, temperature, and substituent effects on the product distribution were discovered.

Substituent-Guided Palladium-Ene Reaction for the Synthesis of Carbazoles and Cyclopenta[ b]indoles

An efficient palladium-catalyzed intramolecular Trost-Oppolzer type Alder-ene strategy was developed for the synthesis of carbazoles and cyclopenta[ b]indoles from easily accessible(3-allyl-1 H-indol-2-yl)methyl acetates. This strategy was extended for the synthesis of naphthalenes and dibenzobenzofurans as well. In addition, a short synthesis of antibacterial and antifungal natural product glycozoline and its analogues was also achieved.

Synthesis of Polysubstituted Cyclopentene and Cyclopenta[ b]carbazole Analogues from Unsymmetrical 4-Arylidene-3,6-diarylhex-2-en-5-ynal and Indole Derivatives via an Iodine Mediated Electrocyclization Reaction

An efficient method for the synthesis of polysubstituted cyclopentene and cyclopenta[ b]carbazole derivatives through the iodine-promoted electrocyclization of substituted indoles and 4-arylidene-3,6-diarylhex-2-en-5-ynal derivatives is reported. Polysubstituted cyclopentene derivatives were produced through 4π electrocyclization reactions with indole, 7-methylindole, and 5-bromoindole as coupling partners, whereas cyclopenta[ b]carbazole derivatives were produced via 6π electrocyclization in the case of methoxy (-OMe)-substituted indoles. The methods reported herein diastereo- and regioselectively proceed under straightforward and mild conditions.

Metal- and Hydride-Free Pentannulative Reductive Aldol Reaction

Traditionally, the reductive aldol reaction is a metal-catalyzed and hydride-promoted coupling between enones and aldehydes. We present a phosphine-mediated diastereoselective intramolecular reductive aldol reaction of α-substituted dienones and aldehydes, which is metal-free and hydride-free. The synthetic utility of the reductive aldol adducts is demonstrated by elaborating them in one step to indeno[1,2- b]furanones, indeno[1,2- b]pyrans, and dibenzo[ a, h]azulen-8-ones.

Synthesis of Cyclopropanoids via Substrate-Based Cyclization Pathways

A series of unexpected reactions triggered by the dimethyloxosulfonium methylide led to the discovery of unconventional approaches for the synthesis of cyclopropa-fused tetralones and indeno-spirocyclopropanes. These highly functionalized structures were further elaborated in one step to privileged scaffolds such as tetralones, indenones, and fluorenones. As a whole, the results presented herein establish new diversity-oriented folding pathways.

Phosphine- and water-promoted pentannulative aldol reaction

An intramolecular aldol reaction promoted by tributylphosphine and water is described. Thereby, metal-free access to highly functionalised cyclopenta-fused arenes and heteroarenes is achieved.