Selectivity in Ring‐Closing Metathesis: Synthesis of Propellanes and Angular Aza‐tricycles

Efficient Synthesis of Cyclohepta[b]indoles and Cyclohepta[b]indole-Indoline Conjugates via RCM, Hydrogenation, and Acid-Catalyzed Ring Expansion: A Biomimetic Approach

Cyclohepta[b]indoles, prevalent in natural products and pharmaceuticals, are conventionally accessed via metal or Lewis acid-mediated cycloadditions with prefunctionalized substrates. Our study introduces an innovative sequential catalytic assembly for synthesizing cyclohepta[b]indoles from readily available isatin derivatives. The process involves three catalytic sequences: ring-closing metathesis, catalytic hydrogenation, and acid-catalyzed ring expansion. The RCM of 2,2-dialkene-3-oxindoles, formed by butenyl Grignard addition to 3-allyl-3-hydroxy-2-oxindoles, yields versatile spirocyclohexene-3-oxindole derivatives. These derivatives undergo further transformations, including dibromination, dihydroxylation, epoxidation, Wacker oxidation at the double bond. Hydrogenation of spirocyclohexene-3-oxindole yields spirocyclohexane-3-oxindoles. Their subsequent acid-catalyzed ring expansion/aromatization, dependent on the acid catalyst, results in either cyclohepta[b]indoles or cyclohepta[b]indole-indoline conjugates, adding a unique synthetic dimension. The utility of this methodology is exemplified through the synthesis of an A-FABP inhibitor, showcasing its potential in pharmaceutical applications.

Synthetic Approach toward Diverse Oxa-Cages via Olefin Metathesis

This article demonstrates a late-stage modification of the cage propellanes that are transformed into intricate oxa-cycles via ring-rearrangement metathesis (RRM) and regioselective ring-closing metathesis (RCM) as crucial steps. In addition, we also report the extended pentacycloundecane (PCUD)-based oxa-cages involving the domino cycloetherification followed by olefin metathesis. These oxa-cages involve a domino sequence in which the PCUD core unit remains intact. [Ru]-based Grubbs catalysts are used to execute the metathesis step to assemble these higher-order oxa-cage systems. Spectroscopic data assigned structures of various products and were further supported by single-crystal X-ray diffraction analysis. The synthetic approach to these cage polycycles involves high complexity generating processes such as Diels-Alder reaction, [2 + 2] photocycloaddition, and RRM as well as RCM.

Modular Approach to Benzofurans, 2H-Chromenes and Benzoxepines via Claisen Rearrangement and Ring-Closing Metathesis: Access to Phenylpropanoids

Benzofurans, 2 H -chromenes and benzoxepines are key structural elements present in several natural products and pharmaceuticals. Here, we describe an easy-to-execute strategy for the synthesis of benzofurans, 2 H -chromenes and benzoxepines, by employing Claisen rearrangement and ring-closing metathesis as key steps. A variety of phenols were converted into useful oxacycles in good to excellent yields. The ring-closing metathesis approach has been used to produce phenylpropanoid natural products. Examples described here include, the naturally occurring benzofurans such as 7-methoxywutaifuranal, 7-methoxywutaifuranol, 7-methoxywutaifuranate and the O -prenylated natural products like boropinic acid, boropinols A and C.

Rapid Construction of Hexacyclic Indolines via the Ru(II)-Catalyzed C-H Activation Initiated Cascade Cyclization of Phenidones with Enynones

A highly efficient cascade cyclization of phenidones and enynones has been developed via a Ru(II)-catalyzed C-H activation initiated indole formation/Diels-Alder reaction/iminium ion cyclization sequence, which afforded hexacyclic indolines as single diastereomer in good to excellent yields with a broad substrate scope under mild conditions. The reaction features the simultaneous generation of five new chemical bonds and four new rings in one pot, providing a rapid and concise approach toward polycyclic indoline alkaloids and their analogues.

Ring closing metathesis for the construction of carbazole and indole-fused natural products

The synthesis and functionalization of carbazole ring systems have received considerable attention in organic synthesis due to their widespread occurrence in biologically active compounds. One of the classical methods for the synthesis of carbazoles involves C-C bond formation of a biaryl amine moiety by oxidizing agents. Over the last few years, various new strategies have evolved for the synthesis of carbazole ring systems. During the past two decades, ring-closing metathesis (RCM) based approaches have been efficiently employed for the synthesis of nitrogen containing heteroaromatic systems including carbazoles. Herein, we discuss the construction of carbazole ring systems using RCM and the application of RCM based methods in the preparation of other indole-fused heterocycles. The application of these methods in the synthesis of carbazole alkaloids and bioactive indole-fused natural products has been discussed to highlight the importance of RCM in total synthesis.

Photoredox-catalyzed hydroxymethylation of β-ketoesters: application to the synthesis of [3.3.3] propellane lactones

Photoredox-catalysed hydroxymethylation of β-ketoesters substituted by an allyl subunit on the α-position afforded directly the corresponding bicyclic lactones possessing both a hydroxy group and an unsaturation. A subsequent regioselective iodoetherification led to the formation of original [3.3.3] propellane structures. Substitution of the iodine atom by various nucleophiles afforded highly functionnalized structures including triazolomethyl derivatives.