Construction of Carbocyclic Ring of Indoles Using Ruthenium-Catalyzed Ring-Closing Olefin Metathesis

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.

Synthesis of functionalized indoles via cascade benzannulation strategies: a decade's overview

Indoles are one of the most prominent aromatic heterocycles in the organic chemistry field. Due to their widespread presence in various natural products, alkaloids, drugs, approved medicines, etc. the synthesis and functionalization of indoles are of great interest. This review emphasizes recent developments and techniques in the domino cascade cyclization process in the last decade starting from the various building blocks. In particular, this review depicts several intriguing benzannulation methods of creating a benzene ring on a pre-existing pyrrole nucleus in an inter/intramolecular fashion under metal-catalyzed/metal-free approaches. Different subsections focus on gradual timely developments in this complementary area and a detailed analysis of the mechanisms and reactivity patterns. As a complementary method, this review gives a significant incentive to various annulation strategies and also gives an overview of the remaining challenges and upcoming possibilities.

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.

Synthesis of 13C/19F/2H labeled indoles for use as tryptophan precursors for protein NMR spectroscopy

Synthesis of indoles labeled with 13C-1H and 13C-19F spin pairs is described. All syntheses utilize inexpensive carbon-13C dioxide as the 13C isotope source. Ruthenium-mediated ring-closing metathesis is the key step in construction of the 13C containing indole carbocycle. Fluorine is introduced via electrophilic fluorination at the 7-position and via palladium-mediated cross-coupling at the 4-position. Indole and fluoroindoles are viable tryptophan precursors for in vivo protein expression. We show that they are viable also in in vitro protein synthesis using standard E. coli S30 extracts. Incorporation of the synthesized 13C-1H and 13C-19F spin pair labeled tryptophans into proteins enables high-resolution and high-sensitivity nuclear magnetic resonance (NMR) spectroscopy.

A one-pot "back-to-front" approach for the synthesis of benzene ring substituted indoles using allylboronic acids

Synthesis of only benzene ring functionalized indoles and poly-substituted carbazoles is reported via a one-pot triple cascade benzannulation protocol. Usage of differently substituted and readily accessible allylboronic acids as a 3-carbon annulating partner enables diverse aliphatic and aromatic substitution patterns, which is still a daunting task. This scalable synthetic protocol tolerates broad scope, thus enabling further downstream modifications. As an application, carbazole based natural products glycozoline and glycozolinol were synthesized.

Highly selective Diels-Alder and Heck arylation reactions in a divergent synthesis of isoindolo- and pyrrolo-fused polycyclic indoles from 2-formylpyrrole

A highly regio-, chemo- and stereoselective divergent synthesis of isoindolo- and pyrrolo-fused polycyclic indoles is herein described, starting from 2-formylpyrrole and employing Diels-Alder and Heck arylation reactions. 3-(N-Benzyl-2-pyrrolyl)acrylates and 4-(pyrrol-2-yl)butenones underwent a highly endo-Diels-Alder cycloaddition with maleimides to furnish octahydropyrrolo[3,4-e]indoles, which served as precursors in the regioselective synthesis of aza-polycyclic skeletons via an intramolecular Heck arylation reaction. Through the latter reaction, the 3-(N-benzyl-2-pyrrolyl)acrylates give rise to 3-(pyrrolo[2,1-a]isoindol-3-yl)acrylates. A further oxidative aromatization of the polycyclic intermediates provides the corresponding polycyclic pyrrolo-isoindoles and isoindolo-pyrrolo-indoles. A theoretical study on the stereoselective Diels-Alder reactions, carried out by calculating the endo/exo transition states, revealed the assistance of non-covalent interactions in governing the endo stereocontrol.

Synthesis of α-Formylated N-Heterocycles and Their 1,1-Diacetates from Inactivated Cyclic Amines Involving an Oxidative Ring Contraction

A novel synthesis of pyrrolidine-2-carbaldehydes or tetrahydropyridine-2-carbaldehydes from the cascade reactions of N-arylpiperidines or N-arylazepanes is presented. Mechanistically, the formation of the title compounds involves an unprecedented oxidative ring contraction of inactivated cyclic amines via Cu(OAc)₂/KI/O₂-promoted oxidative cleavage and reformation of the C-N bond. Interestingly, when PhI(OAc)₂ was used in place of KI, 1,1-diacetates of the corresponding aldehydes were directly obtained with good efficiency. To the best of our knowledge, this is the first example of regioselective C(sp³)-H bond functionalization and C(sp³)-N bond activation of saturated cyclic amines using copper salt and oxygen.

Amine-Triggered Highly Facile Oxidative Benzannulation Reaction for the Synthesis of Anthranilates under Solvent-Free Calcium(II) Catalysis

An amine-triggered facile synthetic approach of anthranilates has been described through benzannulation of readily available chemicals under one-pot solvent-free conditions using Ca(OTf)₂ as the sustainable catalyst. In this regioselective approach, we described a reasonably longer cascade, which proceeds through β-enamino ester formation/Michael addition/intramolecular aldol reaction/elimination/aromatization/oxidative debenzylation/lactonization with a broad substrate scope and high yields. The isolation of intermediates authenticated the mechanism, and the synthetic utility of the products was also demonstrated.

Enantioselective synthesis of planar-chiral ferrocene-fused 4-pyridones and their application in construction of pyridine-based organocatalyst library

A couple of planar-chiral ferrocene-fused 4-pyridone derivatives 2a and 2b were synthesized in enantiomerically pure form by scalable asymmetric transformations. Pyridones 2 are versatile precursors to various ferrocene-fused pyridine derivatives, which are useful nucleophilic asymmetric organocatalysts.

Novel [4 + 2]-benzannulation to access substituted benzenes and polycyclic aromatic and benzene-fused heteroaromatic compounds

A common [4 + 2]-benzannulation of Morita-Baylis-Hillman acetates of acetylenic aldehydes with boronic acids has been developed for the synthesis of aromatic and heteroaromatic compounds through tandem allylic substitution/hydroarylative cycloisomerization process. This method provides a facile and general route to substituted benzenes, naphthalenes, other polycyclic aromatics, and various benzene-fused heteroaromatic compounds such as benzofuran, benzothiophene, indole, and carbazoles.

Synthesis of substituted styrenes and 3-vinylphenols using ruthenium-catalyzed ring-closing enyne metathesis

The synthesis of substituted styrenes 3 and 3-vinylphenols 9 was achieved by ring-closing enyne metathesis (RCEM)/dehydration of 7 and RCEM/tautomerization of 8, respectively. Those methods provide selective access to unique aromatic compounds and solve the problem of regioisomer formation.

Rhodium enalcarbenoids: direct synthesis of indoles by rhodium(II)-catalyzed [4+2] benzannulation of pyrroles

Disclosed herein is the design of an unprecedented electrophilic rhodium enalcarbenoid which results from rhodium(II)-catalyzed decomposition of a new class of enaldiazo compounds. The synthetic utility of these enalcarbenoids has been successfully demonstrated in the first transition-metal-catalyzed [4+2] benzannulation of pyrroles, thus leading to substituted indoles. The new benzannulation has been applied to the efficient synthesis of the natural product leiocarpone as well as a potent adipocyte fatty-acid binding protein inhibitor.

(±)-2,2-Dimethyl-5-oxotetrahydrofuran-3-carboxylic acid (terebic acid): a racemic layered structure

A racemic crystalline form of terebic acid, C7H10O4, which is an important industrial chemical compound, is reported for the first time. The crystal structure is stabilized by O—H...O and C—H...O hydrogen bonds which form racemic double layers parallel to (001).