An approach to cyclohepta[b]indoles through an allenamide (4 + 3) cycloaddition-Grignard cyclization-Chugaev elimination sequence

A RhII-Catalyzed [4 + 3]-Cycloaddition via the Stereoselective Cyclopropanation of Vinyl Allenes En Route to Oxepino[b]indoles and Subsequent Elaboration to Spirooxindole Frameworks

Oxepino[b]indoles were obtained in good to excellent yields employing a [4 + 3]-cycloaddition initiated by a stereo- and regioselective, RhII-catalyzed cyclopropanation between a vinyl allene and diazooxindole to afford an intermediate cyclopropyl allene that engaged the oxindole carbonyl in a spontaneous hetero-[3,3]-rearrangement. A survey of functional group tolerance revealed a diverse array of substrates amenable to oxepino[b]indole formation. In addition to the intriguing architecture of the cycloadducts, exposure to either Brønsted acid or base enables the assembly of functionalized spirroxindoles via the unusual conversion of a 5-7 fused ring system to a 5-5 spirocycle.

General one-step access to unsymmetric propargylic acetals via alcohol functionalization with allenyl ethers

We report a straightforward and robust protocol for synthesizing unsymmetric propargylic acetals through single-step alkoxypropargylation of aliphatic alcohols. This method employs allenyl ethers and hypervalent iodine reagents to achieve direct functionalization under mild conditions, producing 26 distinct acetals in 60-94% yields. The reaction's broad substrate compatibility accommodates diverse hydroxyl-containing molecules, offering a versatile and scalable platform for installing alkynyl moieties in complex molecular architectures.

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.

Cascade radical cyclization on 3-propargyl-2-alkenyl indole gives stereoselective access to cyclohepta[b]indole over carbazole

A protecting group-dependent diastereoselective synthesis of cyclohepta[b]indole over carbazole derivatives is developed. This strategy involves a regioselective 6-exo-trig radical cyclization-cyclopropanation-ring expansion cascade of 3-propargyl-2-alkenyl indole. The cascade radical cyclization was also performed on indole derivatives possessing alkyne, acrylate and vinylogous carbamate moieties, which delivered pyridocarbazole giving credence to the mechanistic hypothesis. Furthermore, cyclohepta[b]indole could be selectively converted to sulfone and sulfoxide as well as benzoazulenoindole via intramolecular Friedel-Crafts acylation.

Chiral Aldehyde-Palladium Catalysis Enables Asymmetric Synthesis of α-Alkyl Tryptophans via Cascade Heck-Alkylation Reaction

The first catalytic asymmetric cascade Heck-alkylation reaction of NH₂-unprotected amino acid esters with N-(2-iodophenyl)allenamides is reported in this work. Under the promotion of a combining catalytic system comprising a chiral aldehyde, a chiral palladium complex, and the Lewis acid ZnCl₂, the title reaction takes place smoothly, giving optically active α-alkyl tryptophan derivatives in moderate to good yields and excellent enantioselectivities. The target products can be converted into other structurally complex chiral indoles without the loss of enantioselectivities.

Stereoselective (4 + 3) cycloadditions of allenyl ethers-furans by chiral auxiliaries inducing and evaluation of anti-breast cancer activity

The medicinal plants were wildly library of natural products in drug discovery. The most active molecules with seven-membered rings skeleton represent a challenge for construction. A stereoselectivity (4 + 3) cycloadditions between allenyl ethers and substituted furans induced by chiral auxiliaries has been investigated. And the results showed significant stereoselectivities and regioselectivities. The optical cycloadducts with an oxygen-substituted seven-membered ring framework were generated by removing chiral auxiliaries under acidic conditions. The antiproliferative activity of the novel compounds displayed moderate antiproliferative effects toward T47D cells.

One-pot multi-step cascade protocols toward β-indolyl sulfoximidoyl amides via intermolecular trapping of an α-indolylpalladium complex by CO

Various β-indolyl sulfoximidoyl amides were efficiently prepared from ortho-iodoanilines, propargyl bromides, 1 atm of CO, and substituted NH-sulfoximines, through a palladium-catalyzed indole annulation/carbonyl insertion/C-N bond formation cascade. Mostly good to high yields of the products were obtained through this multi-step, one-pot reaction protocol under very gentle reaction conditions. The obtained β-indolyl sulfoximidoyl amides could be converted into biologically interesting sulfoximine analogues that contain a tryptamine moiety.

Construction of Bridged Polycyclic Skeletons via Transition-Metal Catalyzed Carbon-Carbon Bond-Forming Reactions

Transition-metal catalysis has become one of most important methods for constructing molecules with diverse architectures. Bridged polycyclic skeletons are often considered one of most challenging structures in organic synthesis. This Minireview summarizes the recent progress on synthesis of bridged polycyclic skeletons by transition-metal-catalyzed carbon-carbon bond-forming reaction. Four main ring-forming strategies including connection via olefin or carbonyl functionality, enolate intermediacy, C-H functionality, and aryl functionality are detailed and some effective methods are discussed with particular emphasis on reaction design and mechanism.

Recent Advances in Transition-Metal-Free (4+3)-Annulations

(4+3)-Annulations are incredibly versatile reactions which combine a 4-atom synthon and a 3-atom synthon to form both 7-membered carbocycles as well as heterocycles. We have previously reviewed transition-metal-catalyzed (4+3)-annulations. In this review, we will cover examples involving bases, NHCs, phosphines, Lewis and Brønsted acids as well as some rare examples of boronic acid catalysis and photocatalysis. In analogy to our previous review, we exclude annulations involving cyclic dienes like furan, pyrrole, cyclohexadiene or cyclopentadiene, as Chiu, Harmata, Fernándes and others have recently published reviews encompassing such substrates. We will however discuss the recent additions (2010–2020) to the literature on (4+3)-annulations involving other types of 4-atom-synthons.

1 Introduction

2 Bases

3 Annulations Using N-Heterocyclic Carbenes

3.1 N-Heterocyclic Carbenes (NHCs)

3.2 N-Heterocyclic Carbenes and Base Dual-Activation

4 Phosphines

5 Acids

5.1 Lewis Acids

5.2 Brønsted Acids

6 Boronic Acid Catalysis and Photocatalysis

7 Conclusion

Synthesis of Cyclohepta[b]indoles by (4 + 3) Cycloaddition of 2-Vinylindoles or 4H-Furo[3,2-b]indoles with Oxyallyl Cations

The synthesis of cyclohepta[b]indole derivatives through the dearomative (4 + 3) cycloaddition reaction of 2-vinylindoles or 4H-furo[3,2-b]indoles with in situ generated oxyallyl cations is reported. Oxyallyl cations are generated from α-bromoketones in the presence of a base and a perfluorinated solvent. Cyclohepta[b]indole scaffolds are obtained under mild reaction conditions, in the absence of expensive catalysts, starting from simple reagents, in good to excellent yields and with complete diasteroselectivity. Preliminary expansion of the scope to 3-vinylindoles and to aza-oxyallyl cations is reported.

(4 + 3) cycloadditions of allenyl ether-derived oxygen-stabilized oxyallyls with furans

(4 + 3) Cycloadditions between allenyl ethers and furans are described. The reaction features anin situformation of oxygen-stabilized oxyallylsviaepoxidations of allenyl ethers in the presence of H₂PO₄⁻.

Divergent oriented synthesis (DOS) of aza-heterocyclic amides through palladium-catalyzed ketenimination of 2-iodo-N-(propa-1,2-dien-1-yl)anilines

A palladium-catalyzed tandem reaction of N-(2-iodophenyl)-4-methyl-N-(propa-1,2-dien-1-yl)benzenesulfonamide with isocyanide is described to divergently produce aza-heterocyclic amides.

Access to N-unprotected 2-amide-substituted indoles from Ugi adducts via palladium-catalyzed intramolecular cyclization of o-iodoanilines bearing furan rings

A variety of N-unprotected 2-amide-substituted indoles were synthesized from readily available furfural-based Ugi adducts in moderate to good yields via palladium-catalyzed intramolecular cyclization of o-iodoanilines bearing furan rings. These reactions involved a cascade sequence consisting of dearomatizing arylation, opening of the furan ring, and deprotection of the N atom.

A variety of N-unprotected 2-amide-substituted indoles were synthesized from readily available furfural-based Ugi adducts in moderate to good yields via palladium-catalyzed intramolecular cyclization of o-iodoanilines bearing furan rings.

The Chemical Synthesis and Applications of Tropane Alkaloids

Tropanes are an important class of alkaloid natural products that are found in plants all over the world. These compounds can exhibit significant biological activity and are among the oldest known medicines. In the early 19th century, tropanes were isolated, characterized, and synthesized by notable chemical researchers. Their significant biological activities have inspired tremendous research efforts toward their synthesis and the elucidation of their pharmacological activity both in academia and in industry. In this chapter, which addresses the developments in this field since 1994, the focus is on the synthesis of these compounds, and several examples of sophisticated synthetic protocols involving both asymmetric and catalytic approaches are described. In addition, the structures of more than 100 new alkaloids are included as well as the applications and pharmacological properties of some tropane alkaloids.

Thermal conversion of primary alcohols to disulfidesviaxanthate intermediates: an extension to the Chugaev elimination

Heating primaryO-alkylS-difluoro(ethoxycarbonyl)methyl xanthates yields disulfides. This extends to the Chugaev elimination.

Cyclohepta[b]indoles: A Privileged Structure Motif in Natural Products and Drug Design

Seven-membered rings fused with an indole are termed cyclohepta[b]indoles. Compounds exhibiting this structure motif display a broad spectrum of biological activities, ranging from inhibition of adipocyte fatty-acid-binding protein (A-FABP), deacetylation of histones, inhibition of leukotriene production p53, antituberculosis activities, and anti-HIV activities. These biological profiles are found in natural products containing the cyclohepta[b]indole motif, as well as in pharmaceuticals that contain this structure motif. Therefore, the biology of molecules derived from the skeleton of cyclohepta[b]indoles, as well as cyclopenta- and cyclohexa[b]indoles, has attracted considerable interest from the pharmaceutical industry as potential therapeutics in recent years. This is reflected by more than two dozen patents that have been issued in the past decade, solely based on the cyclohepta[b]indole structure motif. The efficient preparation of highly functionalized and unsymmetrically substituted cyclohepta[b]indoles has therefore become of central interest for synthetic organic chemists. Historically, this structure motif most often has been prepared by means of a Fischer indole synthesis. Although very robust and useful, this reaction poses certain limitations. Especially unsymmetrically functionalized cyclohepta[b]indoles are not suitable for a Fischer indole type synthesis, since product mixtures are inevitable. Therefore, novel methodologies to overcome these synthetic obstacles have been developed in recent years. This Account introduces all natural products and pharmaceutical compounds exhibiting the cyclohepta[b]indole motif. The structural variability within cyclohepta[b]indole alkaloids in combination with the broad range of organisms where these alkaloids have been isolated from, strongly suggests that the cyclohepta[b]indole is somehow a "privileged" structure motif. The organisms producing these compounds range from evergreen trees (actinophyllic acid) to cyanobacteria (ambiguinines). The synthetic methodologies to construct these molecular scaffolds (natural and unnatural in origin) are in turn highlighted and discussed with regard to their potential to access highly functionalized and unsymmetrical cyclohepta[b]indoles, for which they specifically have been designed. The methods are classified with respect to reaction type and whether or not they are enantioselective. Finally, the syntheses of cyclohepta[b]indole natural products are presented, thereby in each case, focusing on the construction of this structure motif in the course of the respective total synthesis. As a conclusion, we end by contrasting the methodological progress in the field with the actual successful application of the newly developed methods to the synthesis of complex structures to pinpoint the urgent requirement for further synthetic development for efficient synthetic design of this "privileged" structure motif.

Stereocontrolled Syntheses of Seven-Membered Carbocycles by Tandem Allene Aziridination/[4+3] Reaction

A tandem allene aziridination/[4+3]/reduction sequence converts simple homoallenic sulfamates into densely functionalized aminated cycloheptenes, where the relative stereochemistry at five contiguous asymmetric centers can be controlled through the choice of the solvent and the reductant. The products resulting from this chemistry can be readily transformed into complex molecular scaffolds which contain up to seven contiguous stereocenters.

Rhodium-Catalyzed Regio- and Stereoselective [2 + 2] Cycloaddition of Allenamides

A highly regio- and stereoselective Rh-catalyzed intermolecular head-to-head [2 + 2] cycloaddition of allenamides was developed. The intermolecular cycloadducts, trans-dimethylenecyclobutane-1,2-diamine derivatives, were achieved in good yields with high regioselectivity and stereoselectivity.

Exploiting the σ-phylic properties of cationic gold(i) catalysts in the ring opening reactions of aziridines with indoles

Gold(i) species catalyze the ring opening reactions of aziridines with indoles as nucleophiles for the synthesis of tryptamines and pyrroloindolines.

(3+2)-Cycloaddition Reactions of Oxyallyl Cations

The (3+2)-cycloaddition reaction involving oxyallyl cations has proven to be a versatile and efficient approach for the construction of five-membered carbo- and heterocycles, which are prevalent frameworks in natural products and pharmaceuticals. The following article will provide a brief summary of recent disclosures on this process featuring chemo-, regio- and diastereoselective oxyallyl cycloadditions with both electron-rich and electron-deficient 2π partners.

One pot cascade synthesis of fused heterocycles from furan-tethered terminal alkynes and aldehydes in the presence of amines and CuBr

A novel one-pot protocol for the construction of complex heterocycles through furan tethered terminal alkynes, aldehydes, amines and CuBr upon heating has been developed, giving the cycloadducts in moderate to high yields along with moderate to good regioselectivities.

Mild arylboronic acid catalyzed selective [4 + 3] cycloadditions: access to cyclohepta[b]benzofurans and cyclohepta[b]indoles

The first example of arylboronic acid catalyzed [4 + 3] cycloaddition reaction is reported.