Counter-Ion-Dependent Alkyne Iminium Ion Cyclization for Divergent Synthesis of N-Fused Indolylidine, Indole, and Indoline Derivatives Promoted by the Lewis/Bronsted Acid

Alkynyl Halo-Aza-Prins Annulative Couplings

This article is a comprehensive report describing our studies in the field of aza-alkynyl Prins chemistry, comparing and contrasting the different reaction partners and reactivities observed during method development. The synthetic strategies combine an alkynyl aza-Prins coupling with an annulation, enabling the preparation of different nitrogen-containing heterocycles. Different iminium ions are explored as viable electrophiles for an alkynyl Prins cyclization, terminated by capture with a halogen nucleophile to form a vinyl halide. The synthetic utility of this functional handle is exploited through a number of Suzuki cross-couplings, allowing for the preparation of a modest library of compounds. In most cases, the Prins couplings are highly selective for the vinyl halides with E geometry, resulting from anti-addition across the alkyne.

Stereoselective Synthesis of Pyrrolo/Pyrido[2,1-a]isoindoles via Alkyne Iminium Ion Cyclization of Vinylogous Carbamates

An efficient, acid-mediated, intramolecular alkyne iminium ion cyclization of oxoisoindolidene for the diastereoselective synthesis of pyrrolo/pyridoisoindole is described. This protocol features broad substrate scope and easy scalability. An unusual N to C-1,3-alkyl shift is observed with substrates bearing strong electron donating group at the phenyl ring attached to alkyne with concomitant hydration of alkyne to the ketone.

Electron Donor-Acceptor Complex Induced Fused Indoles with Hypervalent Iodine(III) Reagents

An operationally simple and efficient method for the cyclization of tertiary amines and hypervalent iodine reagents enabled by an EDA complex has been developed. A series of [1,2-α]indoles derivatives were obtained in good yields, including some key intermediates for the synthesis of biologically active molecules. In addition, this established strategy features a broad substrate scope and good functional group tolerance.

Leveraging Alkyne-Oximium Cyclization for the Stereoselective Synthesis of Isoxazolidine

A TMSOTf-mediated highly diastereoselective synthesis of isoxazolidine bearing three contiguous stereocenters is described. This method utilizes O-propargyl hydroxylamines as a novel building block for the rapid assembly of the isoxazolidines via alkyne-oximium cyclization. The strategy could be used in the synthesis of enantiomerically enriched isoxazolidines. Reductive cleavage of the N-O bond efficiently gives the corresponding 1,3-aminodiols with precise control over all stereocenters formed.

Synthesis of Spiro[furan-2,1'-isoindolin]-3'-ones from 2-(4-Hydroxybut-1-yn-1-yl)benzonitriles and Aryl Aldehydes under the Action of Triflic Acid

The synthesis of spiro[furan-2,1'-isoindolin]-3'-ones from 2-(4-hydroxybut-1-yn-1-yl)benzonitriles and aryl aldehydes is demonstrated. It involves the initial formation of dihydrofuranylideneisoindolinone via intramolecular sequential Prins and Ritter reactions, followed by the ring opening of the furanyl moiety to generate N-acyliminium ions and alcohols for the final cyclization reaction, and the spiro-cyclic compounds are produced in moderate to good yields. It is a one-pot, three-component reaction in which one new quaternary carbon, two five-membered rings, one C-N bond, two C-O bonds, and one C-C bond are formed. The reaction is carried out with a Brønsted acid from 0 °C to room temperature within a short period of time.

AgNTf2 catalyzed cycloaddition of N-acyliminium ions with alkynes for the synthesis of the 3,4-dihydro-1,3-oxazin-2-one skeleton

A catalyzed process for the synthesis of the 4,6-substituted 3,4-dihydro-1,3-oxazin-2-one skeleton has been developed through cycloaddition of in situ generated acyliminium intermediates with alkynes. A range of chain N,O-acetals and terminal alkynes were amenable for this mild transformation. As a result, a series of desired cycloaddition products were obtained in moderate to good yields.

Cu(OTf)2-catalyzed C3 aza-Friedel-Crafts alkylation of indoles with N,O-acetals

An efficient approach to access functionalized indole derivatives has been developed through Cu(OTf)₂-catalyzed C3 aza-Friedel-Crafts alkylation of substituted indoles 5a-5m, N-methyl-pyrrole with linear N,O-acetals 4a-4l. As a result, a series of C3 amide aza-alkylated indole derivatives 6a-6ag and 7 were synthesized in moderate to excellent yields.

Recent advances in the synthesis of pyrrolo[1,2-a]indoles and their derivatives

The pyrrolo[1,2-a]indole unit is a privileged heterocycle found in numerous natural products and has been shown to exhibit diverse pharmacological properties. Thus, recent years have witnessed immense interest from the synthesis community on the synthesis of this scaffold. In light of the ever-increasing demand for pyrrolo[1,2-a]indoles in drug discovery, this review provides an overview of recent synthesis methods for the preparation of pyrrolo[1,2-a]indoles and their derivatives. The mechanistic pathway and stereo-electronic factors affecting the yield and selectivity of the product are briefly explained. Furthermore, we have attempted to demonstrate the utility of the developed methods in the synthesis of bioactive molecules and natural products, wherever offered.

Synthesis of α-(aminoethyl)-α,β-enones via alkyne aza-Prins cyclization and their synthetic application to pyrrolidines

We developed a synthetic method for α-(aminoethyl)-α,β-enones from aryl-substituted homopropargyl sulfonamides and aldehydes, representing the first synthesis of conjugated enones via alkyne aza-Prins cyclization. These products could be converted into pyrrolidines by a formal 5-endo-trig cyclization.

Intramolecular Formal [4 + 2] Cycloadditions: Synthesis of Spiro Isoindolinone Derivatives and Related Molecules

Acid-catalyzed intramolecular reactions of isoindolinone-derived hydroxylactam derivatives bearing enones or enals that afford spiro isoindolinone derivatives and related molecules have been developed. From the hydroxylactam moieties, N-acylenamides were generated in situ and reacted with the enone and the enal moieties via formal [4 + 2] cycloaddition reactions to construct cyclohexanone- and dihydropyran-fused ring systems and the spiro ring systems.

Application of Cp2TiCl-Promoted Radical-Induced Cyclization: An Expeditious Access to [a]-Annelated Indoles

An efficient and novel route for assembling pyrrolo/piperido[1,2-a]indoles is portrayed involving a radical-mediated reductive epoxide opening reaction of N-tethered epoxy-indoles that trigger facile intramolecular cyclization followed by an oxidative quenching step. Capitalizing on the operational simplicity of the method involving just two steps and use of an efficient C-C bond-forming reaction, this radical-based protocol enables the modular assembly of an important class of N-fused indole derivatives with versatile functional and structural diversity.

Substituted α-Alkylidene Cyclopentenones via the Intramolecular Reaction of Vinyl Cations with Alkenes

Substituted α-alkylidene cyclopentenones are formed in up to 93% yield by the intramolecular capture of vinyl cations with pendent alkenes. An increased level of substitution at the β-position of the β-hydroxy-α-diazoketone starting material changed the course of the reaction to instead give a lactone product. A reaction path that involves bond reorganization via an acylium ion intermediate is proposed to explain these results. Substrate scope studies showed that more stable vinyl cations gave higher α-alkylidene cyclopentenone yields. This study provides a mild and efficient method to form α-alkylidene cyclopentenones that complements C-H insertion and Nazarov cyclization strategies.

Gold-catalyzed generation of azafulvenium from an enyne sulfonamide: rapid access to fully substituted pyrroles

A one-pot gold-catalyzed protocol for the synthesis of fully substituted pyrroles from enyne sulfonamides has been developed.

A Highly Regio- and Stereoselective Syntheses of α-Halo Enamides, Vinyl Thioethers, and Vinyl Ethers with Aqueous Hydrogen Halide in Two-Phase Systems

A metal-free regio- and stereoselective method is achieved for the preparation of ( E)-configured α-halo enamides, vinyl thioethers, and vinyl ethers using aqueous HX (X = F, Cl, Br, I), which features high functional group compatibility and regio- and stereoselectivity, mild conditions, high efficiency, and rapid transformation. Additionally, the isomers could be yielded readily from the ( E)-configured α-halo enamides via photocatalysis or under Sonogashira coupling conditions.

Novel Hybrid Prins/Aza-Prins Oxocarbenium/N-Acyliminium Cascade: Expedient Access to Complex Indolizidines

Heavy silyl enol ethers (mostly TIPS and TBS) combine with cyclic N-alkenyl N-acyliminium salts generated in situ from their N,O-acetal precursors, to furnish highly functionalized indolizidines through an unprecedented double Mukaiyama-Mannich-Prins cascade transformation. This novel cascade annulation process demonstrates a promising scope, and takes place mostly catalytically with interesting stereocontrol. Furthermore, an appealing facet of this chemistry is emphasized with a bicatalytic approach by which the Mannich-Prins cascade follows a Ru-catalyzed N-allylamide to N-(E)-propenyl isomerization of the aminal counterpart in a one-pot operation.

Synthesis of 3-(5-amino-1H-1,2,4-triazol-3-yl)propanamides and their tautomerism

Two complementary pathways for the preparation of N-substituted 3-(5-amino-1H-1,2,4-triazol-3-yl)propanamides (5) were proposed and successfully realized in the synthesis of 20 representative examples. These methods use the same types of starting materials viz. succinic anhydride, aminoguanidine hydrochloride, and a variety of amines. The choice of the pathway and sequence of the introduction of reagents to the reaction depended on the amine nucleophilicity. The first pathway started with the preparation of N-guanidinosuccinimide, which then reacted with amines under microwave irradiation to afford 5. The desired products were successfully obtained in the reaction with aliphatic amines (primary and secondary) via a nucleophilic opening of the succinimide ring and the subsequent recyclization of the 1,2,4-triazole ring. This approach however failed when less nucleophilic aromatic amines were used. Therefore, an alternative pathway, with the initial preparation of N-arylsuccinimides and their subsequent reaction with aminoguanidine hydrochloride under microwave irradiation, was applied. The annular prototropic tautomerism in the prepared 1,2,4-triazoles 5 was studied using NMR spectroscopy and X-ray crystallography.

Two complementary pathways for the preparation of N-substituted 3-(5-amino-1H-1,2,4-triazol-3-yl)propanamides were proposed and successfully realized in the synthesis of 20 representative examples.

Stereoselective synthesis of thiazino[4,3-a]indoles using the thia-Pictet-Spengler reaction of indoles bearing N-tethered thiols and vinylogous thiocarbonates

An inter- as well as intra-molecular thia-Pictet-Spengler cyclization of N-tethered thiols and vinylogous thiocarbonates is described for the stereoselective synthesis of N-fused thiazinoindole derivatives. The strategy is extended to one-pot, sequential Friedel-Crafts alkylation - Pictet-Spengler cyclization and the synthesis of thiazino-oxepino-indole.

Reactivity of alkynylindole-2-carboxamides in [Pd]-catalysed C–H activation and phase transfer catalysis: formation of pyrrolo-diindolones vs. β-carbolinones

[Pd]-Catalysed reaction of 3-alkynylindole-2-carboxamidesviaC–H activation leads to pyrrolodiindolones; in contrast, under phase-transfer catalysis, the same precursors afford β-carbolinones.

Stereoselective synthesis of 2,3-disubstituted indoline, pyrrolidine and cyclic ether-fused 1,2-dihydroquinoline derivatives using alkyne iminium ion cyclization of vinylogous carbamates: switch of regioselectivity using an internal hydroxy group as a nucleophile

TMSOTf promoted alkyne iminium ion cyclization of vinylogous carbamates gives stereoselective access to indolines, pyrrolidines and dihydroquinolines.