Total Synthesis of (+)-Gliocladin C Based on One-Pot Construction of a 3a-(3-Indolyl)pyrroloindoline Skeleton by Sulfonium-Mediated Cross-Coupling of Tryptophan and Indole

Palladium-Catalyzed Enantioselective Intramolecular Heck Dearomative Annulation of Indoles with N-Tosylhydrazones

An elegant Pd(dba)2-catalyzed enantioselective Heck dearomative annulation of indoles and N-tosylhydrazones for the straightforward assembly of structurally diverse optically active indoline scaffolds containing the quaternary carbon centers at the C2 position has been developed. The tandem protocol, which utilized a Pd(dba)2/BINOL-based phosphoramidite ligand as the catalytic system, proceeded smoothly through successive oxidative addition, intramolecular carbon palladation, migratory insertion, and β-elimination sequences, leading to the chiral indoline derivatives in moderate to excellent yields, with excellent enantioselectivities and diastereoselectivities. In addition, the synthetic practicability of the catalytic system was underlined by a scaled-up experiment and the late-stage derivatization of the products, thus highlighting the potential applications in synthetic chemistry, medicinal chemistry, and material science.

Preparation of Alkyl Di(p-tolyl)sulfonium Salts and Their Application in Metal-Free C(sp3)-C(sp3) and C(sp3)-C(sp2) Bond Formations

Alkyldiarylsulfonium salts were synthesized by a combination of active sulfonium species, prepared through the activation of diarylsulfoxide, and alkyl nucleophiles. The isolated sulfonium salts were subjected to the allylation and cyclopropanation of the active methylene compounds and metal-free C(sp³)-C(sp²) couplings via oxyallyl cation intermediates under mild conditions. The series of reactions included an umpolung strategy for the coupling of alkyl nucleophiles and metal-free C-C bond formation using sulfonium salts.

Enantioselective Synthesis of Quaternary Oxindoles: Desymmetrizing Staudinger-Aza-Wittig Reaction Enabled by a Bespoke HypPhos Oxide Catalyst

This paper describes a catalytic asymmetric Staudinger-aza-Wittig reaction of (o-azidoaryl)malonates, allowing access to chiral quaternary oxindoles through phosphine oxide catalysis. We designed a novel HypPhos oxide catalyst to enable the desymmetrizing Staudinger-aza-Wittig reaction through the PIII/PV═O redox cycle in the presence of a silane reductant and an IrI-based Lewis acid. The reaction occurs under mild conditions, with good functional group tolerance, a wide substrate scope, and excellent enantioselectivity. Density functional theory revealed that the enantioselectivity in the desymmetrizing reaction arose from the cooperative effects of the IrI species and the HypPhos catalyst. The utility of this methodology is demonstrated by the (formal) syntheses of seven alkaloid targets: (-)-gliocladin C, (-)-coerulescine, (-)-horsfiline, (+)-deoxyeseroline, (+)-esermethole, (+)-physostigmine, and (+)-physovenine.

Palladium-Catalyzed Intramolecular Heck Dearomative Alkenylation of Indoles with N-Tosylhydrazones

An elegant Pd-catalyzed intramolecular Heck dearomative alkenylation of aryl iodides with functionalized N-tosylhydrazones proceeded through a sequential dearomative carbopalladation, migratory insertion, and β-hydride elimination in the presence of Pd(CF₃COO)₂ (10 mol %), PPh₃ (30 mol %), and Cs₂CO₃ (2.0 equiv) in 1,4-dioxane (2.0 mL) at 120 °C for 14 h under an argon atmosphere. This cascade cycloaddition protocol provided a reliable and versatile approach to a sequence of structurally diverse indolines in moderate to good yields with good functional group compatibility. In addition, the synthetic robustness of the methodology is highlighted by a scaled-up experiment and derivatization of products via epoxidation and reduction reactions.

Triflic Anhydride (Tf2O)-Activated Transformations of Amides, Sulfoxides and Phosphorus Oxides via Nucleophilic Trapping

Trifluoromethanesulfonic anhydride (Tf2O) is utilized as a strong electrophilic activator in a wide range of applications in synthetic organic chemistry, leading to the transient generation of a triflate intermediate. This versatile triflate intermediate undergoes nucleophilic trapping with diverse nucleophiles to yield novel compounds. In this review, we describe the features and applications of triflic anhydride in organic synthesis reported in the past decade, especially in amide, sulfoxide, and phosphorus oxide chemistry through electrophilic activation. A plausible mechanistic pathway for each important reaction is also discussed.

1 Introduction

2 Amide Chemistry

2.1 Carbon Nucleophiles

2.2 Hydrogen Nucleophiles

2.3 Nitrogen Nucleophiles

2.4 Oxygen and Sulfur Nucleophiles

2.5 hosphorus Nucleophiles

2.6 A Vilsmeier-Type Reagent

2.7 Umpolung Reactivity in Amides

3 Sulfoxide Chemistry

3.1 Oxygen Nucleophiles

3.2 Carbon Nucleophiles

3.3 Nitrogen Nucleophiles

3.4 Thionium Reagents

4 Phosphorus Chemistry

4.1 Hendrickson’s Reagent

4.2 Diaryl Phosphine Oxides

4.3 Phosphonates, Phosphates and Phosphinates

5 Conclusion and Outlook

Palladium-Catalyzed Asymmetric Intramolecular Dearomative Heck Annulation of Aryl Halides to Furnish Indolines

An unprecedented Pd-catalyzed asymmetric intramolecular cascade cyclization of aryl halides with readily available arylboronic acids proceeds through a Heck-type dearomative cyclization terminated with arylation in the presence of Pd₂(dba)₃ (10 mol %), Cu₂O (5 mol %), and Cs₂CO₃ (2.0 equiv) in 1,2-dichloroethane (1.0 mL) at 100 °C for 15 h in air using BINOL-based phosphoramidite as the chiral ligand. This dearomative Heck protocol, which tolerates a broad variety of functional groups, is amenable to the generation of optically active indoline derivatives bearing all-carbon quaternary stereogenic centers in one step in moderate to excellent yields, with excellent diastereoselectivities (>20:1) and enantioselectivities (up to >99% ee). It is worth mentioning that no decrease in the enantiopurity of the indoline derivatives was observed during the synthetic transformations of the products.

Electrophilic Cyanative Alkenylation of Arenes

A variety of appropriately substituted internal alkynes were transformed into the corresponding cyano-substituted phenanthrenes, dihydronaphthalenes, and cyclohepta-1,3,5-trienes in moderate to excellent yields by treatment with imidazolium thiocyanate 1, which serves as an easy to handle [CN]⁺ precursor, in the presence of BCl₃. The synthetic value of the method is additionally demonstrated by the transformation of the primarily obtained products into heavily substituted quinolines. Additionally, the dynamic properties of the prepared dibenzocyclohepta-1,3,5-trienes have been investigated.

Enantioselective synthesis of mixed 3,3'-bisindoles via a phosphine-catalyzed umpolung γ-addition of 3'-indolyl-3-oxindoles to allenoates

An enantioselective umpolung γ-addition reaction of 3'-indolyl-3-oxindoles to allenoates catalyzed by amino acid-derived bifunctional phosphine catalysts has been developed. A wide range of chiral mixed 3,3'-bisindole scaffolds containing an all-carbon quaternary stereogenic center were obtained in high yields and with excellent enantioselectivities. 3,3'-Bisindoles are valuable synthetic intermediates, the employment of which led to formal total syntheses of (+)-Chimonanthine, (+)-Folicanthine and (-)-Calycanthine, as well as facile creation of useful pyrrolidinoindoline core structure.

Oxidative cyanation of 2-oxindoles: formal total synthesis of (±)-gliocladin C

Efficient oxidative direct cyanations of 3-alkyl/aryl 2-oxindoles using Cyano-1,2-BenziodoXol-3(1H)-one (CBX) (2a) have been reported under 'transition metal-free' conditions to synthesize a wide variety of 3-cyano 3-alkyl/aryl 2-oxindoles sharing an all-carbon quaternary center under additive-free conditions. The application of this process is shown by the formal total synthesis of (±)-gliocladin C (11c) in a few steps.

5-(Cyano)dibenzothiophenium Triflate: A Sulfur-Based Reagent for Electrophilic Cyanation and Cyanocyclizations

The synthesis of 5-(cyano)dibenzothiophenium triflate 9, prepared by activation of dibenzo[b,d]thiophene-5-oxide with Tf2 O and subsequent reaction with TMSCN is reported, and its reactivity as electrophilic cyanation reagent evaluated. The scalable preparation, easy handling and broad substrate scope of the electrophilic cyanation promoted by 9, which includes amines, thiols, silyl enol ethers, alkenes, electron rich (hetero)arenes and polyaromatic hydrocarbons, illustrate the synthetic potential of this reagent. Importantly, Lewis acid activation of the reagent is not required for the transfer process. We additionally report herein biomimetic cyanocyclization cascade reactions, which are not promoted by typical electrophilic cyanation reagents, demonstrating the superior ability of 9 to trigger challenging transformations.

Construction of Indole Structure on Pyrroloindolines via AgNTf2-Mediated Amination/Cyclization Cascade: Application to Total Synthesis of (+)-Pestalazine B

An N-linked indole structure was constructed on the 3a-position of pyrroloindoline derivatives via a cascade process involving silver-mediated amination of bromopyrroloindolines with 2-ethynylanilines with subsequent 5- endo-dig cyclization. In this reaction, AgNTf₂ was used as a tandem reagent, which activated the bromo group as a σ-Lewis acid and the alkyne moiety as a π-Lewis acid. Switching from the initial step to the second step was conducted by controlling the temperature. This protocol was applied to the synthesis of various pyrroloindolines, α-carboline, and furoindolines and the total synthesis of a dimeric indole alkaloid, (+)-pestalazine B.

Revisiting Furodiindolines: One-Pot Synthesis of Furodiindolines Using Indole 2,3-Epoxide Surrogates and Their Synthetic Applications

Furodiindolines have emerged as versatile intermediates in various transformations, which are easily obtained from indole 2,3-epoxide surrogates via a one-pot procedure and allowed access to heterocyclic compounds with structural diversity and complexity. Four types of transformations from the furodiindolines have been achieved: (i) dehydrative rearrangement to afford 2,3'-biindoles, (ii) hydrolysis/cyclization to give 3,4-disubstituted quinolines, (iii) ring-opening/cyclization to give pyrroloindolines, and (iv) aminal cleavage to give a 3,3-disubstituted 2-oxindole.

A metal-free biaryl coupling reaction activated by a sulfonium salt

A metal-free biaryl coupling reaction of phenol or aryl ethers mediated by a sulfonium salt formed from diphenyl sulfoxide and trifluoromethanesulfonic anhydride has been developed. The method is a rapid, one-pot reaction, leaving no traces of the sulfonium moiety in the product.