Synthesis of Naphthols by Rh(III)-Catalyzed Domino C–H Activation, Annulation, and Lactonization Using Sulfoxonium Ylide as a Traceless Directing Group

Divergent Access to α-Carbonyl-α'-vinyl Sulfoxonium Ylides and Polysubstituted Furans from β-Ketosulfoxonium Ylides and Ynone-Esters

Here, we describe the synthesis of novel α-carbonyl-α'-vinyl sulfoxonium ylides under ambient, catalyst-free, and additive-free conditions, demonstrating broad substrate scope and scalability using β-ketosulfoxonium ylides and α-ynone-esters. Furthermore, these ylides serve as versatile intermediates for the synthesis of highly substituted furans via Brønsted acid (p-TsOH) catalysis at 130 °C. This approach offers key advantages, including wide substrate compatibility, divergent product formation from common precursors, gram-scale feasibility, and good to excellent yields.

Catalyst-Free [4+1] Annulation of α-Imidoyl Sulfoxonium Ylides and Diazo Compounds Enabling the Modular Synthesis of 2-Indanones and 3(2H)-Furanones

A novel substrate-regulated [4+1] annulation of α-imidoyl sulfoxonium ylides with diazoketones under catalyst-free conditions is described. The reaction proceeds through a coupling of sulfoxonium ylides and in situ-generated ketenes to form the key reactive zwitterionic intermediates, followed by selective formation of C-C or C-O bonds to achieve five-membered ring systems. The cascade reaction permits the direct synthesis of synthetically useful 2-indanones and 3(2H)-furanones, which expands the reaction pattern of sulfoxonium ylides in annulation transformation.

Isocyanide-Based Multicomponent Reaction: Cascade α-Acyloxylation/Carboxamidation and [3 + 1+1] Cyclization of I(III)/S(VI)-Ylides

A metal-free cascade of α-acyloxylation/carboxamidation of I(III)/S(VI)-ylides, carboxylic acids, and isonitriles via a Passerini-like multicomponent reaction is reported. Unexpectedly, [3 + 1+1] cyclization involving I(III)/S(VI)-ylides and two molecules of ethyl isocyanoacetate was observed. The strategy allows for the synthesis of unsymmetrical α,α-disubstituted ketones and functionalized pyrroles with up to 99% yield and wide substrate compatibility. Notably, the procedure has been extended to the late-stage modification of drugs and natural products, offering an elegant complement to the classic Passerini reaction.

Electrochemical Synthesis of Spirolactones from α-Tetralone Derivatives with Methanol as a C1 Source

Spirolactones are widely found in pharmaceuticals and bioactive natural products. However, efficient and environmentally friendly approaches to accessing spirolactones are still highly desirable. Herein, a novel electrochemical synthesis of spirolactones from α-tetralone derivatives with methanol as a C1 source is described. This electrochemical reaction exhibits a high efficiency and good functional group tolerance.

Structurally diverse deformed phenanthrenes from Strophioblachia fimbricalyx with cytotoxic activities by inducing cell apoptosis

A group of phenanthrene derivatives with different deformed types, including four previously undescribed derivatives (1-4), an undescribed natural product (5) and five known compounds (6-10), were isolated from the leaves and stems of Strophioblachia fimbricalyx by molecular networking based on UPLC-MS/MS method. Their structures were established by 1D/2D NMR spectroscopy, HRESIMS, quantum chemistry calculation, and single crystal X-ray diffraction. In biogenic pathways, series of deformed phenanthrenes were all suspected to be derived from 6/6/6 tricyclic phenanthrenes with a gem-dimethyl unit in one ring as characteristic components of Strophioblachia. Fimbricalyxone (1) and trigoxyphin M (6) with a 6/6/5 tricyclic carbon skeleton were reported for the first time from the genus and fimbricalyxanhydride C (2) is the first example of anhydride type bearing a rare 8,9-oxycycle. All the isolates were evaluated for their cytotoxic activity against three tumor cell lines, and compounds 8 and 10 exhibited significant activity with IC50 values of 4.65-9.02 μM, and the structure-activity relationship of the deformed phenanthrenes was discussed. In addition, the X-ray structure of 8 and 10 and the antineoplastic activity of 10 are reported herein for the first time. Trigohowilol G (10) inhibiting the proliferation of A549 cells might be related to cell cycle distribution and the induction of S phase arrest, and it induced cell apoptosis through Bad/Bax/Cleaved PARP1 pathway.

Intramolecular 1,2-Aroyl Migration in Spiro Donor-Acceptor Cyclopropanes: Formation of 1,4-Naphthoquinones and 1-Naphthols as Ring-Expansion Products

Most of the known rearrangement reactions of donor-acceptor cyclopropanes (DACs) involve the migration of cationic carbon atom to anionic carbon or heteroatoms in 1,3- or 1,4-positions. In the present work, we observed that spiro DACs based on 1,3-indanedione or 1-indanone moiety undergo intramolecular 1,2-aroyl migration when treated with titanium(IV) chloride to afford 1,4-naphthoquinones and α-naphthols readily. The reactions take place through the formation of putative 1,3-dipolar intermediates, followed by cleavage and migration of the aroyl group to the adjacent carbon to afford the ring-expansion products.

Copper-Mediated Cyclization of Terminal Alkynes with CF3-Imidoyl Sulfoxonium Ylides To Construct 5-Trifluoromethylpyrroles

A copper-mediated [3 + 2] cyclization of CF3-imidoyl sulfoxonium ylides and terminal alkynes has been demonstrated. This work provides a practical approach for assembling 5-trifluoromethylpyrroles with the merits of a broad substrate scope, good functional tolerance, and mild reaction conditions. Control experiments and DFT studies indicate that this reaction may involve the addition of π-bonds of terminal alkynes by copper-carbene radicals and hydrogen migration.

Pd-Catalyzed Chelation-Assisted Regioselective and Site Selective Cyclative C-H Annulation of Alkynyl Oximes with Activated Alkynes

Electrophilic cyclization and concomitant C-H annulation constitute an expedient cascade strategy for the construction of multicyclic scaffolds with precise substitutional patterns. We report here a novel Pd-catalyzed cyclative annulation of ynone oxime with activated alkynes. The cascade features a dual regioselectivity including site selective C-H activation and chelation-assisted selective insertion of alkynes. Control experiments together with kinetic experiments give insights into the mechanism.

Expedient C-H allylation of sulfoxonium ylides: merging C-H and C-C/C-het bond activation

Sulfoxonium ylide chelation-assisted C-H allylation of arenes has been accomplished utilizing strained vinyl carbo/heterocycles as the allyl surrogates via sequential C-H and C-C/het bond activation. Broad substrate scope, Co-catalysis, selectivity, and late-stage drug mutation are the important practical features.

Rh-catalyzed chemo-, stereo- and regioselective C-H cascade annulation of indolyloxopropanenitriles for pyranoindoles

Selective annulations of alkynes represent a powerful tool for constructing multicyclic scaffolds while installing desired substitution patterns with precision. Herein, we report a Rh-catalyzed unique annulation of indolyl oxopropanenitrile with hydroxy-alkynoates to access pyranoindole cyclic motifs, featuring enol oxygen as a rare chemoselective reactive terminal. The reaction proceeds via a five-membered oxy-rodacycle through C-H activation by a rhodium complex guided by enolic- and propargyloxy dual co-ordination to enable a regio- and stereoselective modular assembly.

Rh(III)-catalyzed [3 + 2] spiroannulation of 2,3-dihydro-1,4-benzoxazines with 4-hydroxy-2-alkynoates through ortho-C-H bond functionalization

Rhodium(III)-catalyzed [3 + 2]-spiroannulation of 2-aryl-1,4-benzoxazines with 4-hydroxy-2-alkynoates has been developed for the synthesis of highly rigid spirolactones in good yields with high regioselectivity. The reaction proceeds through a cascade of C-H activation followed by C-H annulation and lactonization. In this approach, two C-C and C-O bonds are formed in a single step. This is the first report on the spiroannulation of 2,3-dihydro-1,4-benzoxazines with 4-hydroxy-2-alkynoates.

Cascade C-H Activation/Annulation of Sulfoxonium Ylides with Vinyl Cyclopropanes: Access to Cyclopropane-Fused α-Tetralones

Rh-catalyzed weak and traceless directing-group-assisted cascade C-H activation and annulation of sulfoxonium ylides with vinyl cyclopropanes as a coupling partner have been accomplished to furnish functionalized cyclopropane-fused tetralones at moderate temperature. The C-C bond formation, cyclopropanation, functional group tolerance, late-stage diversifications of drug molecules, and scale-up are the important practical features.

Rh(III)-Catalyzed C-H Annulation of Sulfoxonium Ylides and 1,3-Diynes: A Rapid Access to Alkynyl-1-Naphthol Derivatives

An effective redox-neutral strategy to synthesize aryl/alkynyl and alkyl/alkynyl substituted 1-naphthol derivatives has been efficaciously developed by Rh(III)-catalyzed [4+2]-annulation of sulfoxonium ylides and 1,3-diynes with excellent regio- and chemoselectivity. Subsequently, the same strategy was extended to furnish various unsymmetrical binaphthol motifs in one-pot manner. Interestingly, the TMS-derived 1,3-diyne predominantly delivered the 3-alkynyl-1-naphthol via desilylation pathway. The salient features such as traceless directing group, broad substrate scope, good functional group tolerance, and operationally simple conditions made the present protocol more valuable and appealing.

α-Carbonyl sulfoxonium ylides in transition metal-catalyzed C-H activation: a safe carbene precursor and a weak directing group

Transition metal-catalyzed cross-coupling of sp² C-H bonds with diazo compounds via carbene migratory insertion represents an efficient strategy for the construction of C-C and C-heteroatom bonds in organic synthesis. Despite the popularity of diazo compounds as coupling partners in C-H activation, they pose serious safety and stability issues due to potential exothermic reactions linked with the release of N₂ gas. However, compared with diazo compounds, sulfoxonium ylides are generally crystalline solids, more stable, widely used in industrial scales, and easier/safer to prepare. Therefore, recent years have witnessed an upsurge in employing α-carbonyl sulfoxonium ylides as an alternative carbene surrogate in transition metal-catalyzed C-H activation. Unlike diazo compounds, α-carbonyl sulfoxonium ylides contain inherent potential to serve as a coupling partner as well as a weak directing group. This review will summarize the progress made in both categories of reactions.

Synthesis of Tetrahydro-2H-thiopyran 1,1-Dioxides via [1+1+1+1+1+1] Annulation: An Unconventional Usage of a Tethered C-S Synthon

An unprecedented [1+1+1+1+1+1] annulation process has been developed for the construction of tetrahydro-2H-thiopyran 1,1-dioxides. Notably, rongalite acted as a tethered C-S synthon in this reaction and can be chemoselectively used as triple C1 units and as a source of sulfone. Mechanistic investigation indicated that two different carbon-increasing models are involved in this reaction in which rongalite serves as C1 units.

Rhodium(III)-Catalyzed Redox-Neutral [4 + 1]-Annulation of Unactivated Alkenes with Sulfoxonium Ylides

A novel methodology for redox-neutral [4 + 1] annulation of unactivated alkenes with sulfoxonium ylides leads to the synthesis of a diverse library of indanone compounds. The developed annulation reaction was found to be highly versatile due to its compatibility with various unactivated alkenes functionalized with various sensitive functional groups as well as substituted sulfoxonium ylides. Further, multiple transformations such as ring-expansion, reduction, aldol condensation, and Wittig reaction were carried out with indanones. Using this way, highly useful cyclic heterocycles such as indene, dihydroisocoumarin, and 1-indanilidene were prepared in a single step. A possible reaction mechanism was supported by deuterium labeling studies, competitive studies, and kinetic isotopic studies.

Synthesis of Indenone Derivatives by Rh(III)-Catalyzed C-H Functionalization of Sulfoxonium Ylides with 1,3-Diynes

The transition-metal-catalyzed C-H functionalization of sulfoxonium ylides with alkynes formally participates in [4 + 2] annulations to deliver the naphthol scaffolds. In contrast, herein we disclose the first Rh(III)-catalyzed C-H activation, followed by redox-neutral [3 + 2] annulation of sulfoxonium ylides with 1,3-diynes, which delivers the alkynated indenone derivatives. This protocol features a good functional group tolerance, a broad substrate scope, moderate to excellent yields, and mild reaction conditions. The reaction mechanism was supported through ESI-HRMS by characterizing key intermediates in the catalytic cycle.

Rhodium(III)-Catalyzed Atroposelective Synthesis of C-N Axially Chiral Naphthylamines and Variants via C-H Activation

Reported herein is the efficient and atroposelective construction of two categories of C-N atropisomers via rhodium(III)-catalyzed C-H activation of sulfoxonium ylides en route to [4+2] annulation with sterically hindered, electron-rich alkynes. This reaction proceeds with high regio- and enantioselectivity under redox-neutral conditions via a double-substrate activation strategy, providing a novel entry to C-N axially chiral 4-functionalized 1-naphthols.

Cobalt(III)-Catalyzed Chemo- and Regioselective [4 + 2]-Annulation of Aromatic Sulfoxonium Ylides with 1,3-Diynes

Air-stable, highly abundant, and cost-effective Co(III)-catalyzed redox-neutral [4 + 2]-annulation of aromatic sulfoxonium ylides with 1,3-diynes providing useful substituted 1-naphthol derivatives in a regioselective manner is described. Further, the prepared 1-naphthols having internal alkyne were converted into useful polycarbocyclic molecules and spiro-dienone derivatives in good-to-excellent yields. A possible reaction mechanism involving ortho C-H activation as a key step was proposed and supported by deuterium labeling and kinetic isotope labeling studies.

Rhodium(III)-Catalyzed Cascade Reactions of Imines/Imidates with 4-Hydroxy-2-alkynoates to Synthesize Regioselective Furanone-Fused Isoquinoline Scaffolds

A regioselective synthesis of furanone-fused isoquinoline heterocycles is developed in a single step using a Rh(III) catalyst. In this reaction, a cascade C-H activation, regioselective annulation, and lactonization occur in one pot. A wide range of alkynoates was examined, which showed good regioselectivity. The regioselectivity was guided by steric bulk at the C4 position of the 4-hydroxy-2-alkynoates. The synthetic utility was exemplified, and the model reaction was scaled up to a 1 g scale.

Rhodium-Catalyzed Annulation of Phenacyl Ammonium Salts with Propargylic Alcohols via a Sequential Dual C-H and a C-C Bond Activation: Modular Entry to Diverse Isochromenones

Given their omnipresence in natural products and pharmaceuticals, isochromenone congeners are one of the most privileged scaffolds to synthetic chemists. Disclosed herein is a dual (ortho/meta) C-H and C-C activation of phenacyl ammonium salts (acylammonium as traceless directing group) toward annulation with propargylic alcohols to accomplish rapid access for novel isochromenones by means of rhodium catalysis from readily available starting materials. This operationally simple protocol features broad substrate scope and wide functional group tolerance. Importantly, the protocol circumvents the need of any stoichiometric metal oxidants and proceeds under aerobic conditions.

Recent advances in directed sp2 C-H functionalization towards the synthesis of N-heterocycles and O-heterocycles

Heterocyclic compounds are widely present in the core structures of several natural products, pharmaceuticals and agrochemicals, and thus great efforts have been devoted to their synthesis in a mild and simpler way. In the past decade, remarkable progress has been made in the field of heterocycle synthesis by employing C-H functionalization as an emerging synthetic strategy. As a complement to previous protocols, transition metal catalyzed C-H functionalization of arenes using various directing groups has recently emerged as a powerful tool to create different classes of heterocycles. This review is mainly focussed on the recent key progress made in the field of the synthesis of N,O-heterocycles from olefins and allenes by using nitrogen based and oxidizing directing groups.

Pd-Catalyzed Sequential Formation of C-C Bonds: A New Strategy for the Synthesis of (E)-α,β-Unsaturated Carbonyl Compounds from Sulfoxonium Ylides and 1-Iodo-2-((2-methylallyl)oxy)benzene Compounds

α,β-Unsaturated carbonyl compounds are significant moieties in many biological molecules and have attracted considerable attention in organic synthetic chemistry. A Pd-catalyzed cascade cyclization for the synthesis of (E)-α,β-unsaturated carbonyl compounds with the sequential formation of C-C bonds was developed. This method offers high efficiency, good functional group tolerance, and moderate to excellent yields and generally displays high stereoselectivity.

Harnessing Rhodium-Catalyzed C-H Activation: Regioselective Cascade Annulation for Fused Polyheterocycles

In the realm of transition-metal catalyzed arene functionalization, rhodium(III) catalysis is considered as exemplary due to its propensity to activate C-H bonds to obtain comprehensive molecular assembly. Herein, we demonstrate a new rhodium(III) catalyzed assembly of polyheterocyclic scaffolds via C-H activation and regioselective annulation of 4-arylbut-3-yn-1-amines with 4-hydroxy-2-alkynoates. Heterocyclization and trans-metalation prior to annulation is the key for initiation of this relay redox-neutral catalytic cascade.

Rh(III)-Catalyzed olefination to build diverse oxazole derivatives from functional alkynes

A novel Rh(iii)-catalyzed olefination reaction of oxazoles to generate diverse oxazole skeleton derivatives has been realized by directly using oxazole as the directing group. The reaction could tolerate many functional groups, affording complex oxazole derivatives with long chain alkenyls in moderate to good yields, which might find applications in the construction of diverse compounds.

Metal-Free Intramolecular [3+2] Cycloaddition of γ-Hydroxy Acetylenic Ketones with Alkynes for the Synthesis of Naphtho[1,2-c]furan-5-ones and Its Derivatization via a Selective C(sp2)-H Deuteration Reaction

A metal-free intramolecular [3+2] cycloaddtion has been achieved by treating benzene-linked propynol-ynes with AcOH/H₂O in a one-pot manner. The reaction provides greener, 100% atom-economic, highly regioselective, and more practical access to functionalized naphtho[1,2-c]furan-5-ones with valuable and versatile applications. The regioselective α-deuteration of naphtho[1,2-c]furan-5-ones has been also presented with excellent deuterium incorporation and chemical yields. Moreover, the fluorescent properties of naphtho[1,2-c]furan-5-one products have been investigated in solution.

Synthesis of C3-sulfone substituted naphthols via rhodium(III)-catalyzed annulation of sulfoxonium ylides with alkynylsulfones

C-H activation of sulfoxonium ylides catalyzed by rhodium(iii) with subsequent annulation by alkynylsulfones was accomplished. This methodology offers a step-economical approach for assembling C3-sulfone-substituted naphthols with a high level of regioselectivity that is complementary to previous protocols. The approach has an extensive substrate spectrum and broad functional group tolerance.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

Ruthenium-catalyzed coupling of α-carbonyl phosphoniums with sulfoxonium ylides via C–H activation/Wittig reaction sequences

A Ru(ii)-catalyzed coupling of various α-carbonyl phosphoniums with sulfoxonium ylides has been realized for the facile synthesis of 1-naphthols in good to excellent yields.

Synthesis of tetracyclic indenopyrazolopyrazolones through cascade reactions of aryl azomethine imines with propargyl alcohols

Presented herein is a novel and efficient synthesis of tetracyclic indenopyrazolopyrazolone derivatives from aryl azomethine imines and propargyl alcohols through an initial inert C(sp²)–H bond alkenylation followed by the cascade formation of two five-membered rings.

Traceless directing groups: a novel strategy in regiodivergent C-H functionalization

The use of functional groups as internal ligands for assisting C-H functionalization, termed the chelation assisted strategy, is emerging as one of the most powerful tools for construction of C-C and C-X bonds from inert C-H bonds. However, there are various directing groups which cannot be either removed after functionalization or require some additional steps or reagents for their removal, thereby limiting the scope of structural diversity of the products, and the step and atom economy of the system. These limitations are overcome by the use of the traceless directing group (TDG) strategy wherein functionalization of the substrate and removal of the directing group can be carried out in a one pot fashion. Traceless directing groups serve as the most ideal chelation assisted strategy with a high degree of reactivity and selectivity without any requirement for additional steps for their removal. The present review overviews the use of various functional groups such as carboxylic acids, aldehydes, N-oxides, nitrones, N-nitroso amines, amides, sulfoxonium ylides and silicon tethered directing groups for assisting transition metal catalyzed C-H functionalization reactions in the last decade.

Ruthenium(II)-Catalyzed Homocoupling of α-Carbonyl Sulfoxonium Ylides Under Mild Conditions: Methodology Development and Mechanistic DFT Study

A mild ruthenium(II)-catalyzed homocoupling of α-carbonyl sulfoxonium ylides was developed and the detailed mechanism was understood based on DFT calculations in the current report. The catalytic system utilizes the α-carbonyl sulfoxonium ylide as both the directing group for ortho-sp2 C-H activation and the acylmethylating reagent for C-C coupling. Various substituents are compatible in the transformation and a variety of isocoumarin derivatives were synthesized at room temperature without any protection. The theoretical results disclosed that the full catalytic cycle contains eight elementary steps, and in all the cationic Ru(II) monomer is involved as the catalytic active species. The acid additive is responsible for protonation of the ylide carbon prior to the intramolecular nucleophilic addition and C-C bond cleavage. Interestingly, the intermediacy of free acylmethylation intermediate or its enol isomer is not necessary for the transformation.