Rhodium(III)-Catalyzed Intramolecular Redox-Neutral Annulation of Tethered Alkynes: Formal Total Synthesis of (±)-Goniomitine

Isatoic anhydride as a masked directing group and internal oxidant for Rh(III)-catalyzed decarbonylative annulation through C-H activation: insights from DFT calculations

Density functional theory calculations uncovered a new mechanism for the rhodium-catalyzed decarbonylative annulation of isatoic anhydride with alkynes, in which the acyloxy group formed from the N-H deprotonation and C-O bond cleavage of isatoic anhydride acts as the directing group to assist the ortho C-H activation. From the generated five-membered rhodacycle intermediate, the final aminoisocoumarin product could be formed by subsequent steps of alkyne insertion, reductive elimination, decarbonylation, and protonation. The isocyanate moiety contained in the annulation intermediate was uncovered as a novel internal oxidant for the reaction, which oxidizes the Rh(I) to Rh(III) by decarbonylation.

One-pot formal [3 + 2] annulation of 2-pyridinyl-substituted p-quinone methides and arynes for the synthesis of 3-aryl pyrido[1,2-a]indoles

An efficient and metal-free method for the synthesis of 3-aryl pyrido[1,2-a]indoles from aryne intermediates and 2-pyridinyl-substituted p-QMs was successfully developed under ambient conditions. The reaction offered a novel and practical protocol to access some diverse functional molecules in good to excellent yields. The proposed mechanism indicated that the reaction proceeded via a formal [3 + 2] cycloaddition step.

Advances and Strategies towards Synthesis of Aspidosperma Indole Alkaloids Goniomitine

Goniomitine is of the aspidosperma alkaloid family, with an angularly fused tetracyclic skeleton housing an all-carbon quaternary carbon chiral center alongside an aminal functional group. This natural product has garnered attention as a synthetic target due to its intriguing molecular architecture and anti-proliferative activity in recent years. Following the first synthesis of (-)-goniomitine by Takano in 1991, synthetic chemists have developed various methods. This review provides an overview of the methodologies used in the synthesis of goniomitine in racemic and enantiopure forms via divergent construction indole framework, indole functionalization, and the integrated oxidation/reduction/cyclization (iORC) sequence from 1991 to 2023.

Visible-light-induced radical cascade cyclization: a catalyst-free synthetic approach to trifluoromethylated heterocycles

A visible-light-promoted research protocol for constructing dihydropyrido[1,2-a]indolone skeletons is herein described proceeding through a cascade cyclization mediated by trifluoromethyl radicals. This method allows the efficient synthesis of various indole derivatives without the need of photocatalysts or transition-metal catalysts. Mechanism experiments indicate that the process involves a radical chain process initiated by the homolysis of Umemoto's reagent. This straightforward method enables a rapid access to heterocycles containing a trifluoromethyl group.

Transition-metal catalyzed C-H activation as a means of synthesizing complex natural products

Over the past few decades, the advent of C-H activation has led to a rethink among chemists about the synthetic strategies employed for multi-step transformations. Indeed, deploying innovative and masterful tricks against the numerous classical organic transformations has been the need of the hour. Despite this, the immense importance of C-H activation remains unfulfilled unless the methodology can be deployed for large-scale industrial processes and towards the concise, step-economic synthesis of prodigious natural products and pharmaceutical drugs. Lately, the growing potential of C-H activation methodology has indeed driven the pioneers of synthetic organic chemists into finding more efficient methods to accelerate the synthesis of such complex molecular scaffolds. This review aims to draw a general overview of the various C-H activation procedures that have been adopted for synthesizing these vast majority of structurally complicated natural products. Our objective lies in drawing a complete picture and taking the readers through the synthesis of a series of such complex organic compounds by simplified techniques, making it step-economic on a larger scale and thus instigating the readers to trigger the use of such methodology and uncover new, unique patterns for future synthesis of such natural products.

Visible-light-induced indole synthesis via intramolecular C-N bond formation: desulfonylative C(sp2)-H functionalization

Despite significant advances made on the synthesis of indole derivatives through photochemical strategies during the past several years, the requirement of equivalent amounts of oxidants, bases or other additional additives has limited their practical applications in the synthesis of natural products and pharmaceuticals as environment-friendly processes. Herein, we report LED visible-light-induced redox neutral desulfonylative C(sp2)-H functionalization for the synthesis of N-substituted indoles with a broad scope through γ-fragmentation under mild conditions in the absence of any additional additive. The reaction mechanism paradigm has been investigated on the basis of deuterium labeling experiments, kinetic analysis, Hammett plotting analysis and DFT calculations.

Rapid construction of indole-fused 8-10 membered lactones via a tandem reaction

An intramolecular anaerobic Mukaiyama hydration-initiated tandem reduction/condensation/acyl migration/aromatization reaction was developed, which enabled the rapid construction of indole-fused 8-10 membered lactones starting from cyclic 2-allyl-2-(2-nitrophenyl)-1,3-diketones. A nitro substituent in the substrates acted as both an oxygen source in the Mukaiyama hydration step and a nitrogen source in a tandem indole ring construction step. Our reaction features mild conditions, atom economy, and inexpensive reagents and it can be conveniently scaled up to a gram scale in modest yields. A rational reaction mechanism was also proposed based on previous reports and control experiments.

Facile synthesis of the spiro-pyridoindolone scaffold via a gold-catalysed intramolecular alkynol cyclisation/hydroindolylation

A simple approach for the synthesis of pyridoindolone scaffolds with a spiroannulated tetrahydrofuran ring is described. The overall process comprises intramolecular sequential gold-catalysed 5-endo-dig alkynol cycloisomerization and subsequent addition of indole C2 to the in situ generated oxocarbenium cation.

Transition Metal-Free, Methoxide-Catalyzed Synthesis of Pyridoindolones

A simple transition metal-free strategy for the synthesis of pyrido[1,2-a]indolone derivatives has been devised through sodium methoxide-catalyzed intramolecular cyclization of 2-alkenylated N-pyrimidyl indoles. The reactions involved a Smiles rearrangement/cyclization cascade, which resulted in a new series of N-fused indoles, potentially applicable skeletons in medicinal chemistry. This reaction presents simple eco-friendly reaction conditions, a high atom- and cost-economy, a short reaction time, and a broad range of substrate scope with high reaction efficiency.

Rhodium-Catalyzed Annulation of N-Acetoxyacetanilide with Substituted Alkynes: Conversion of Nitroarenes to Substituted Indoles

A general and efficient rhodium-catalyzed redox-neutral annulation of N-acetoxyacetanilides, readily accessible from nitroarenes, with alkynes has been accomplished for the synthesis of substituted indole derivatives. A wide range of substituted 2,3-diarylindoles were achieved from various substituted N-acetoxyacetanilides and symmetrical/unsymmetrical alkynes in good to excellent yields. The developed method was successfully integrated with the synthesis of N-acetoxyacetanilides for the efficient one-pot synthesis of indoles from nitroarenes. The important features are the introduction of N-acetoxyacetamide as a new directing group, redox-neutral annulation, an additive-free approach, wide functional group tolerance, an intramolecular version, and a one-pot reaction of nitroarenes. The method was further extended to the synthesis of potent higher analogues of indole, viz., pyrrolo[3,2-f]indoles and dibenzo[a,c]carbazoles. In addition, a plausible mechanism was proposed based on the isolation and stoichiometric study of a potential aryl-Rh intermediate.

Rhodium(III)-Catalyzed Sequential C-H Activation and Cyclization from N-Methoxyarylamides and 3-Diazooxindoles for the Synthesis of Isochromenoindolones

An efficient synthetic method for structurally various isochromenoindolones has been demonstrated through Rh(III)-catalyzed C-H activation followed by a cyclization reaction of N-methoxyarylamides with 3-diazooxindoles. The sequential reaction involves the streamlined formation of C-C and C-O bonds in one pot. The present method provides a broad range of isochromenoindolones as a new privileged scaffold in moderate to good yields with the release of methoxyamine and molecular nitrogen and has the benefits of a broad substrate scope and good functional group tolerance.

Nonbiomimetic total synthesis of indole alkaloids using alkyne-based strategies

Biomimetic natural product synthesis is generally straightforward and efficient because of its established feasibility in nature and utility in comprehensive synthesis, and the cost-effectiveness of naturally derived starting materials. On the other hand, nonbiomimetic strategies can be an important option in natural product synthesis since (1) nonbiomimetic synthesis offers more flexibility and can demonstrate the originality of chemists, and (2) the structures of derivatives accessible by nonbiomimetic synthesis can be considerably different from those that are synthesised in nature. This review summarises nonbiomimetic total syntheses of indole alkaloids using alkyne chemistry for constructing core structures, including ergot alkaloids, monoterpene indole alkaloids (mainly corynanthe, aspidosperma, strychnos, and akuammiline), and pyrroloindole and related alkaloids. To clarify the differences between alkyne-based strategies and biosynthesis, the alkynes in nature and the biosyntheses of indole alkaloids are also outlined.

Recent advances in the synthesis of pyrido[1,2-a]indoles

This review deals with the different accesses leading to the pyrido[1,2-a]indole nucleus in the last 20 years.

Synthesis of 1H-Indole-2,3-dicarboxylates via Rhodium-Catalyzed C-H Annulation of Arylhydrazines with Maleates

This work describes a one-step synthesis of 1H-indole-2,3-dicarboxylates through C-H activation. Rhodium-catalyzed tandem C-H activation and annulation of 2-acetyl-1-phenylhydrazines with maleates proceeded smoothly in the presence of additive NaOAc and oxidant Ag₂CO₃ and produced the corresponding indole derivatives, 1H-indole-2,3-dicarboxylates, in satisfactory to good yields. A variety of useful functional groups were tolerated on the benzene ring including halogen atoms (F, Cl, Br, and I) and methoxycarbonyl groups.

Synthesis of 2,3-disubstituted indoles via a tandem reaction

A wide array of 2,3-disubstituted indoles were accessed in modest to good yields via a tandem reduction/condensation/fragmentation/cyclization sequence. Differential fragmentation made the reaction more complicated.

Rhodium(III)-Catalyzed Intramolecular Annulations of Acrylic and Benzoic Acids to Alkynes

Rh(III) catalysts can promote a formal (4 + 2) intramolecular oxidative annulation between acrylic or benzoic acid derivatives and alkynes. The reaction, which involves a C-H activation process, allows for a rapid assembly of appealing bicyclic pyran-2-ones and tricyclic isocoumarin derivatives in moderate to good yields. The α-pyrone moiety of the products provides for further manipulations to obtain relatively complex cyclic skeletons in a very simple manner.

Tryptophol and derivatives: natural occurrence and applications to the synthesis of bioactive compounds

This report presents some fundamental aspects related to the natural occurrence and bioactivity of tryptophol as well as the synthesis of tryptophols and their utilization for the preparation of naturally occurring alkaloid metabolites embedding the indole system.

Rhodium-catalyzed intramolecular cascade sequence for the formation of fused carbazole-annulated medium-sized rings by cleavage of C(sp2)-H/C(sp3)-H bonds

The rhodium(iii)-catalyzed intramolecular annulation of alkyne-tethered 3-(indol-3-yl)-3-oxopropanenitriles for the synthesis of fused carbazole scaffolds via C-H activation has been developed. A series of six-, seven-, and eight-membered hydroazepino[3,2,1-jk]carbazoles were achieved. This reaction proceeded under mild reaction conditions and with a broad substrate scope. The reaction involved sequential cleavage of C(sp2)-H/C(sp3)-H bonds and annulation with the tethered alkyne.

Rhodium(III)-Catalyzed Redox-Neutral C-H Activation/Annulation of N-Aryloxyacetamides with Alkynyloxiranes: Synthesis of Highly Functionalized 2,3-Dihydrobenzofurans

Alkynyloxiranes have been employed for the first time as effective coupling partners in Cp*Rh^III-catalyzed C-H functionalization reactions. Their annulation with N-aryloxyamides then offers a redox-neutral and efficient synthesis of functionalized 2,3-dihydrobenzofurans bearing an exocyclic E-allylic alcohol and a tetrasubstituted carbon center in good yields with a broad substrate scope. The products can be easily converted to molecules with more complexity, which provides an opportunity for rapid assembly of structurally diverse heterocycles.

The regioselective synthesis of 2-phosphinoylindoles via Rh(iii)-catalyzed C–H activation

A step-economical route for the synthesis of 2-phosphinoylindoles via the Rh(iii)-catalyzed redox-neutral annulation of N-nitrosoanilines with 1-alkynylphosphine oxides was developed.

Recent advances on the total synthesis of alkaloids in mainland China

Alkaloids are a large family of natural products that mostly contain basic nitrogen atoms. Because of their intriguing structures and important functions, they have long been popular targets for synthetic organic chemists. China's chemists have made significant progress in the area of alkaloid synthesis over past decades. In this article, selected total syntheses of alkaloids from research groups in mainland China during the period 2011–16 are highlighted.

A FISCHER INDOLIZATION STRATEGY TOWARD THE TOTAL SYNTHESIS OF (-)-GONIOMITINE

A Fischer indolization strategy toward the core of (-)-goniomitine is reported. Initial investigations into the Pd-catalyzed asymmetric allylic alkylation of dihydropyrido[1,2-a]indolone (DHPI) substrates are also discussed.

Rhodium(iii)-catalyzed intramolecular annulation through C–H activation: concise synthesis of rosettacin and oxypalmatime

A flexible and efficient rhodium(iii)-catalyzed intramolecular annulation of benzamides bearing tethered alkynes for the synthesis of indolizinones and quinolizinones is reported.

Facile one-pot synthesis of 2,3-dihydro-1H-indolizinium derivatives by rhodium(iii)-catalyzed intramolecular oxidative annulation via C–H activation: application to ficuseptine synthesis

Various substituted indolizidinium, quinolizinium and pyrido[1,2-a]azepinium salts were synthesized from benzaldehydes (α,β-unsaturated aldehydes) and alkyne–amines via Rh-catalyzed C–H activation.