Development of an Alkyne Analogue of the de Mayo Reaction: Synthesis of Medium-Sized Carbacycles and Cyclohepta[b]indoles

Charge-recombinative triplet sensitization of alkenes for DeMayo-type [2 + 2] cycloaddition

Synthetic photochemistry has undergone significant development, largely owing to the development of visible-light-absorbing photocatalysts (PCs). PCs have significantly improved the efficiency and precision of cycloaddition reactions, primarily through energy or electron transfer pathways. Recent research has identified photocatalysis that does not follow energy- or electron-transfer formalisms, indicating the existence of other, undiscovered photoactivation pathways. This study unveils an alternative route: a charge-neutral photocatalytic process called charge-recombinative triplet sensitization (CRTS), a mechanism with limited precedents in synthetic chemistry. Our investigations revealed CRTS occurrence in DeMayo-type [2 + 2] cycloaddition reactions catalyzed by indole-fused organoPCs. Our mechanistic investigations, including steady-state and transient spectroscopic analyses, electrochemical investigations, and quantum chemical calculations, suggest a mechanism involving substrate activation through photoinduced electron transfer, followed by charge recombination, leading to substrate triplet state formation. Our findings provide valuable insights into the underlying photocatalytic reaction mechanisms and pave the way for the systematic design and realization of innovative photochemical processes.

Chemoselective light-induced reactivity of β-enaminones

The irradiation of β-enaminones, generated in situ from cyclic 1,3-diketones and activated alkenes leads to polyheterocyclic skeletons. The photoproduct chemoselectivity depends on the type of cyclic 1,3-diketones employed viz., 2-acetylcyclopentanone and 2-acetylcyclohexanone. The observed chemoselectivity was rationalized based on the Dieckmann-Kon rule.

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.

Development of a Strategy for the Total Synthesis of Aspidosperma Alkaloids via the Cyclobutenone-Based PET-Initiated Cationic Radical-Driven [2+2]/Retro-Mannich Reaction

A novel strategy for the synthesis of Aspidosperma alkaloids has been achieved via a photoredox-initiated [2+2]/retro-Mannich reaction of tryptamine-substituted enaminones as a key step. The developed chemistry has been applied to the construction of the core tetracycle of Aspidosperma alkaloids (±)-aspidospermidine and (±)-limaspermidine.

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.

Intramolecular Alkyne-de Mayo Reaction Using the Enol Methylene Acetal Linker: Experimental Implementation and Computational Simulation

The enol-linked intramolecular alkyne-de Mayo reaction is a photochemically triggered cascade reaction suitable for the synthesis of substituted dihydrotropones by two-carbon ring expansion of enol ethers of cyclopentane-1,3-dion. We report on the implementation of the methylene acetal linker and the isolation of the initial (2 + 2) photocycloadduct in substances. We have investigated in depth the modus operandi of the ring-opening of the π-donor-π-acceptor cyclobutene derivatives by computational chemistry.

Enantioselective Synthesis of Medium-Sized-Ring Lactones via Iridium-Catalyzed Z-Retentive Asymmetric Allylic Substitution Reaction

Medium-sized rings are important structural units, but their synthesis, especially in a highly enantioselective manner, has been a great challenge. Herein we report an enantioselective synthesis of medium-sized-ring lactones by an iridium-catalyzed Z-retentive asymmetric allylic substitution reaction. The reaction features mild conditions and a broad substrate scope. Various eight- to 11-membered-ring lactones can be afforded in moderate to excellent yields (up to 88%) and excellent enantioselectivity (up to 99% ee). The utilization of both Z-allyl precursors and an Ir catalyst is critical for the medium-sized-ring formation.

Gold/scandium bimetallic relay catalysis of formal [5+2]- and [4+2]-annulations: access to tetracyclic indole scaffolds

Regiodivergent formal [5+2]- and [4+2]-annulation reactions of indole derivatives with 2-(2-alkynyl)aryl cyclopropane-1,1-diesters (ACPs) have been developed. A series of tetracyclic indole derivatives were delivered in a 77% average yield with excellent regioselectivities enabled by Au(I)/Sc(III) bimetallic relay catalysis. A gram-scale reaction and further transformation of the resulting tetracyclic indoles demonstrated the practical utility of this protocol. Moreover, the photophysical properties of the obtained multicyclic compounds were also investigated.

The Deuterated "Magic Methyl" Group: A Guide to Site-Selective Trideuteromethyl Incorporation and Labeling by Using CD3 Reagents

In the field of medicinal chemistry, the precise installation of a trideuteromethyl group is gaining ever-increasing attention. Site-selective incorporation of the deuterated "magic methyl" group can provide profound pharmacological benefits and can be considered an important tool for drug optimization and development. This review provides a structured overview, according to trideuteromethylation reagent, of currently established methods for site-selective trideuteromethylation of carbon atoms. In addition to CD3 , the selective introduction of CD2 H and CDH2 groups is also considered. For all methods, the corresponding mechanism and scope are discussed whenever reported. As such, this review can be a starting point for synthetic chemists to further advance trideuteromethylation methodologies. At the same time, this review aims to be a guide for medicinal chemists, offering them the available C-CD3 formation strategies for the preparation of new or modified drugs.

Alkyl Carbagermatrane Enabled Synthesis of Seven-Membered Carbocycle-Fused Aromatics through Catellani Strategy

Synthesis of seven-membered carbocycle-fused aromatics was realized by Catellani reaction using terminally brominated alkyl carbagermatranes through intermolecular cyclization manner. Various functional groups were well tolerated, and this transformation was also expanded to the synthesis of carbocycles of other size. The utility of this method was demonstrated by modification of natural product derivatives and synthesis of bioactive molecules.

Small rings in the bigger picture: ring expansion of three- and four-membered rings to access larger all-carbon cyclic systems

The release of the inherent ring strain of cyclobutane and cyclopropane derivatives allows a rapid build-up of molecular complexity. This review highlights the state-of-the-art of the ring expansions of three- and four-membered cycles and is organised by types of reactions with emphasis on the reaction mechanisms. Selected examples are discussed to illustrate the synthetic potential of this elegant synthetic tool.

Stereoselective Synthesis of Cyclohepta[b]indoles by Visible-Light-Induced [2+2]-Cycloaddition/retro-Mannich-type Reactions

A novel method for the concise synthesis of cyclohepta[b]indoles in high yields was developed. The method involves a visible-light-induced, photocatalyzed [2+2]-cycloaddition/ retro-Mannich-type reaction of enaminones. Experimental and computational studies suggested that the reaction is a photoredox process initiated by single-electron oxidation of an enaminone moiety, which undergoes subsequent cyclobutane formation and rapidly fragmentation in a radical-cation state to form cyclohepta[b]indoles.

Photochemical Approach to the Cyclohepta[b]indole Scaffold by Annulative Two-Carbon Ring-Expansion

We report on the implementation of the concept of a photochemically elicited two-carbon homologation of a π-donor-π-acceptor substituted chromophore by triple-bond insertion. Implementing a phenyl connector between the slide-in module and the chromophore enabled the synthesis of cylohepta[b]indole-type building blocks by a metal-free annulative one-pot two-carbon ring expansion of the five-membered chromophore. Post-irradiative structural elaboration provided founding members of the indolo[2,3-d]tropone family of compounds. Control experiments in combination with computational chemistry on this multibond reorganization process founded the basis for a mechanistic hypothesis.

A.E.Favorskii’s scientific legacy in modern organic chemistry: prototropic acetylene – allene isomerization and the acetylene zipper reaction

Alexei Evgrafovich Favorskii was an outstanding organic chemist who left a great scientific legacy as a result of long time and fruitful work. Most of the theoretically and practically important discoveries of A.E.Favorskii were made in the chemistry of acetylene and its derivatives. Nowadays, the reactions discovered by him, which include acetylene – allene isomerization, the Favorskii and retro-Favorskii reactions, the Favorskii rearrangement and the vinylation reaction, are widely used in industry and in laboratory synthesis. This review summarizes the main scientific achievements of A.E.Favorskii, as well as their development in modern organic chemistry. Much consideration is given to acetylene – allene isomerization as a convenient method for the synthesis of methyl-substituted acetylenes and to the acetylene zipper reaction as a synthetic tool for obtaining terminal acetylenes. The review presents examples of the application of these reactions in modern organic synthesis of complex molecules, including natural compounds and their analogues.

The bibliography includes 266 references.

Regiodivergent Photocyclization of Dearomatized Acylphloroglucinols: Asymmetric Syntheses of (-)-Nemorosone and (-)-6-epi-Garcimultiflorone A

Regiodivergent photocyclization of dearomatized acylphloroglucinol substrates has been developed to produce type A polycyclic polyprenylated acylphloroglucinol (PPAP) derivatives using an excited-state intramolecular proton transfer (ESIPT) process. Using this strategy, we achieved the enantioselective total syntheses of the type A PPAPs (-)-nemorosone and (-)-6-epi-garcimultiflorone A. Diverse photocyclization substrates have been investigated leading to divergent photocyclization processes as a function of tether length. Photophysical studies were performed, and photocyclization mechanisms were proposed based on investigation of various substrates as well as deuterium-labeling experiments.

Photochemically Produced Aminocyclobutanes as Masked Dienes in Thermal Electrocyclic Cascade Reactions

Cyclobutane products of a triplet sensitized enamide-alkene intramolecular [2 + 2] photocycloaddition have been shown to undergo fragmentation under acidic conditions. This lability has been exploited by inducing a complexity-generating thermal electrocyclic cascade sequence involving the in situ formation of a cyclobutene, followed by electrocyclic ring opening, Diels-Alder cycloaddition, and subsequent lactamization. This combination of excited state photochemistry and thermal electrocyclic cascade reactions allows simple planar molecules to be rapidly transformed into sp³-rich scaffolds.