Synthesis of 9,10-Phenanthrenes via Palladium-Catalyzed Aryne Annulation by o-Halostyrenes and Formal Synthesis of (±)-Tylophorine

Total Synthesis of Phenanthroindolizidines Using Strained Azacyclic Alkynes

We report a concise approach to phenanthroindolizidine alkaloids, wherein strained azacyclic alkynes are intercepted in Pd-catalyzed annulations. Two types of strained intermediates were evaluated: a functionalized piperidyne and a new strained intermediate, an indolizidyne. We show that each can be employed, ultimately allowing access to three natural products: tylophorine, tylocrebine, and isotylocrebine. These efforts demonstrate the successful merger of strained azacyclic alkyne chemistry with transition-metal catalysis for the construction of complex heterocycles.

Tandem Ene/[4 + 2] Cycloaddition Reaction for the Synthesis of 9-Benzylphenanthrenes from Arynes and α-(Bromomethyl)styrenes

A tandem reaction for the synthesis of phenanthrenes from arynes and α-(bromomethyl)styrenes is reported. The transformation proceeds via an ene reaction of α-(bromomethyl)styrenes with arynes, followed by a [4 + 2] cycloaddition reaction. The reaction generates 9-benzylphenanthrene derivatives in moderate to excellent yields.

A Three-Step Method for the Preparation of N-Substituted 3,4-Dihydroisoquinolin-1(2H)-ones and Heteroaryl-Fused 3,4-Dihydropyridin-2(1H)-ones from 2-Bromobenzoate Precursors

We demonstrate a general method for the preparation of diverse N-substituted 3,4-dihydroisoquinolin-1(2H)-one compounds through an overall three-step cross-coupling/cyclization/N-deprotection/N-alkylation sequence. In the first step, ethyl 2-bromobenzoates and 2-bromo-1-carboxyethyl heterocycles are cross-coupled with commercially available potassium (2-((tert-butoxycarbonyl)amino)ethyl)trifluoroborate to produce (hetero)aryl-substituted 3-[(N-Boc-2-carboxyethyl)phenyl]ethylamines. In a subsequent two-stage process, these (hetero)arylethylamines undergo base-mediated ring closure followed by N-deprotection and N-alkylation to produce N-substituted 3,4-dihydroisoquinolin-1(2H)-ones and heteroaryl-fused N-benzyl 3,4-dihydropyridin-2(1H)-ones. Mechanistic work was performed to elucidate the order of transformations for the latter two-stage process. The method was also extended to the production of N-benzyl isoindolin-1-one and N-benzyl 2,3,4,5-tetrahydro-1H-benzo[c]azepin-1-one.

Enamine-mediated Mannich reaction of cyclic N,O-acetals and amido acetals: the multigram synthesis of pyrrolidine alkaloid precursors

The cooperative L-proline/Brønsted acid/base promoted reaction of 2-ethoxypyrrolidines or N-substituted 4,4-diethoxybutan-1-amines with methyl(alkyl/aryl)ketones for the synthesis of 2-(acylmethylene)pyrrolidine derivatives is reported. The key features of the developed protocol are gram-scale synthesis of the target compounds, easily available starting materials, operational simplicity and usage of non-expensive reagents.

Transannular Approach to 2,3-Dihydropyrrolo[1,2-b]isoquinolin-5(1H)-ones through Brønsted Acid-Catalyzed Amidohalogenation

A transannular approach has been developed for the construction of pyrrolo[1,2-b]isoquinolinones starting from benzo-fused nine-membered enelactams. This process takes place in the presence of a halogenating agent and under Brønsted acid catalysis and proceeds via a transannular amidohalogenation, followed by elimination. The reaction has been found to be wide in scope, enabling the formation of a variety of tricyclic products in good overall yield, regardless of the substitution pattern in the initial lactam substrate. The reaction has also been applied to the total synthesis of a reported topoisomerase I inhibitor and to the formal synthesis of rosettacin. Further extension of this methodology allows the preparation of 10-iodopyrrolo[1,2-b]isoquinolinones by using an excess of halogenating agent and these compounds can be further manipulated through standard Suzuki coupling chemistry into a variety of 10-aryl-substituted pyrrolo[1,2-b]isoquinolinones.

Aryne Annulations for the Synthesis of Carbocycles and Heterocycles: An Updated Review

Carbocycles and heterocycles are the most relevant structural moieties, which find boundless applications in interdisciplinary areas. In this regard, the interesting chemistry of aryne is highly embraced by the researchers for the synthesis of structurally different polycyclic systems i.e. carbocycles and heterocycles. In this review, we intended to provide a comprehensive overview on annulative approach considering Kobayashi's aryne intermediate as the key reactive species with diverse nucleo-/electrophilic partners. The review highlights the different aryne annulation reactions adopted for the synthesis of carbocycles and heterocycles (covering articles mainly from middle of 20’s), showcasing their viability towards broad range of substrates bearing diverse functionalities.

Reactivity of Vinyl Epoxides/Oxetanes/Cyclopropanes toward Arynes: Access to Functionalized Phenanthrenes

The reactivity of vinyl epoxides/oxetanes/cyclopropanes toward arynes has been demonstrated under mild conditions to give the corresponding phenanthrenes in moderate to good yields. This transition-metal-free cascade process involves a series of Diels-Alder reaction, ring-opening aromatization, and ene reaction. Various functionalized phenanthrenes could be synthesized utilizing the versatile hydroxy group. Interestingly, vinyl epoxides/oxiranes experience preferentially the Diels-Alder reaction toward arynes over nucleophilic attack of epoxides/oxiranes.

Palladium-Catalyzed Sequential Vinyl C-H Activation/Dual Decarboxylation: Regioselective Synthesis of Phenanthrenes and Cyclohepta[1,2,3-de]naphthalenes

The application of a C(vinyl), C(aryl)-palladacycle from vinyl-containing substrates is challenging due to the interference of a reactive double bond in palladium catalysis. This Letter describes a [4 + 2] or [4 + 3] cyclization based on a C(vinyl), C(aryl)-palladacycle by employing α-oxocarboxylic acids as the insertion units under a palladium/air system. The reaction proceeded through the key vinyl C-H activation and dual decarboxylation sequence, forming phenanthrenes and cyclohepta[1,2,3-de]naphthalenes regioselectively in good yields. The synthetic versatility of this protocol is highlighted by the gram-scale synthesis and synthesizing functional material molecule.

Concise Synthesis of Chromene/Chromane-Type Aryne Precursors and Their Applications

The gram-scale synthesis of 5,6-, 6,7-, and 7,8-chromene/chromane-type aryne precursors and their applications in regioselective transformation to other functional derivatives is reported. Chromene/chromane-type arynes are generated under mild conditions, which can further undergo [2 + 2], [3 + 2], and [4 + 2] cycloaddition reactions, nucleophilic addition reactions, and σ-insertion reactions to produce structurally novel substituted chromenes and chromanes. The excellent regioselectivity of the reaction is facilitated by the oxygen-containing guiding groups at the ortho-position of the triple bond, which can be removed or switched to other functional groups including alkenyl, aryl, heteroaryl, and arylamino groups.

Palladium-Catalyzed Three-Component Carbocarbonation of Arynes: Expeditious Synthesis of o-Alkylated Arylacrylates and Stilbenes

A palladium-catalyzed multicomponent reaction involving olefin-tethered aryl iodides, arynes, and electrophilic alkenes has been developed for straightforward synthesis of o-alkylated arylacrylates and stilbenes through tandem intramolecular Heck cyclization/aryne dicarbofunctionalization.

Photochemical synthesis of 3-hydroxyphenanthro[9,10-c]furan-1(3H)-ones from α-keto acids and alkynes

A novel and efficient method for the synthesis of 3-hydroxyphenanthro[9,10-c]furan-1(3H)-ones has been achieved from α-keto acids and alkynes through photo-initiated transformation, providing a range of products in good to excellent yields.

N-Bromosuccinimide-Induced C-H Bond Functionalization: An Intramolecular Cycloaromatization of Electron Withdrawing Group Substituted 1-Biphenyl-2-ylethanone for the Synthesis of 10-Phenanthrenol

An NBS-induced intramolecular cycloaromatization for the synthesis of 10-phenanthrenols from electron-withdrawing group substituted 1-biphenyl-2-ylethanones is described. The in situ generated bromide was designed to act as an initiator for the radical C-H bond activation. An oxidative cross-dehydrogenative coupling reaction of a highly active C-H bond with an inert C-H bond readily occurs under mild conditions without the need for transition metals or strong oxidants.

Aryne-based strategy in the total synthesis of naturally occurring polycyclic compounds

This review has outlined the strategies and tactics of using arynes in the total syntheses of polycyclic natural products.

Expedient synthesis of 9,10-phenanthrenes via LiOPiv-promoted and palladium-catalysed aryne annulation by vinyl triflates

An expedient synthesis of 9,10-phenanthrenes via LiOPiv-promoted and palladium-catalysed aryne annulation by vinyl triflates.

Total synthesis of the reported structure of 13a-hydroxytylophorine

The first total synthesis of the reported structure of 13a-hydroxytylophorine was accomplished. The key step was an unprecedented NaBH₄-promoted one-pot reductive cyclization cascade that efficiently yielded a hydroxyl azonane intermediate. The indolizidine framework was obtained by means of oxidation and a subsequent unexpected protecting-group migration. This total synthesis revealed that the reported structure of the naturally isolated compound is incorrect.

Decarbethoxylative Arylation Employing Arynes: A Metal-Free Pathway to Arylfluoroamides

An efficient, metal-free decarbethoxylative arylation protocol for the synthesis of α-aryl-α-fluoroamides from fluoromalonamates, under ambient reaction conditions using aryne as an electrophilic arylating agent, is reported. This decarbethoxylative arylation proceeds under mild conditions and provides a practical and effective entry to a wide range of α-aryl-α-fluoroacetamides. Interestingly, the use of the tert-butyl ester of fluoromalonamate prevented the otherwise rapid decarboxylation step, affording the arylated fluoromalonamate in moderate yield.

Diversity-Oriented Synthesis of Spiroannulated Benzofuran-3-one Scaffold of Leptosphaerin C and Congeners via Aryne Insertion

A concise synthesis of functionalized cyclohexenone-fused spirobenzofuran-3-ones under mild reaction conditions was developed. The reaction proceeds via insertion of aryne into the C-O bond followed by a regioselective intramolecular conjugate addition. The use of silyl-protected acid was crucial for the transformation. This protocol was successfully applied for the synthesis of leptosphaerin C core and its novel analogues.

Transition metal-free N-arylation of secondary amides through iodonium salts as aryne precursors

A transition metal-free approach for N-arylation of secondary amides was developed via diaryliodonium salts as aryne precursors.