Expedient Synthesis of Phenanthrenes via In(III)-Catalyzed 6-Exo-Dig Cycloisomerization

Enantioselective Synthesis of α-Quaternary Isochromanes by Oxidative Aminocatalysis and Gold Catalysis

A novel strategy that combines oxidative aminocatalysis and gold catalysis allows the preparation of chiral α-quaternary isochromanes, a motif that is prevalent in natural products and synthetic bioactive compounds. In the first step, α-branched aldehydes and propargylic alcohols are transformed into α-quaternary ethers with excellent optical purities (>90 % ee) via oxidative umpolung with DDQ and an amino acid-derived primary amine catalyst. Subsequent gold(I)-catalyzed intramolecular hydroarylation affords the isochromane products with retention of the quaternary stereocenter. A second approach explores the use of allylic alcohols as reaction partners for the oxidative coupling to furnish α-quaternary ethers with generally lower enantiopurities. Stereoretentive cyclization to isochromane products is achieved via intramolecular Friedel-Crafts type alkylation with allylic acetates as a reactive handle. A number of synthetic elaborations and a biological study on these α-quaternary isochromanes highlight the potential applicability of the presented method.

Cr-Catalyzed Intramolecular Arylative Cross-Coupling of Unactivated C-H Bonds with C-Halide Bonds

We report here a chromium-catalyzed intramolecular arylation of unactivated C-H bonds with C-halide bonds under mild conditions. This reaction was enabled by using a low-cost CrCl2 salt as the precatalyst in combination with allylmagnesium bromide and E/Z-mixed 1-halo-2-styrylarenes as substates, providing a strategy for the construction of functionalized phenanthrene compounds without using external ligands.

Domino Reaction of 2,4-Diyn-1-ols with 1,3-Dicarbonyl Compounds: Direct Access to Aryl/Heteroaryl-Fused Benzofurans and Indoles

A domino propargylation/furanylation (intramolecular exo-dig-cyclization)/benzannulation reaction of 2,4-diyn-1-ols with 1,3-dicarbonyl compounds has been developed for the first time. This provides a novel and effective method for the preparation of aryl/heteroaryl-fused benzofurans from easily accessible starting materials in a single step. The methodology was extended to pyrrolyl-benzannulation to obtain aryl/heteroaryl-fused indoles. Further, application of this approach in the synthesis of eustifoline D and dictyodendrin structural frameworks has been demonstrated.

Cu(I)-Catalyzed Ligand-Free Tandem One-Pot or Sequential Annulation via Knoevenagel Intermediates: An Entry into Multifunctional Naphthalenes, Phenanthrenes, Quinolines, and Benzo[b]carbazoles

A simple but efficient one-pot or sequential copper-catalyzed protocol using 2-bromoaldehydes and active methylene group containing substrates that affords multifunctional naphthalenes, phenanthrenes, quinolines, and benzo[b]carbazoles via Knoevenagel condensation, C-arylation, and decarboxylation, followed by aromatization, is developed. The reaction utilizes the potential of Knoevenagel intermediates and does not require any ancillary ligand. The phenanthrene products thus obtained show moderate fluorescence activity. Structural elaboration of the products to obtain dihydrobenzoquinazolines is also highlighted.

In(OTf)3-catalyzed intramolecular hydroarylation of α-phenylallyl β-ketosulfones – synthesis of sulfonyl 1-benzosuberones and 1-tetralones

In(OTf)₃-catalyzed intramolecular hydroarylation of α-phenylallyl β-ketosulfones provides sulfonyl 1-benzosuberones and 1-tetralones in moderate to good yields in refluxing (CH₂Cl)₂ under open-vessel and easy-operation reaction conditions. A plausible mechanism is proposed and discussed. This highly regioselective protocol provides an atom-economic ring-closure route.

In(OTf)₃-catalyzed intramolecular hydroarylation of α-phenylallyl β-ketosulfones provides sulfonyl 1-benzosuberones and 1-tetralones in moderate to good yields in refluxing (CH₂Cl)₂ under open-vessel and easy-operation reaction conditions.

A palladium-catalyzed multi-component annulation approach towards the synthesis of phenanthrenes

An efficient palladium-catalyzed three-component cascade reaction has been developed for the facile construction of phenanthrene frameworks. The transformation is driven by a controlled reaction sequence of Suzuki-Miyaura coupling followed by the insertion of alkynes, and finally, annulation to yield phenanthrene derivatives via C-H activation. This methodology is able to accommodate a variety of substrates and affords the anticipated products in good to excellent yields.

Phenanthrene Synthesis by Palladium(II)-Catalyzed γ-C(sp2)-H Arylation, Cyclization, and Migration Tandem Reaction

Phenanthrene is an important structural motif in chemistry and materials science, and many synthetic routes have been developed to construct its skeleton. However, synthesis of unsymmetric phenanthrenes remains a challenge. Here, an efficient one-pot tandem reaction for the preparation of phenanthrenes via sequential γ-C(sp²)-H arylation, cationic cyclization, dehydration, and 1,2-migration was developed. A wide range of symmetric and unsymmetric phenanthrenes with diversified functional groups were synthesized with good to excellent yields.

Copper-catalyzed direct couplings of terminal alkynes with primary and secondary benzyl bromides

We have developed a facile Cu-catalyzed benzylation of terminal alkynes from easily available benzyl bromides.

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.

Oxidative cyclization of 2-allenyl-1,1'-biphenyls with α-carbonyl alkyl bromides: facile access to functionalized phenanthrenes

A new copper-facilitated oxidative cyclization of 2-allenyl-1,1'-biphenyls with α-carbonyl alkyl bromides for producing functionalized phenanthrenes is presented, which represents the first allene 2,3-dicarbofunctionalization triggered by oxidative radical-medicated C3-addition of the terminal allene moiety and C-Br/C-H functionalization.

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.

Indium(iii) as π-acid catalyst for the electrophilic activation of carbon–carbon unsaturated systems

This review focuses on indium(iii) as a π-acid for the activation of C–C unsaturated systems (alkynes, alkenes, and allenes) in organic synthesis.

Enhancing polo-like kinase 1 selectivity of polo-box domain-binding peptides

An important goal in the development of polo-like kinase 1 (Plk1) polo-box domain (PBD) binding inhibitors is selectivity for Plk1 relative to Plk2 and Plk3. In our current work we show that Plk1 PBD selectivity can be significantly enhanced by modulating interactions within a previously discovered "cryptic pocket" and a more recently identified proximal "auxiliary pocket."

A new route to π-extended polycyclic aromatic hydrocarbons via cross-dehydrogenative coupling

A rare example of fruitful construction of highly π-extended PAHs expressly via palladium-catalyzed CDC reaction is demonstrated.

Mechanisms of metal-catalyzed cycloisomerizations of o-propargylbiaryls and o-allenylbiaryls to phenanthrenes: a DFT study

DFT calculation results reveal that InCl₃-catalyzed cycloisomerization of o-propargylbiphenyl proceeds via 6-exo cyclization followed by intermolecular chloride-assisted H-abstraction/proto-demetalation and intramolecular chloride-assisted H-abstraction/proto-demetalation to afford a phenanthrene skeleton.

Ascorbic acid promoted [4 + 2] benzannulation: a mild, operationally simple approach to the synthesis of phenanthrenes

Ascorbic acid-promoted radical [4 + 2] benzannulation for the generation of substituted phenanthrenes at room temperature has been developed.

Preparation of phenanthrenes from ortho-amino-biphenyls and alkynes via base-promoted homolytic aromatic substitution

Phenanthrenes are prepared from ortho-aminobiphenyls and acetylenes via base-promoted homolytic aromatic substitution.

Indium-catalyzed intramolecular hydroarylation of aryl propargyl ethers

Indium(iii) halides are efficient and selective catalysts for the intramolecular hydroarylation of aryl propargyl ethers.

Synthesis of quinoxalines or quinolin-8-amines from N-propargyl aniline derivatives employing tin and indium chlorides

The utility of stannous chloride dihydrate as well as indium/InCl₃ in combination with “ortho-nitrogen”-N-propargylanilines for the construction of either quinoxaline- or quinolin-8-amine-scaffolds is demonstrated. Depending on the nature of the alkyne, different bicyclic nitrogen heterocycles are formed under aerobic conditions.

A highly efficient tandem [3 + 2] “click” cycloaddition/6-exo-cyclization strategy for the construction of triazole fused pyrazines

The pharmaceutically important tetrahydro-[1,2,3]triazolopyrazine heterocyclic architecture has been synthesized via a concise tandem “click”/6-exo-dig cyclization strategy under mild conditions in mixed aqueous–organic media.