Palladium-catalyzed heteroannulation of cyclic alkenes by functionally substituted aryl iodides

Reusable Supported Pyridine-Mediated Cascade Synthesis of trans-2,3-Dihydroindoles via In Situ-Generated N-Ylide

Merrifield resin-anchored pyridines were prepared and applied as reusable mediators for trans-selective cascade synthesis of 2,3-dihydroindoles. The developed approach relied on in situ N-ylide formation followed by Michael substitution reactions. The cascade reaction was also carried out efficiently with simple pyridine. The products were further transformed into synthetically valuable compounds, and supported pyridine was reused for multiple cycles. Density functional theory calculations confirmed the trans-selectivity as the lower-energy pathway.

[3 + 2] Coupling of Quinone Monoacetals with Vinyl Ethers Effected by Tetrabutylammonium Triflate: Regiocontrolled Synthesis of 2-Oxygenated Dihydrobenzofurans

The synthesis of 2-oxygenated dihydrobenzofurans involving the [3 + 2] coupling of quinone monoacetals with vinyl ethers has been realized by tetrabutylammonium triflate catalysis. The reaction involves a new activation method of the acetal moiety in quinone monoacetals under acid-free conditions affording the highly oxygenated dihydrobenzofurans. This new activation mode was achieved by using the triflate anion catalyst for stabilization of the highly reactive cationic intermediate.

Anti-selective [3+2] (Hetero)annulation of non-conjugated alkenes via directed nucleopalladation

2,3-Dihydrobenzofurans and indolines are common substructures in medicines and natural products. Herein, we describe a method that enables direct access to these core structures from non-conjugated alkenyl amides and ortho-iodoanilines/phenols. Under palladium(II) catalysis this [3 + 2] heteroannulation proceeds in an anti-selective fashion and tolerates a wide variety of functional groups. N-Acetyl, -tosyl, and -alkyl substituted ortho-iodoanilines, as well as free -NH2 variants, are all effective. Preliminary results with carbon-based coupling partners also demonstrate the viability of forming indane core structures using this approach. Experimental and computational studies on reactions with phenols support a mechanism involving turnover-limiting, endergonic directed oxypalladation, followed by intramolecular oxidative addition and reductive elimination.

Pd/Xiang-Phos-catalyzed enantioselective intermolecular carboheterofunctionalization of norbornene and norbornadiene

A highly enantioselective Pd/Xiang-Phos-catalyzed carbohetero-functionalization of norbornene is described, giving a direct access to various chiral norbornane-fused dihydrofurans and dihydro-pyrroles. This synthetic methodology provides the first example of asymmetric carboetherification of norbornene, and also tolerates norbornadiene well.

Pd/Xiang-Phos-catalyzed enantioselective intermolecular carboheterofunctionalization under mild conditions

A mild and practical Pd/Xiang-Phos-catalyzed enantioselective intermolecular carboheterofunctionalization reaction of 2,3-dihydrofurans is developed, leading to various optically active fused furoindolines and tetrahydrofurobenzofurans. The key to this transformation is employing two newly modified N-Me-Xiang-Phos ligands ((S, R S)- N-Me-X4/X5) as chiral ligands under mild conditions. Moreover, this synthetic methodology can be efficiently applied to a variety of complex polysubstituted heterocycles with high chemo-, regio-, and enantio-selectivities via introducing diverse substituents on furan rings, which were hard to access by other routes.

Structurally Modified Cyclopenta[b]benzofuran Analogues Isolated from Aglaia perviridis

Four new cyclopenta[b]benzofuran derivatives based on an unprecedented carbon skeleton (1-4), with a dihydrofuran ring fused to dioxanyl and aryl rings, along with a new structural analogue (5) of 5‴-episilvestrol (episilvestrol, 7), were isolated from an aqueous extract of a large-scale re-collection of the roots of Aglaia perviridis collected in Vietnam. Compound 5 demonstrated mutarotation in solution due to the presence of a hydroxy group at C-2‴, leading to the isolation of a racemic mixture, despite being purified on a chiral-phase HPLC column. Silvestrol (6) and episilvestrol (7) were isolated from the most potently cytotoxic chloroform subfraction of the roots. All new structures were elucidated using 1D and 2D NMR, HRESIMS, IR, UV, and ECD spectroscopic data. Of the five newly isolated compounds, only compound 5 exhibited cytotoxic activity against a human colon cancer (HT-29) and human prostate cancer cell line (PC-3), with IC50 values of 2.3 μM in both cases. The isolated compounds (1-5) double the number of dioxanyl ring-containing rocaglate analogues reported to date from Aglaia species and present additional information on the structural requirements for cancer cell line cytotoxicity within this compound class.

Palladium-catalyzed cascade carboesterification of norbornene with alkynes

An efficient and convenient palladium-catalyzed cascade carboesterification of norbornenes (NBE) with alkynes has been accomplished to afford functionalized α-methylene γ-lactone and tetrahydrofuran derivatives in good to excellent yields. This new strategy exhibits excellent atom- and step-economy, good functional group tolerance and broad substrate scope. In particular, NBE-palladium species was proposed to be the key intermediate in the catalytic cycle to suppress the β-H elimination process. Notably, the developed protocol provides a straightforward and practical tool for the construction of diverse oxygen-containing heterocycle frameworks, illustrating a promising application in synthetic and pharmaceutical chemistry.

Pd-NHC catalyzed cyclopentannulation of diazabicyclic alkenes with ortho-aryl halides

Pd-NHC catalyzed cyclopentannulation of diazabicyclic alkenes with ortho- functionalized aryl halides is described. In all the reactions, a single diastereomer of the cyclopentannulated product is observed, and this reaction is very efficient under microwave irradiation.

New pterocarpanquinones: synthesis, antineoplasic activity on cultured human malignant cell lines and TNF-alpha modulation in human PBMC cells

A new pterocarpanquinone (5a) was synthesized through a palladium catalyzed oxyarylation reaction and was transformed, through electrophilic substitution reaction, into derivatives 5b-d. These compounds showed to be active against human leukemic cell lines and human lung cancer cell lines. Even multidrug resistant cells were sensitive to 5a, which presented low toxicity toward peripheral blood mononuclear cells (PBMC) cells and decreased the production of TNF-alpha by these cells. In the laboratory these pterocarpanquinones were reduced by sodium dithionite in the presence of thiophenol at physiological pH, as NAD(P)H quinone oxidoredutase-1 (NQO1) catalyzed two-electron reduction, and the resulting hydroquinone undergo structural rearrangements, leading to the formation of Michael acceptors, which were intercepted as adducts of thiophenol. These results suggest that these compounds could be activated by bioreduction.

Rhodium-Catalyzed Asymmetric Cyclodimerization of Oxabenzonorbornadienes and Azabenzonorbornadienes: Scope and Limitations

Cationic rhodium(I)-catalyzed cyclodimerization of oxabenzonorbornadienes produced naphtho[1,2-b]furan ring systems in a single step with excellent yields and excellent enantioselectivities. The effect of various Rh(I) catalysts, Ag(I) salts, solvents, and phosphine ligands on the yield and enantioselectivity of the reaction was investigated, and the scope and limitations of this reaction with various oxabicyclic alkenes were studied. Similar results were obtained with the azabenzonorbornadiene analogues, providing the corresponding cyclodimerization products in excellent yields and excellent enantioselectivities.

Joining the rings: the preparation of 2- and 3-indenyl-triptycenes, and curious related processes

The indenyltriptycenes, and , where the 3- or 2-indenyl, respectively, is attached at the 9-position of the triptycene, are attractive prototypes of molecular gearing systems that can also incorporate a brake. These molecules have been prepared from their respective indenylanthracenes, and , by the [4 + 2] cycloaddition of benzyne to the anthracene fragment, and the rotational barriers about the indenyl-triptycenyl single bonds in (12 kcal mol(-1)) and (<9 kcal mol(-1)) have been measured. The precursor anthracenes, and , were prepared by using palladium-catalysed coupling reactions. Unexpectedly, the Heck-type reaction of 9-bromoanthracene, , with indene leads to the formation of 3-indenylanthracene ; moreover, this process is accompanied by a novel palladium-catalysed carbocyclisation reaction leading to the indenophenanthrylene . The addition of benzyne to 9-(3-indenyl)anthracene, , yields the corresponding indenyltriptycene, , and, surprisingly, the anthracenyl methano-bridged phenanthrene . It has been demonstrated that 2-arylindenes can act as 1,3-dienes in the [4 + 2] cycloadditions of benzyne. The products , , and have been characterised by X-ray crystallography.