Stereospecific Synthesis of Tetrahydronaphtho[2,3-b]furans Enabled by a Nickel-Promoted Tandem Reductive Cyclization

Photocatalytic Tandem Radical Cyclization Enables Expeditious Total Synthesis of Epoxyhinokiol Analogues for Anticancer Activity Evaluation

A photocatalytic radical cascade with an unusual endo-trig cyclization was developed, which enables the efficient assembly of divergent tricyclic diterpenoid frameworks. The first total synthesis of abietane 10-epi-epoxyhinoliol was thus achieved in six steps by a subsequent reductive coupling of i-PrBr under photoredox/nickel dual catalysis. Inhibitory tests of chiral 10-epi-epoxyhinoliol and its analogues in 4T1 cancer cells demonstrated the critical role of the C12 hydroxyl group, leading to a discovery of the simplified analogue with better activity.

Intramolecular Ni-catalyzed reductive coupling enables enantiodivergent synthesis of linoxepin

A nickel-catalyzed reductive tandem cyclization of the elaborated β-bromo acetal with a dibenzoxepin scaffold was invented to strategically construct the remaining two rings in linoxepin. The generated diasterodivergent intermediates could be easily converted to both enantiomers of this unique cyclolignan molecule via facile oxidations, thus realizing enantiodivergent total synthesis of linoxepin for the first time.

New Preparation of Ferrocene Carboxylic Acid Benzotriazol-1-yl Ester

Ferrocene and its derivatives are very useful in the fields of chemistry, biomedicine and materials. Herein, a ferrocene derivative was synthesized in one step from benzotriazol-1-yl-oxytripyrrolidino-phosphonium hexafluorophosphate and ferrocenecarboxylic acid. Its accurate structure was determined by 1H and 13C NMR and further confirmed by X-ray diffraction analysis of the corresponding single crystal.

Palladium-Catalyzed Sequential Heck Reactions of Olefin-Tethered Aryl Iodides with Alkenes

An efficient approach to functionalized (E)-3-cinnamyl-3-methyl-2,3-dihydrobenzofurans and (E)-(3-methyl-2,3-dihydrobenzofuran-3-yl)but-2-enones has been developed through a Pd-catalyzed one-pot cascade process involving two sequential Heck reactions, that is, an intramolecular Heck reaction of olefin-tethered aryl iodides and an intermolecular Heck reaction with substituted styrenes and α,β-unsaturated ketones. As a result, a series of desired products were obtained in moderate to good yields and with exclusive E-form selectivities.

Synthesis of Vicinal Carbocycles by Intramolecular Nickel-Catalyzed Conjunctive Cross-Electrophile Coupling Reaction

A nickel-catalyzed intramolecular conjunctive cross-electrophile coupling reaction has been established. This method enables the synthesis of 3,5-vicinal carbocyclic rings found in numerous biologically active compounds and natural products. We provide mechanistic experiments that indicate this reaction proceeds through alkyl iodides formed in situ, initiates at the secondary electrophilic center, and proceeds through radical intermediates.

Total synthesis of linoxepin facilitated by Ni-catalyzed tandem reductive cyclization

A nickel-catalyzed reductive cyclization was developed to construct the tricyclic core embedded in linoxepin, a cyclolignan with a unique benzoxepin ring. The generated diasterodivergent acetals could be converted to the...

Total syntheses of (+)-adunctins C and D: assignment of their absolute configurations

The first total synthesis of (+)-adunctin C (ent-1) and (+)-adunctin D (2), two monoterpene-substitued dihydrochalcones isolated from Piper aduncum (Piperaceae), was achieved. A regioselective oxidative [3 + 2] cycloaddition of acylphloroglucinol with (-)-β-phellandrene was developed to construct their unique spirobenzofuran skeleton. The absolute configurations of natural adunctins 1 and 2 were thus assigned through these endeavors.

Intermolecular alkene arylcyanation using BnSCN as a cyanide source via a reductive strategy: access to 3,3-disubstituted oxindoles

Herein, a nickel-catalyzed two-component reductive arylcyanation of aryl (pseudo)halide tethered alkenes using benzyl thiocyanate as a cyanide source via C–S bond activation is developed.

Efficient construction of diverse 3-cyanoindoles under novel tandem catalysis

A novel and rapid construction of 3-cyanoindoles by palladium-catalyzed tandem reactions has been developed. "N-H" free unprotected, N-alkyl and N-aryl 3-cyanoindoles are obtained with good to excellent yields. The usefulness of this synthetic approach is further demonstrated by the successful synthesis of practical compounds such as the therapeutic estrogen receptor ligand A precursor. Mechanism study shows that the tandem catalysis exploits a Suzuki cross-coupling with subsequent base-induced isoxazole fragmentation, followed by the aldimine condensation.

Nickel-Catalyzed Reductive Aryl Thiocarbonylation of Alkene via Thioester Group Transfer Strategy

Herein reported is a nickel-catalyzed reductive aryl thiocarbonylation of alkene via thioester group transfer strategy by using simple and readily available thioesters. In contrast to traditional activation of weaker C(acyl)-S bond, the C(acyl)-C bond of thioester was selectively cleaved to enable this reaction under mild conditions. Furthermore, this approach features operational simplicity and broad substrate scope, providing a complementary and practical route for thioester synthesis without requiring toxic thiol or CO gas.

Nickel-Catalyzed Dicarbofunctionalization of Alkenes

1,2-Dicarbofunctionalization of alkenes has emerged as an efficient synthetic strategy for preparing substituted molecules by coupling readily available alkenes with electrophiles and/or nucleophiles. Nickel complexes serve as effective catalysts owing to their tendency to undergo facile oxidative addition and slow β-hydride elimination, and their capability to access both two-electron and radical pathways. Two-component alkene functionalization reactions have achieved high chemo-, regio-, and stereoselectivities by tethering one of the coupling partners to the alkene substrate. Three-component reactions, however, often incorporate directing groups to control the selectivity. Only a few examples of directing-group-free difunctionalizations of unactivated alkenes have been reported. Therefore, great opportunities exist for the development of three-component difunctionalization reactions with broad substrate scopes and tunable chemo-, regio-, and stereoselectivities.

Artemlavanins A and B from Artemisia lavandulaefolia and Their Cytotoxicity Against Hepatic Stellate Cell Line LX2

Two new sesquiterpenoids, artemlavanins A (1) and B (3), together with fifteen known compounds (2 and 4-17) were isolated from the EtOH extract of Artemisia lavandulaefolia. The structures of new compounds were elucidated by extensive spectroscopic analyses (HRESIMS, 1D and 2D NMR) and ECD calculations. Compound 1 was a sesquiterpenoid lactone possessing a rearranged eudesmane skeleton; compounds 2-5, 6-8, 9 and 10-12 belonged to the eudesmane, guaiane, oppositane and farnesane sesquiterpenoids, respectively; compounds 13-17 were the phenyl derivatives with a 4-hydroxyacetophenone moiety. Twelve compounds (1-3, 5-7, 10-12, 14, 15 and 17) displayed cytotoxicity against hepatic stellate cell line LX2 (HSC-LX2) with IC50 values ranging from 35.1 to 370.3 μM. Compounds 2, 7, 10-12 and 17 exhibited the stronger cytotoxicity than silybin (IC50, 169.6 μM) with IC50 values of 82.1, 35.1, 95.0, 83.8, 81.6 and 90.1 μM. Compound 7 as the most active one showed significant inhibition on the deposition of human collagen type I (Col I), human hyaluronic acid (HA) and human laminin (HL) with IC50 values of 10.7, 24.5 and 13.3 μM.

Mechanisms of Nickel-Catalyzed Coupling Reactions and Applications in Alkene Functionalization

Nickel complexes exhibit distinct properties from other group 10 metals, including a small nuclear radius, high paring energy, low electronegativity, and low redox potentials. These properties enable Ni catalysts to accommodate and stabilize paramagnetic intermediates, access radical pathways, and undergo slow β-H elimination. Our research program investigates how each of these fundamental attributes impact the catalytic properties of Ni, in particular in the context of alkene functionalization.Alkenes are versatile functional groups, but stereoselective carbofunctionalization reactions of alkenes have been underdeveloped. This challenge may derive from the difficulty of controlling selectivity via traditional two-electron migratory insertion pathways. Ni catalysts could lead to different stereodetermining steps via radical mechanisms, allowing access to molecular scaffolds that are otherwise difficult to prepare. For example, an asymmetric alkene diarylation reaction developed by our group relies upon the radical properties of Ni(III) intermediates to control the enantioselectivity and give access to a library of chiral α,α,β-triarylethane molecules with biological activity.Mechanistic studies on a two-component reductive 1,2-difunctionalization reaction have shed light on the origin of the cross-electrophile selectivity, as C sp² and C sp³ electrophiles are independently activated at Ni(I) via two-electron and radical pathways, respectively. Catalyst reduction has been identified to be the turnover-limiting step in this system. A closer investigation of the radical formation step using a (Xantphos)Ni(I)Ar model complex reveals that Ni(I) initiates radical formation via a concerted halogen-abstraction pathway.The low redox potentials of Ni have allowed us to develop a reductive, trans-selective diene cyclization, wherein a classic two-electron mechanism operates on a Ni(I)/Ni(III) platform, accounting for the chemo- and stereoselectivity. This reaction has found applications in the efficient synthesis of pharmaceutically relevant molecules, such as 3,4-dimethylgababutin.The tendency of Ni to undergo one-electron redox processes prompted us to explore dinuclear Ni-mediated bond formations. These studies provide insight into Ni-Ni bonding and how two metal centers react cooperatively to promote C-C, C-X, and N-N bond forming reductive elimination.Finally, isolation of β-agostic Ni and Pd complexes has allowed for X-ray and neutron diffraction characterization of these highly reactive molecules. The bonding parameters serve as unambiguous evidence for β-agostic interactions and help rationalize the slower β-H elimination at Ni relative to Pd. Overall, our research has elucidated the fundamental properties of Ni complexes in several contexts. Greater mechanistic understanding facilitates catalyst design and helps rationalize the reactivity and selectivity in Ni-catalyzed alkene functionalization reactions.

Nickel-Catalyzed Asymmetric Reductive 1,2-Carboamination of Unactivated Alkenes

Starting from diverse alkene-tethered aryl iodides and O-benzoyl-hydroxylamines, the enantioselective reductive cross-electrophilic 1,2-carboamination of unactivated alkenes was achieved using a chiral pyrox/nickel complex as the catalyst. This mild, modular, and practical protocol provides rapid access to a variety of β-chiral amines with an enantioenriched aryl-substituted quaternary carbon center in good yields and with excellent enantioselectivities. This process reveals a complementary regioselectivity when compared to Pd and Cu catalysis.

Nickel-Catalyzed Reductive Arylalkylation via a Migratory Insertion/Decarboxylative Cross-Coupling Cascade

Reported is a nickel-catalyzed reductive arylalkylation of unactivated alkenes tethered to aryl iodides with redox active N-hydroxyphthalimide esters as the alkyl source through successful merging of migratory insertion and decarboxylative cross-coupling in a cascade. This new method avoids the use of pregenerated organometallic reagents and thus enables the synthesis of diverse benzene-fused carbo- and heterocyclic compounds with high tolerance of a wide range of functional groups.

Oxidative Transformations of Lignans

Numerous oxidative transformations of lignan structures have been reported in the literature. In this paper we present an overview on the current findings in the field. The focus is put on transformations targeting a specific structure, a specific reaction, or an interconversion of the lignan skeleton. Oxidative transformations related to biosynthesis, antioxidant measurements, and total syntheses are mostly excluded. Non-metal mediated as well as metal mediated oxidations are reported, and mechanisms based on hydrogen abstractions, epoxidations, hydroxylations, and radical reactions are discussed for the transformation and interconversion of lignan structures. Enzymatic oxidations, photooxidation, and electrochemical oxidations are also briefly reported.

Ni-catalysed reductive arylalkylation of unactivated alkenes

In this protocol Ni-catalysed reductive arylalkylation of unactivated alkenes tethered to aryl bromides with primary alkyl bromides has been accomplished, providing a new path to construct diverse benzene-fused carbo- and heterocyclic cores including indanes, tetrahydroisoquinolines, indolines and isochromanes. Notably, this new method circumvents the pregeneration of organometallics and demonstrates high tolerance to a wide range of functional groups. The preliminary mechanistic investigations suggest a reaction pathway with an intermediate reduction.

Concise Synthesis of (+)-β- and γ-Apopicropodophyllins, and Dehydrodesoxypodophyllotoxin

Herein, we present an expeditous synthesis of bioactive aryldihydronaphthalene lignans (+)-β- and γ-apopicropodophyllins, and arylnaphthalene lignan dehydrodesoxypodophyllotoxin. The key reaction is regiocontrolled oxidations of stereodivergent aryltetralin lactones, which were easily accessed from a nickel-catalyzed reductive cascade approach developed in our group.

Transition Metal-Catalyzed Dicarbofunctionalization of Unactivated Olefins

Transition metal (TM)-catalyzed difunctionalization of unactivated olefins with two carbon-based entities is a powerful method to construct complex molecular architectures rapidly from simple and readily available feedstock chemicals. While dicarbofunctionalization of unactivated olefins has a long history typically with the use of either carbon monoxide to intercept C(sp³ )-[M] (alkyl-TM) species or substrates lacking in β-hydrogen (β-Hs), development of this class of reaction still remains seriously limited due to complications of β-H elimination arising from the in situ-generated C(sp³ )-[M] intermediates. Over the years, different approaches have been harnessed to suppress β-H elimination, which have led to the development of various types of olefin dicarbofunctionalization reactions even in substrates that generate C(sp³ )-[M] intermediates bearing β-Hs with a wide range of electrophiles and nucleophiles. In this review, these developments will be discussed both through the lens of historical perspectives as well as the strategies scrutinized over the years to address the issue of β-H elimination. However, this review article by no means is designed to be exhaustive in the field, and is merely presented to provide the readers an overview of the key reaction developments.

Ni-Catalyzed 1,2-iminoacylation of alkenes via a reductive strategy

The reductive difunctionalization strategy was successfully applied in the Ni-catalyzed 1,2-iminoacylation reaction of oxime ester-tethered olefins with electrophilic acylating reagents, providing an efficient entry to diverse pyrrolines under safe and mild reaction conditions.

Stereoselective synthesis of a Podophyllum lignan core by intramolecular reductive nickel-catalysis

Ni-catalyzed intramolecular coupling for a stereodivergent construction of a THN[2,3-c]furan core embedded in bioactive Podophyllum lignans, is developed.

Nickel-Catalyzed Reductive Dicarbofunctionalization of Alkenes

An intermolecular, three-component reductive dicarbofunctionalization of alkenes is presented here. The combination of Ni catalysis with TDAE as final reductant enables the direct formation of Csp³-Csp³ and Csp³-Csp² bonds across a variety of π-systems using two different electrophiles that are sequentially activated with exquisite selectivity under mild reaction conditions.

Ni-catalyzed reductive coupling of α-halocarbonyl derivatives with vinyl bromides

This work describes the vinylation of α-halo carbonyl compounds with vinyl bromides under Ni-catalyzed reductive coupling conditions. While aryl-conjugated vinyl bromides entail pyridine as the sole labile ligand, the alkyl-substituted vinyl bromides require both bipyridine and pyridine as the co-ligands.