Stereoselective Double Spirocyclization of 2-Benzyl-3-alkynyl Chromone with Nitrone via Gold-Catalyzed Cascade Reactions

A novel, highly stereoselective gold-catalyzed spirocyclization of 2-benzyl-3-alkynyl chromone with nitrone is described. This cascade reaction involves gold-catalyzed cycloisomerization, nitrone-olefin [3 + 2]-annulation, alkene oxidation, and rearrangement for the formation of spirocyclic products. Interestingly, the isoxazolidine ring generated from [3 + 2]-annulation donates oxygen to alkene to generate a new pyran-3(4H)-one and azetidine ring for dispiro-benzofuran formation upon heating. This work demonstrates the one-pot, gold-catalyzed, multiple-step reaction, and the reaction temperature directly affects the formation of spirocyclic products.

Gold-Catalyzed Cascade Reaction of Yne-Enones with Iminooxindoles, Access to 3,2'-Pyrrolidinyl-Spirooxindole Derivatives

Herein, a gold-catalyzed cascade reaction of yne-enones with iminooxindoles has been developed through a cascade cycloisomerization/(3 + 2) annulation process. This approach provides a straightforward and efficient route for the synthesis of functionalized 3,2'-pyrrolidinyl-spirooxindoles in high reactivity and broad substrate scope with excellent cis-selectivity. Moreover, the subsequent functionalization of furan units allows for the diverse synthesis of spirooxindole derivatives, which have demonstrated good antitumoral activity.

Access to Chiral Tetrahydroquinazolines/1,3-Benzoxazines via Iridium-Catalyzed Asymmetric [4 + 2] Cycloaddition

An iridium-catalyzed asymmetric [4 + 2] cycloaddition of 1,3,5-triazinanes with 2-(1-hydroxyallyl)anilines/2-(1-hydroxyallyl)phenols has been developed, providing a straightforward and efficient approach to a wide range of tetrahydroquinazolines in good yields and excellent enantioselectivities (up to >99% ee). Typically, chiral 1,3-benzoxazines, which are challenging substrates in asymmetric [4 + 2] cycloaddition, could be obtained in excellent enantioselectivities via this protocol.

Lewis Acid-Catalyzed Annulations of Geminally Disubstituted Cyclopropanes with Aldehydes or 1,3,5-Triazinanes

Under the catalysis of simple Lewis acid catalysts, 2-(1-aroylcyclopropyl)malonates demonstrated unique reactivities in annulation reactions with aryl/alkyl aldehydes, paraformaldehyde, and 1,3,5-triazinanes. Three types of structurally diverse cyclic products that are otherwise not easy to obtain were generated in moderate to good yields and excellent diastereoselectivities. Possible reaction pathways leading to these products were proposed on the basis of the results of control experiments.

Construction of Diverse N-Heterocycles by Formal (3 + 3) Cycloaddition of Naphthol/Thionaphthol/Naphthylamine and 1,3,5-Triazinanes

Here, we report a facile and metal-free method for the construction of dihydrooxazine derivatives via a formal (3 + 3) annulation reaction of naphthols and 1,3,5-triazinanes. The 1,3,5-triazinanes were utilized as a formal three-atom synthon (C-N-C) for cycloaddition. In addition, dihydrothiazine and tetrahydrobenzoquinazoline derivatives could also be produced in good yields by this strategy under catalyst-free and additive-free conditions.

Synthesis of Chromeno[2,3-d]pyrimidin-5-one Derivatives from 1,3,5-Triazinanes via Two Different Reaction Pathways

1,3,5-Triazinanes, as a kind of versatile building block, are applied in the synthesis of chromeno[2,3-d]pyrimidin-5-one derivatives via two different reaction modes, which perfectly exhibits the powerful function of 1,3,5-triazinane as a three-atom synthon along with the structure variation of another substrate. The two annulation reactions proceed under mild conditions and bear broad substrate scope and high yield.

Synthesis of α-Amino Tertiary Alkylperoxides by Lewis Acid-Catalyzed Peroxidation of 1,3,5-Triazines

α-Substituted peroxides have been found in natural products and are widely used as anti-malarial agents. Zn(OTf)₂ -catalyzed peroxidation of 1,3,5-triazines has been developed, accessing diversely substituted α-amino tertiary alkylperoxides with high efficiency. Mechanistic investigations and useful synthetic application of the products have also been presented.

Indium-Catalysed Transfer Hydrogenation for the Reductive Cyclisation of 2-Alkynyl Enones towards Trisubstituted Furans

Indium tribromide catalysed the transfer hydrogenation from dihydroaromatic compounds, such as the commercially available γ-terpinene, to enones, which resulted in the cyclisation to trisubstituted furan derivatives. The reaction was initiated by a Michael addition of a hydride nucleophile to the enone subunit followed by a Lewis-acid-assisted cyclisation and the formation of a furan-indium intermediate and a Wheland intermediate derived from the dihydroaromatic starting material. The product was formed by protonation from the Wheland complex and replaced the indium tribromide substituent. In addition, a site-specific deuterium labelling of the dihydroaromatic HD surrogates resulted in site specific labelling of the products and gave useful insights into the reaction mechanism by H-D scrambling.

Highly enantioselective tandem cycloisomerization/Diels-Alder reaction of 2-(1-alkynyl)-2-alken-1-ones and enals: dual catalysis with platinum and amines

Herein, we disclosed a highly efficient strategy of enantioselective synthesis of 2,3-furan-fused carbocycles bearing three-contiguous stereocenters. This transformation is catalyzed by dual catalysis of PtCl4/chiral amines via tandem dehydrogenative annulation/Diels-Alder reaction of 2-(1-alkynyl)-2-alken-1-ones and enals. The in situ generation of the furan-based ortho-quinodimethane intermediates and the iminium activation of enals are crucial to this transformation.

Gold-Catalyzed Cascade Cyclization and 1,3-Difunctionalization To Access Polysubstituted Furans

Gold-catalyzed cascade cyclization and 1,3-difunctionalization of 2-(1-alkyny)-2-alken-1-ones with N,O-acetals are described, leading to the discovery of novel 1,3-oxyaminomethylation and 1,3-aminomethylamination. Typically, by varying the reaction conditions especially the gold catalysts, these two distinct reaction pathways can be controlled in a selective manner. Moreover, tandem cyclization and 1,4-oxyaminomethylation have also been achieved.

Silver(i)-catalyzed tandem reaction of enynones and 4-alkynyl isoxazoles: regioselective synthesis of highly functionalized 4H-furan[3,4-c]pyrroles

This work reports a silver(i)-catalyzed tandem reaction of enynones with 4-alkynyl isoxazoles.

Gold Catalysis and Furans: A Powerful Match for Synthetic Connections

This review summarizes the advances made on the synthesis and functionalization of furans via gold catalysis during the period between 2016 and 2020. A separate section is dedicated to the tandem gold-catalyzed synthesis and functionalization of furans.

1 Introduction

2 Gold-Catalyzed Synthesis of Furans

2.1 Cycloisomerizations of Alkynyl and Cumulenyl Alcohols

2.2 Cycloisomerizations of Alkynyl and Allenyl Ketones

2.3 Reactions with External Oxidants

2.4 Miscellaneous

3 Gold-Catalyzed Functionalization of Furans

3.1 Cycloadditions

3.2 Furan Ring Decorations

3.3 Reactions Involving Furan Ring Opening

4 Gold-Catalyzed Tandem Synthesis and Functionalization of Furans­

4.1 Cycloisomerizations Followed by Gold-Catalyzed Cycloaddition

4.2 Cycloisomerizations to a Gold 1,3- or 1,4-Dipole and Intermolecular Annulation

4.3 Cycloisomerizations to a Gold Carbene and Intermolecular Trapping

5 Conclusion

Transition-Metal-Catalyzed Cycloaddition Reactions to Access Seven-Membered Rings

The efficient and selective synthesis of functionalized seven-membered rings remains an important pursuit within synthetic organic chemistry, as this motif appears in numerous drug-like molecules and natural products. Use of cycloaddition reactions remains an attractive approach for their construction within the perspective of atom and step economy. Additionally, the ability to combine multiple components in a single reaction has the potential to allow for efficient combinatorial strategies of diversity-oriented synthesis. The inherent entropic penalty associated with achieving these transformations has impressively been overcome with development of catalysis, whereby the reaction components can be pre-organized through activation by transition-metal-catalysis. The fine-tuning of metal/ligand combinations as well as reaction conditions allows for achieving chemo-, regio-, diastereo- and enantioselectivity in these transformations. Herein, we discuss recent advances in transition-metal-catalyzed construction of seven-membered rings via combination of 2-4 components mediated by a variety of metals. An emphasis is placed on the mechanistic aspects of these transformations to both illustrate the state of the science and to highlight the unique application of novel processes of transition-metals in these transformations.

Yne-Enones Enable Diversity-Oriented Catalytic Cascade Reactions: A Rapid Assembly of Complexity

A small-molecule collection with structural diversity and complexity is a prerequisite to using either drug candidates or chemical probes for drug discovery and chemical-biology investigations, respectively. Over the past 12 years, we have engaged in developing efficient diversity-oriented cascade strategies for the synthesis of topologically diverse skeletons incorporating biologically relevant structural motifs such as O- and N-heterocycles, fused polycycles, and multifunctionalized allenes. In particular, we have highlighted the use of simple, linear, and densely functionalized molecular platforms in these reactions.This account details our efforts in the design of novel molecular platforms for use in metal- and organo-catalyzed cascade reactions, which include 2-(1-alknyl)-2-alken-1-ones (yne-enones) for heterocyclization/cross-coupling cascades, heterocyclization/cycloaddition cascades, nucleophilic addition/cross-coupling cascades, nucleophilic addition/heterocyclization cascades, and so on. Moreover, this Account outlines corresponding mechanistic insights, computational information, and applications of these cascades in the construction of various highly substituted carbo- and heterocycles as well as highly functionalized acyclic compounds, e.g., allenes and dienes. In addition to yne-enones, we evolved the functional groups of our original yne-enones to provide a series of yne-enone variants, which resulted in products with complementary reactivities.The reactivity profile of the yne-enones is defined by the presence of an alkyne moiety and a conjugated enone unit and their mutual through-bond connectivity. Owing to the conceptually rapid development of carbophilic activation, we have identified a series of efficient catalytic systems consisting of metal catalysts, including Pd, Au, and Rh complexes, for diversity-oriented cascade catalysis, allowing various unprecedented reactions to be achieved through different-types of reaction intermediates, including all-carbon metal 1,n-dipoles, furan-based o-quinodimethanes (oQDMs), and allenyl-metal species. In addition to commonly known transition-metal catalytic activity, the Lewis acidity of these complexes is crucial to accomplish the corresponding transformation. In addition, highly enantioselective gold(I)-catalyzed heterocyclization/cycloaddition cascades of yne-enones and their variants were achieved by the application of bisphosphines (e.g., Cn-TunePhos), monophosphines, and our developed "Ming-Phos" as chiral ligands. Importantly, Ming-Phos ligands exhibited excellent performance in gold-catalyzed mechanistically distinct [3 + n]-cycloaddition reactions, in which the chiral sulfinamide moiety is possibly responsible for the interaction with the substrate to control enantioselectivity. Subsequently, we demonstrated that the easily prepared polymer-supported Ming-Phos ligand could be applied for heterogeneously gold(I)-catalyzed asymmetric cycloaddition with good stereocontrol. With metal-free catalysis, the divergent functionalization of yne-enones provides numerous synthetic outlets for structure diversification. For example, yne-enones are particularly attractive for use as precursors of various chiral and achiral heterocycles, such as pyrazoles, isoxazoles, pyrroles, and pyrans, etc.

Synthesis of 3,4-Dihydro-2H-1,3-thiazines from α-Enolic Dithioesters and 1,3,5-Triazinanes via a Formal (3 + 3) Annulation Reaction

The synthesis of 3,4-dihydro-2H-1,3-thiazines from α-enolic dithioesters and 1,3,5-triazinanes has been achieved via a formal (3 + 3) annulation reaction under thermal conditions, where 1,3,5-triazinanes were utilized as three-atom synthons. This transformation is catalyst-free and additive-free.

Recent Advances of 1,3,5-Triazinanes in Aminomethylation and Cycloaddition Reactions

1,3,5-Trisubstituted 1,3,5-triazinanes (hexahydro-1,3,5-triazines), as stable and readily available surrogates for formaldimines, have found extensive applications for the construction of various nitrogen-containing compounds. The formaldimines, formed in situ from this reagent class, can participate in various aminomethylation and cycloaddition­ reactions. This short review presents recent advances in this field with emphasis on the conceptual ideas behind the developed methodologies and the reaction mechanisms.

1 Introduction

2 Aminomethylations with 1,3,5-Triazinanes

3 Cycloadditions with 1,3,5-Triazinanes

3.1 Use of 1,3,5-Triazinanes as Two-Atom Synthons

3.2 Use of 1,3,5-Triazinanes as Three-Atom Synthons

3.3 Use of 1,3,5-Triazinanes as Four-Atom Synthons

3.4 Use of 1,3,5-Triazinanes as Six-Atom Synthons

4 Conclusions

Intramolecular sp2-sp3 Disequalization of Chemically Identical Sulfonamide Nitrogen Atoms: Single Crystal X-Ray Diffraction Characterization, Hirshfeld Surface Analysis and DFT Calculations of N-Substituted Hexahydro-1,3,5-Triazines

In this manuscript, the synthesis and single crystal X-ray diffraction characterization of four N-substituted 1,3,5-triazinanes are reported along with a detailed analysis of the noncovalent interactions observed in the solid state architecture to these compounds, focusing on C–H···π and C–H···O H-bonding interactions. These noncovalent contacts have been characterized energetically by using DFT calculations and also by Hirshfeld surface analysis. In addition, the supramolecular assemblies have been characterized using the quantum theory of “atoms-in-molecules” (QTAIM) and molecular electrostatic potential (MEP) calculations. The XRD analysis revealed a never before observed feature of the crystalline structure of some molecules: symmetrically substituted 1,3,5-triazacyclohexanes possess two chemically identical sulfonamide nitrogen atoms in different sp2 and sp3-hybridizations.

Inverse-electron-demand [4+2] cycloaddition of photogenerated aza-ortho-quinone methides with 1,3,5-triazinanes: access to perfluoroalkylated tetrahydroquinazolines

A [4+2] cycloaddition of photogenerated aza-ortho-quinone methides with formaldimines derived from 1,3,5-triazinanes is described.

Copper-Catalyzed Amino-oxymethylation of Ynamides with N,O-Acetals

A copper-catalyzed 1,2-difunctionalization of ynamides has been developed by using readily available N,O-acetals as the bifunctional reagents. Importantly, contrary to the initially expected 1,2-oxy-aminomethylation reaction, an unprecedented 1,2-amino-oxymethylation has been observed. This reaction is atom-economical and tolerates a broad substrate scope, affording the difunctionalization products in good yield with unique Z configuration.

Cascade Radical Cyclization on Alkynyl Vinylogous Carbonates for the Divergent Synthesis of Tetrasubstituted Furans and Dihydrofurans

A single alkynyl vinylogous carbonate was elaborated to tetrasubstituted furan or dihydrofuran via a cascade inter-intramolecular radical reaction by changing the radical being added. The strategy could be used in the synthesis of polycyclic heterocycles as well as bis-furan exhibiting atropisomerism. Installation of a new furan motif on the existing one was feasible by iteration. Stannyl dihydrofuran derivative was used in Stille coupling, whereas intramolecular Friedel-Crafts acylation on the furan gave furanonaphthol.

Rhodium(ii)-catalyzed annulation of N-sulfonyl-1,2,3-triazoles with 1,3,5-triazinanes to produce octahydro-1H-purine derivatives: a combined experimental and computational study

The Rh(ii)-catalyzed annulation of N-sulfonyl-1,2,3-triazoles with 1,3,5-triazinanes leading to the octahydro-1H-purine derivatives with moderate to good yields is described in this communication.

Preparation of spiro[imidazolidine-4,3′-indolin]-2′-imines via copper(i)-catalyzed formal [2 + 2 + 1] cycloaddition of 3-diazoindolin-2-imines and triazines

We report a facile and efficient synthesis of spiro[imidazolidine-4,3′-indolin]-2′-imines via a copper(i)-catalyzed cascade reaction of 3-diazoindolin-2-imines with 1,3,5-triazines.