Gold-Catalyzed Formal [4+1]/[4+3] Cycloadditions of Diazo Esters with Triazines

Reaction of 1,3,5-Triazinanes with Phosphoryl Diazomethanes: Access to 5-Phosphoryl-1,2,3,4-tetrahydropyrimidines

Conversion of 1,3,5-triazinanes into 5-phosporyl-1,2,3,4-tetrahydropyrimidines is achieved efficiently through the microwave-assisted reaction with phosphoryl diazomethanes. Both trialkyl and triaryl 1,3,5-triazinanes were converted by diazomethyldiarylphosphine oxides, dialkyl diazomethylphosphonates, and alkyl diazomethyl(aryl)phosphinates and functionalized simultaneously in good to excellent yields. The reaction is a sequence of 1,3,5-triazinane fragmentation, tandem nucleophilic addition of the generated formaldimines and phosphoryl diazomethanes, and final N,N-acetalization.

Catalyst-free inverse-electron-demand aza-Diels-Alder reaction of 4,4-dicyano-2-methylenebut-3-enoates and 1,3,5-triazinanes: access to polysubstituted tetrahydropyridines

An inverse-electron-demand aza-Diels-Alder reaction between 4,4-dicyano-2-methylenebut-3-enoates and 1,3,5-triazinanes under catalyst-free and additive-free conditions was developed, which provided a highly convenient and straightforward method to construct a series of polyfunctionalized tetrahydropyridines in high yields. This strategy features numerous advantages, including high efficiency, good functional group tolerance, broad substrate scope, and environmentally friendly conditions.

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.

Access to 3-Aminomethylated Maleimides via a Phosphine-Catalyzed Aza-Morita-Baylis-Hillman Type Coupling

A designed method for the preparation of 3-aminomethylated maleimides via Morita-Baylis-Hillman (MBH) reaction was developed. This phosphine-catalyzed coupling adopted maleimides and 1,3,5-triazinanes as the substrate, giving a series of 3-aminomethylated maleimide derivatives with a double bond retained on the maleimide ring in 41-90% yield. Acylation, isomerization, and Michael addition of the obtained products demonstrated the synthetic application of the present protocol. The results of control experiments indicated that phosphorus ylide formation and elimination take place during the reaction pathway.

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.

Enantioselective [3+2] Cycloaddition of Donor-Acceptor Aziridines and Imines to Construct 2,5-trans-Imidazolidines

An enantioselective [3+2] cycloaddition of donor-acceptor aziridines with N-aryl protected imines was developed with a Ni(ClO₄ )₂  ⋅ 6H₂ O/N,N'-dioxide catalyst system, providing a broad range of chiral trans-substituted imidazolidine compounds with good yields and excellent enantioselectivities (up to 99 % yield, up to 98 % ee). Control experiments indicated that the products could offer excellent diastereoselectivities with the control of chiral Ni(II)-N,N'-dioxide complex and the interaction of the substrates. The possible catalytic process was proposed to rationalize the stereocontrol.

Copper-Catalyzed Cycloadditions of Diazo Compounds with Imidazolidines/Hexahydropyrimidines for the Syntheses of N-Heterocycles

Reported herein are the unprecedented copper-catalyzed formal [n + 1]/[n + 3] (n = 5, 6) cycloadditions of diazo compounds with imidazolidines/hexahydropyrimidines, thus providing a general, economical, and efficient route to construct different sized (six- to nine-membered) diaza-heterocycles in moderate to excellent yields under mild reaction conditions. This strategy features the use of copper catalyst to accomplish such diverse annulations and the utilization of imidazolidines/hexahydropyrimidines as stable 1,5-/1,6-dipoles.

Enyne diketones as substrate in asymmetric Nazarov cyclization for construction of chiral allene cyclopentenones

The Nazarov cyclization is one of the most powerful tools for the stereoselective synthesis of various cyclopentenone scaffolds. Therefore, developing the new classes substrate of Nazarov reaction is an important endeavor in synthetic chemistry. Herein, we report enyne diketones, enables diastereo- and enantioselective construction of chiral allene cyclopentenones in moderate to good yields with good enantioselectivities (up to 97% ee). Importantly, it is a typical example for asymmetric synthesis of cyclopentanones with allene moiety using Nazarov cyclisation. Mechanistic studies indicate that this metal-organo relay catalysis protocol involves a rhodium-catalyzed tandem oxonium ylide formation/[2,3]-sigmatropic rearrangement/reverse benzylic acid rearrangement, followed by organo-catalyzed asymmetric Nazarov cyclization/alkyne-to-allene isomerization to give the final chiral allene cyclopentenones.

Non-globular organic ionic plastic crystal containing crown-ether moiety - Tuning its behaviour using sodium salts

Organic ionic plastic crystals (OIPCs) are a class of soft materials showing positional order while still allowing orientational freedom. Due to their motional freedom in the solid state, they possess plasticity, non‐flammability and high ionic conductivity. OIPC behavior is typically exhibited by ‘simple’ globular molecules allowing molecular rotation, whereas the interactions that govern the formation of OIPC phases in complex non‐globular molecules are less understood. To better understand these interactions, a new family of non‐globular OIPCs containing a 15‐crown‐5 ether moiety was synthetized and characterized. The 15C5BA molecule prepared does not exhibit the sought‐after behavior because of its non‐globular nature and strong intermolecular H‐bonds that restrict orientational motion. However, the OIPC behavior was successfully obtained through complexation of the crown‐ether moiety with sodium salts containing chaotropic anions. Those anions weaken the interactions between the molecules, allowing rotational freedom and tuning of the thermal and morphological properties of the OIPC.

High-order dipolar annulations with metal-containing reactive dipoles

Medium-sized heterocycles are widespread among a spectrum of structurally intriguing and biologically significant natural products and synthetic pharmaceuticals. Metal-catalyzed high-order dipolar annulations resembling reactions of metal-containing reactive dipoles with dipolarophiles constitute a highly efficient and flexible strategy for constructing medium-sized heterocycles. Mechanistically, these annulation reactions usually proceeding through stepwise pathways are different from the classic high-order pericyclic reactions that follow the Woodward-Hoffman rules. More significantly, asymmetric high-order dipolar annulations using chiral organometallic catalysts have been proven successful for constructing chiral medium-sized heterocycles with high enantio- and diastereoselectivity. This review highlights the impressive advances in this area and is focused on the reactivity, scope, mechanisms and applications of high-order dipolar annulation reactions.

Metal-free [2 + 1 + 3] Cycloaddition of Trifluoroacetaldehyde N-Sulfonylhydrazones with Hexahydro-1,3,5-triazines Leading to Trifluoromethylated 2,3,4,5-Tetrahydro-1,2,4-triazines

A transition-metal-free [2 + 1 + 3] cycloaddition of trifluoroacetaldehyde N-sulfonylhydrazone and hexahydro-1,3,5-triazine was described. This operationally simple protocol provides a general synthesis of diverse trifluoromethylated 2,3,4,5-tetrahydro-1,2,4-triazines in 81-97% yield with a broad substrate scope, including aryl, benzyl, and alkyl hexahydro-1,3,5-triazine.

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.

Diastereoselective formation of β-lactams via a three-component reaction

A highly diastereoselective three-component reaction of N-hydroxyanilines, diazo compounds and cyclobutenones to form densely functionalized β-lactams has been realized.

Enantioselective formal [2 + 2 + 2] cycloaddition of 1,3,5-triazinanes to construct tetrahydropyrimidin-4-one derivatives

A chiral Lewis acid-catalyzed enantioselective formal [2 + 2 + 2] cycloaddition of 1,3,5-triazinanes with azlactones or β,γ-unsaturated pyrazole amides was developed to synthesize chiral tertiary/quaternary tetrahydropyrimidin-4-one derivatives with good yields and enantioselectivities. Two competitive reaction pathways were proposed based on experiments.

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.

Brønsted Acid Catalyzed Oxocarbenium-Olefin Metathesis/Rearrangements of 1H-Isochromene Acetals with Vinyl Diazo Compounds

An oxocarbenium-olefin cross metathesis occurs during Brønsted acid catalyzed reactions of 1H-isochromene acetals with vinyl diazo compounds. Formally a carbonyl-alkene [2 + 2]-cyclization between isobenzopyrylium ions and the vinyl group of vinyl diazoesters, the retro-[2 + 2] cycloaddition produces a tethered alkene and a vinyl diazonium ion that, upon loss of dinitrogen, undergoes a highly selective carbocationic cascade rearrangements to diverse products whose formation is controlled by reactant substituents. Polysubstituted benzobicyclo[3.3.1]oxocines, benzobicyclo[3.2.2]oxepines, benzobicyclopropane, and naphthalenes are obtained in good to excellent yields and selectivities. Furthermore, isotopic tracer and control experiments shed light on the oxocarbenium-olefin metathesis/rearrangement process as well as on the origin of the interesting substituent-dependent selectivity.

Relieving the stress together: annulation of two different strained rings towards the formation of biologically significant heterocyclic scaffolds

Small carbo- and heterocycles have become versatile building blocks owing to their intrinsic ring strain and ease of synthesis. However, the traditional approaches of heterocycle synthesis involved the combination of one strained-carbocycle or heterocycle with one unsaturated molecule. On the contrary, there is an exciting possibility of combining two different strained rings to furnish varieties of heterocycles, where one of the strained rings can act as a valuable alternative to the unsaturated molecule. These strategies are also useful to access multi-functionalized rings. Despite these distinctive synthetic benefits, this chemistry has not drawn considerable attention of the community. In this minireview, we explicitly choose this topic to reveal the unexplored possibilities with these different strained rings. This minireview provides comprehensive details with the mechanistic rationale about the reactivity of these pairs of small rings when they are allowed to react together in the presence of different Lewis acids. Subsequently, it will also open a new avenue for heterocycle synthesis.

Challenges in the Highly Selective [3 + 1]-Cycloaddition of an Enoldiazoacetamide to Form a Donor-Acceptor Cis-Cyclobutenecarboxamide

A substituted donor-acceptor cyclobutenecarboxamide is synthesized with modest enantiocontrol through a chiral copper(I) complex catalyzed [3 + 1]-cycloaddition reaction of α-acyl diphenylsulfur ylides with 3-siloxy-2-diazo-3-butenamides. With a methyl substituent on the 4-position of the 3-butenamide, the cis-vicinal-3,4-disubstituted cyclobutenecarboxamide is formed with >20:1 diastereocontrol. Donor-acceptor 3-methyl-2-siloxycyclopropenecarboxamide is rapidly formed from the reactant enoldiazoamide and undergoes catalytic ring opening to give only the Z-γ-substituted metallo-enolcarbene. Elimination from 3-siloxy-2-diazo-3-pentenamide to form the conjugated 3-siloxy-2,4-pentadienamide is competitive but minimized at low temperature.

Divergent Synthesis of Aziridine and Imidazolidine Frameworks under Blue LED Irradiation

We develop a visible light-promoted divergent cycloaddition of α-diazo esters with hexahydro-1,3,5-triazines, leading to a series of aziridine and imidazolidine frameworks in average good yield, by simply changing the reaction media used. It is noteworthy that the reaction occurs under sole visible light irradiation without the need for exogenous photoredox catalysts. More significantly, a reasonable reaction mechanism was proposed on the basis of the control experiments and density functional theory calculation results.

Bifunctional reagents in organic synthesis

Developments in synthetic chemistry are increasingly driven by improvements in the selectivity and sustainability of transformations. Bifunctional reagents, either as dual coupling partners or as a coupling partner in combination with an activating species, offer an atom-economic approach to chemical complexity, while suppressing the formation of waste. These reagents are employed in organic synthesis thanks to their ability to form complex organic architectures and empower novel reaction pathways. This Review describes several key bifunctional reagents by showcasing selected cornerstone research areas and examples, including radical reactions, C-H functionalization, cross-coupling, organocatalysis and cyclization reactions.

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.

Catalyst-free synthesis of oxazol-2(3H)-ones from sulfilimines and diazo compounds through a tandem rearrangement/aziridination/ring-expansion reaction

Oxazol-2(3H)-ones play a significant role in the fields of organic synthesis and drug development.

Strain-release enabled [3 + 2] annulation of 3-aminooxetanes with simple CN bonds: facile synthesis of imidazolidines

An unprecedented [3+2] annulation of readily available 1,3,5-triazinanes and 3-aminooxetanes is accomplished for the first time, enabling the convenient synthesis of a range of structurally diverse 4-hydroxymethyl imidazolidines.

Pd/LA-catalyzed decarboxylation enabled exclusive [5 + 2] annulation toward N-aryl azepanes and DFT insights

A practical approach towards the synthesis of non-fused N-aryl azepane derivatives with diversity is described. DFT calculations revealed the origins of this unusual exclusive [5 + 2] rather than empirical [3 + 2] annulation process.

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.

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.

Aziridinium Ylides: Underutilized Intermediates for Complex Amine Synthesis

Harnessing the chemistry of onium ylide intermediates generated from transition metal catalysis is a powerful strategy to convert simple precursors into complex scaffolds. While the chemistry of onium ylides has been studied for over three decades, transformations of aziridinium ylides have just recently emerged as a versatile way to exploit the strain of these reactive intermediates to furnish densely functionalized N-heterocycles in a highly stereocontrolled manner. Herein, we provide a short overview of the key concepts and recent developments in this area, with a focus on how mechanistic studies to delineate the factors controlling the reactivity of aziridinium ylides can stimulate fruitful future investigations.

Gold-Catalyzed Intermolecular Formal [4 + 2 + 2]-Cycloaddition of Anthranils with Allenamides

The construction of eight-membered rings is a challenging issue due to unfavorable transannular strain and entropic barriers. We report herein a gold-catalyzed formal [4 + 2 + 2] cycloaddition reaction of anthranils with allenamides to deliver oxa-bridged eight-membered heterocycles in accepted yields with unique E/Z configuration. Moreover, the asymmetric [4 + 2 + 2] cycloaddition by using chiral phosphoramidite gold catalyst has also been conducted.

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

Synthesis, structure, and antitumor activity of 2,9-disubstituted perhydro 2,3a,7b,9,10a,14b-hexaazadibenzotetracenes

Catalytic methods for the synthesis of previously unknown 2,9-disubstituted 3bR*,7aR*,10bR*,14aR*-cis-14c,14d-perhydro-2,3a,7b,9,10a,14b-hexaazadibenzotetracenes have been developed. The structures were established by 1D (¹H, ¹³C) and 2D (COSY, HSQC, HMBC) NMR spectroscopy, MALDI TOF/TOF mass spectrometry, and X-ray diffraction analysis. Primary screening of the synthesized perhydro hexaazadibenzotetracenes for antitumor activity was carried out.

Catalytic methods for the synthesis of previously unknown 2,9-disubstituted 3bR*,7aR*,10bR*,14aR*-cis-14c,14d-perhydro-2,3a,7b,9,10a,14b-hexaazadibenzotetracenes with pronounced antitumor activity have been developed.

Gold-Catalyzed [3+2] Carbocycloaddition Reaction of Pinacol Alkenylboronates: Stereospecific Synthesis of Boryl-Functionalized Cyclopentene Derivatives

Gold-catalysis has enabled new synthetic opportunities in the chemistry of vinyldiazo compounds. Herein, we report the gold-catalyzed reaction of stabilized vinyldiazo compounds with pinacol alkenylboronates to provide boryl-functionalized cyclopentene derivatives through a formal [3+2] carbocycloaddition reaction, a very unusual pathway in alkenylboronate chemistry. This reaction proceeds with high regio- and stereoselectivity. The synthetic usefulness of the resulting borylated cyclopentene derivatives toward the synthesis of densely functionalized cyclopentanoids is also demonstrated.

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.

Recent Advances in Transition-Metal-Catalyzed (4+3)-Cycloadditions

A (4+3)-cycloaddition combines a four-atom synthon and three-atom synthon to form seven-membered rings. In the past decade, many improvements have been made to this class of cycloaddition, including excellent diastereo- and enantioselectivities, both intra- and intermolecularly. Through the strategic use of transition-metal catalysts, acids, bases, and organocatalysts, it is possible to perform the cycloaddition on a variety of substrates, generating novel seven-membered rings. With these advances, (4+3)-cycloaddition has also been applied to the synthesis of biologically relevant compounds and natural products. We exclude the cycloadditions of cyclic dienes such as furan, pyrrole, cyclohexadiene or cyclopentadiene as Chiu, Harmata, Mascareñas­ and others have recently published thorough reviews on that topic. We will however discuss the recent additions (2009–2020) to the literature for the (4+3)-cycloadditions involving other types of four-atom synthons.

1 Introduction

2 Rhodium

2.1 Cyclopropanation/Cope Rearrangement

2.2 C–H activation

3 Gold, Silver

4 Copper

5 Palladium, Platinum, Iridium

6 Dual-Activation

7 Conclusion