Regioselectivity in the nucleophile trapping of arynes: the electronic and steric effects of nucleophiles and substituents

Ferrocenyl conjugated oxazepines/quinolines: multiyne coupling and ring-expanding or rearrangement

Ferrocenyl conjugated oxazepine/quinoline derivatives were presented through the reaction of hexadehydro-Diels-Alder (HDDA) generated arynes with ferrocenyl oxazolines under mild conditions via ring-expanding or rearrangement processes. Water molecule participated in this unexpected rearrangement process to produce quinoline skeletons, and DFT calculations supported a ring-expanding and intramolecular hydrogen migration process for the formation of oxazepine derivatives. Two variants of this chemistry, expanded the reactivity between ferrocenyl conjugated substances and arynes, further providing an innovative approach for the synthesis of ferrocene derivatives.

1,3-Butadiynamides the Ethynylogous Ynamides: Synthesis, Properties and Applications in Heterocyclic Chemistry

1,3-butadiynamides-the ethynylogous variants of ynamides-receive considerable attention as precursors of complex molecular scaffolds for organic and heterocyclic chemistry. The synthetic potential of these C4-building blocks reveals itself in sophisticated transition-metal catalyzed annulation reactions and in metal-free or silver-mediated HDDA (Hexa-dehydro-Diels-Alder) cycloadditions. 1,3-Butadiynamides also gain significance as optoelectronic materials and in less explored views on their unique helical twisted frontier molecular orbitals (Hel-FMOs). The present account summarizes different methodologies for the synthesis of 1,3-butadiynamides followed by the description of their molecular structure and electronic properties. Finally, the surprisingly rich chemistry of 1,3-butadiynamides as versatile C4-building blocks in heterocyclic chemistry is reviewed by compiling their exciting reactivity, specificity and opportunities for organic synthesis. Besides chemical transformations and use in synthesis, a focus is set on the mechanistic understanding of the chemistry of 1,3-butadiynamides-suggesting that 1,3-butadiynamides are not just simple alkynes. These ethynylogous variants of ynamides have their own molecular character and chemical reactivity and reflect a new class of remarkably useful compounds.

Development of an Allenyne-Alkyne [4+2] Cycloaddition and its Application to Total Synthesis of Selaginpulvilin A

A new [4+2] cycloaddition of allenyne-alkyne is developed. The reaction is believed to proceed with forming an α,3-dehydrotoluene intermediate. This species behaves as a σπ-diradical to react with a hydrogen atom donor, whereas it displays a zwitterionic reactivity toward weak nucleophiles. The efficiency of trapping α,3-dehydrotoluene depends not only on its substituents but also the trapping agents. Notable features of the reaction are the activating role of the extra alkyne of the 1,3-diyne that reacts with the allenyne moiety and the opposite mode of trapping with oxygen and nitrogen nucleophiles. Oxygen nucleophiles result in the oxygen-end incorporation at the benzylic position of the α,3-dehydrotoluene, whereas with amine nucleophiles the nitrogen-end is incorporated into the aromatic core. Relying on the allenyne-alkyne cycloaddition as an enabling strategy, a concise total synthesis of phosphodiesterase-4 inhibitory selaginpulvilin A is realized.

Hexadehydro-Diels-Alder Reaction: Benzyne Generation via Cycloisomerization of Tethered Triynes

The hexadehydro-Diels-Alder (HDDA) reaction is the thermal cyclization of an alkyne and a 1,3-diyne to generate a benzyne intermediate. This is then rapidly trapped, in situ, by a variety of species to yield highly functionalized benzenoid products. In contrast to nearly all other methods of aryne generation, no other reagents are required to produce an HDDA benzyne. The versatile and customizable nature of the process has attracted much attention due not only to its synthetic potential but also because of the fundamental mechanistic insights the studies often afford. The authors have attempted to provide here a comprehensive compilation of publications appearing by mid-2020 that describe experimental results of HDDA reactions.

Access to Benzoxazepines and Fully Substituted Indoles via HDDA Coupling

The HDDA-derived benzyne intermediate was captured by oxazolines based on the addition reaction of benzyne to the C═N double bond. Benzoxazepine derivatives, fused benzoxazepine derivatives, and fully substituted indoles are synthesized in one step. The reaction does not require any catalyst or additives. Possible reaction mechanisms are presented.

The present and future synthetic strategies of structural modifications of sinomenine

This review summarizes the modifications of sinomenine, a hot compound derived from herbal plants, which possesses diverse biological activities and low cytotoxicity.

Synthesis of Imides, Imidates, Amidines, and Amides by Intercepting the Aryne-Isocyanide Adduct with Weak Nucleophiles

New aryne-based multicomponent coupling reactions for the formation of functionalized aromatic compounds have been developed. Arynes generated from triynes or tetraynes through the hexadehydro Diels-Alder reaction readily react with isocyanide to generate nitrilium intermediate. Intercepting this nitrilium species with various weak nucleophile including carboxylic acids, alcohols, sulfonamides, or water generated the corresponding imides, imidates, amidines, or amides. The high regioselectivity of these transformations was mainly controlled by the substituents of the arynes.

Reactions of Diaziridines with Benzynes Give N-Arylhydrazones

Reactions of thermally generated benzynes with diaziridines are reported. These trapping reactions follow the same pathway as reported earlier by Heine and co-workers with electron-deficient alkynes. The resulting N-arylhydrazones were obtained efficiently in a single step. The preference for the mode of addition of the nucleophilic diaziridine nitrogen atom to the more electrophilic benzyne carbon was consistent with what is predicted on the basis of distortion analysis. The feasibility of converting the hydrazone into a Fisher-indole adduct was demonstrated.

CuI -Mediated Bromoalkynylation and Hydroalkynylation Reactions of Unsymmetrical Benzynes: Complementary Modes of Addition

Benzynes formed by heating a suitable triyne (or tetrayne) substrate are shown to react with in situ generated alkynyl copper species. The latter are compatible with the polyyne substrates and two types of chemistries have been achieved: (i) 1-bromo-1-alkynes efficiently undergo net bromoalkynylation of the (unsymmetrical) benzynes and (ii) in situ generated alkynylcopper species give rise to hydroalkynylation products. The regiochemical preferences of these two modes of reaction are complementary to one another with respect to the position of alkynyl substituent in the final products.

Multiheterocyclic Motifs via Three-Component Reactions of Benzynes, Cyclic Amines, and Protic Nucleophiles

A broadly general, three-component reaction strategy for the construction of compounds containing multiple heterocycles is described. Thermal benzyne formation (by the hexadehydro-Diels-Alder (HDDA) reaction) in the presence of tertiary cyclic amines and a protic nucleophile (HNu) gives, via ring-opening of intermediate ammonium ion/Nu- ion pairs, heterocyclic products. Many reactions are efficient even when the stoichiometric loading of the three reactants approaches unity. Use of HOSO2CF3 as the HNu gives ammonium triflate intermediates, which can then be ring opened by an even wider variety of nucleophiles.

Reactivity of arynes toward functionalized alkenes: intermolecular Alder-ene vs. addition reactions

The selectivity between two different manifolds of reactions of arynes reacting with functionalized alkenes is described.

Mechanistic Duality in Tertiary Amine Additions to Thermally Generated Hexadehydro-Diels-Alder Benzynes

Reported here are studies directed at understanding the mechanism of tertiary amine addition to hexadehydro-Diels-Alder (HDDA)-generated benzynes. Tertiary amines are presumed to engage benzynes by generation of a zwitterionic intermediate. Simple trialkylamines undergo intermolecular protonation by a protic nucleophile to give an aryl ammonium intermediate that is then dealkylated. Amines containing acidified β-protons undergo an intramolecular elimination to give the aniline and an alkene. Finally, amino alcohols react at either of their N- or O atoms, depending upon the extent of internal hydrogen bonding.

Fused Multifunctionalized Chromenes from Tetraynes and α,β-Unsaturated Aldehydes

The synthesis of fused, highly substituted chromenes in a one-pot-three-step cascade hexadehydro-Diels-Alder reaction of tetraynes followed by an intermolecular α,β-unsaturated aldehyde rearrangement is reported. The target compounds were prepared by the condensation of malonates with 3-bromo-1-propyne, and the resulting 2,2-di(1-propyn-3-yl)malonates underwent cross-coupling with phenylethynyl bromides to afford 2,2-di(5-phenyl-2,4-pentadiynyl)malonates, which underwent intramolecular cyclization to produce tricyclic chromenes. The overall transformation involves the formation of four new C-C bonds and one new C-aryl-O-C-aryl bond by both intramolecular cyclization and intermolecular rearrangement reactions. These constitute a key strategy for the construction of a highly substituted natural chromene core and a robust method for producing other chromenes.

Direct Access to Fused Salicylaldehydes and Salicylketones from Tetraynes

A facile method for the synthesis of fused, multifunctionalized salicylaldehydes and salicylketones by a one-pot, three-step cascade hexadehydro-Diels-Alder (HDDA) reaction of tetraynes followed by an intermolecular aldehyde/ketone reaction and hydroxylation is reported. Target compounds were prepared by the condensation of malonates with 3-bromo-1-propyne, and the resulting 2,2-di(1-propyn-3-yl)malonates underwent cross-coupling with phenylethynyl bromides to afford 2,2-di(5-phenyl-2,4-pentadiynyl)malonates, which underwent intramolecular cyclization to produce bicyclic salicylaldehydes. The overall transformation involves the formation of four new C-C bonds and one new C_aryl -O bond through both intramolecular and intermolecular reactions. The reaction is easy to perform, proceeds under mild conditions, and exhibits excellent regioselectivity. Water, a simple and readily available reagent, was employed as the OH source. A plausible reaction mechanism is proposed for this new annulation based on isotopic substitution experiments.

Reactions of hexadehydro-Diels-Alder benzynes with structurally complex multifunctional natural products

An important question in organic chemistry concerns the extent to which benzynes-one of the classical reactive intermediates in organic chemistry-can react in discriminating fashion with trapping reagents. In particular, whether these species can react selectively with substrates containing multiple functional groups and possible sites of reactivity has remained unanswered. Natural products comprise a palette of multifunctional compounds with which to address this question. Here, we show that benzynes produced by the hexadehydro-Diels-Alder (HDDA) reaction react with many secondary metabolites with a preference for one among several pathways. Examples demonstrating such selectivity include reactions with: phenolics, through dearomatizing ortho-substitution; alkaloids, through Hofmann-type elimination; tropolone and furan, through cycloaddition; and alkaloids, through three-component fragmentation-coupling reactions. We also demonstrate that the cinchona alkaloids quinidine and quinine give rise to products (some in as few as three steps) that enable subsequent and rapid access to structurally diverse polyheterocyclic compounds. The results show that benzynes are quite discriminating in their reactivity-a trait perhaps not broadly enough appreciated.

Recent advances in fluoride-free aryne generation from arene precursors

Aryne chemistry has flourished in the past few decades. This review highlights new aryne precursors that operate under fluoride-free conditions as alternative methodologies to the popular fluoride-mediated ortho-silylaryl triflates.

Methodology and applications of the hexadehydro-Diels–Alder (HDDA) reaction

Hexadehydro-Diels–Alder (HDDA) reactions between alkynes and 1,3-diynes readily generate highly reactive and synthetically useful arynes.

Fused multifunctionalized isoindole-1,3-diones via the coupled oxidation of imidazoles and tetraynes

The facile synthesis of fused multifunctionalized isoindole-1,3-diones by a hexadehydro-Diels–Alder domino reaction of various substituted tetraynes and imidazole derivatives is reported.

Hexadehydro-Diels-Alder (HDDA)-Enabled Carbazolyne Chemistry: Single Step, de Novo Construction of the Pyranocarbazole Core of Alkaloids of the Murraya koenigii (Curry Tree) Family

Here we report the use of the hexadehydro-Diels-Alder (HDDA) reaction for the de novo construction of a benzenoid ring in fused polycyclic heteroaromatic carbazole (i.e., [2,3]-benzoindole) skeletons. The strategy allows creation of highly substituted benzenoids. We also describe the HDDA-enabled chemical synthesis of the natural product alkaloids mahanimbine and koenidine. Trapping of the intermediate carbazolyne with a conjugated enal, proceeding through formal [2+2] cycloaddition, 4π-electrocyclic ring opening, and 6π-electrocyclic ring-closing events, constitutes a robust method for producing pyranocarbazoles.

The pentadehydro-Diels-Alder reaction

In the classic Diels–Alder (DA) [4+2] cycloaddition reaction¹, the overall degree of unsaturation of the 4π (diene) and 2π (dienophile) pairs of reactants dictates the oxidation state of the newly formed six-membered carbocycle. For example, in the classic DA reaction, butadiene and ethylene combine to produce cyclohexene. More recent developments include variants in which the hydrogen atom count in the reactant pair and in the resulting product is reduced by², for example, four in the tetradehydro-DA (TDDA) and by six in the hexadehydro-DA (HDDA^3,4,5,6,7) reactions. Any oxidation state higher than tetradehydro leads to the production of a reactive intermediate that is more highly oxidized than benzene. This significantly increases the power of the overall process because trapping of the benzyne intermediate^8,9 can be used to increase the structural complexity of the final product in a controllable and versatile manner. In this manuscript, we report an unprecedented net 4π+2π cycloaddition reaction that generates a different, highly reactive intermediate known as an α,3-dehydrotoluene. This species is at the same oxidation state as a benzyne. Like benzynes, α,3-dehydrotoluenes can be captured by various trapping agents to produce structurally diverse products that are complementary to those arising from the HDDA process. We call this new cycloisomerization reaction a pentadehydro-Diels–Alder (PDDA) reaction—a nomenclature chosen for chemical taxonomic rather than mechanistic reasons. In addition to alkynes, nitriles (RC≡N), although non-participants in aza-HDDA reactions, readily function as the 2π-component in PDDA cyclizations to produce, via trapping of the α,3-(5-aza)dehydrotoluene intermediates, pyridine-containing products.

Reactions of HDDA-Derived Benzynes with Sulfides: Mechanism, Modes, and Three-Component Reactions

We report here reactions of alkyl sulfides with benzynes thermally generated by the hexadehydro-Diels-Alder (HDDA) cycloisomerization. The initially produced 1,3-betaine (o-sulfonium/aryl carbanion) undergoes intramolecular proton transfer to generate a more stable S-aryl sulfur ylide. This can react in various manners, including engaging weak acids (HA) in the reaction medium. This can produce transient ion pairs ArSR2(+)A(-) that proceed to the products ArSR + RA. When cyclic sulfides are used, A(-) opens the ring and is incorporated into the product, an outcome that constitutes a versatile, three-component coupling process.

N-Arylation of Tertiary Amines under Mild Conditions

A transition-metal-free procedure for the N-arylation of tertiary amines to sp(3) quaternary ammonium salts is described. The presented conditions allow for the isolation of trialkylaryl, dialkyldiaryl, and novel triarylalkyl ammonium salts, including N-chiral quaternary ammonium salts. The reaction works at room temperature, open to air with electron-rich or -poor benzyne precursors and different tertiary amines, allowing the synthesis of a broad range of N-aryl ammonium salts that have applications in a variety of fields.

Hydrohalogenative aromatization of multiynes promoted by ruthenium alkylidene complexes

A new functionalization method of arynes promoted by a novel catalytic role of the Grubbs-type ruthenium alkylidene complex is described.

Reactions of arynes promoted by silver ions

This tutorial review describes all known reactions of arynes promoted by silver-based reagents and catalysts focusing on reactivity and selectivity.

Synthesis of Phenolic Compounds by Trapping Arynes with a Hydroxy Surrogate

Trapping of arynes with various nucleophiles provides a range of heteroatom-functionalized arene derivatives, but the corresponding reaction with water does not provide phenol derivatives. Silver trifluroacetate (AgO₂CCF₃) can nicely solve this problem. It was found that in typical organic solvent, AgO₂CCF₃ readily reacts with arynes to generate trifluoroacetoxy organosilver arene intermediate, which, upon treating with silica gel, provides phenolic products. This protocol can be extended to the synthesis of α-halofunctionalized phenol derivatives by simply adding NBS (N-bromosuccinimides) or NIS (N-iodosuccinimides) to the reaction along with silver trifluroacetate, which provided α-bromo or α-iodophenol derivatives in good yield. However, the similar reactions with NCS (N-chlorosuccinimides) afforded only the protonated product instead of the expected α-chlorophenols derivatives. Interestingly, substrates containing silyl substituents on 1,3-diynes resulted in α-halotrifluoroacetates rather than their hydrolyzed product. Additionally, trapping the same arynes with other oxygen-based nucleophiles containing silver counter cation, along with NXS (N-halosuccinimides), generated α-halooxyfunctionalized products.

Competition between classical and hexadehydro-Diels-Alder (HDDA) reactions of HDDA triynes with furan

We report here thermal reactions between furan and one of three related triyne substrates. Each triyne is capable of reacting initially in two modes: (i) unimolecular hexadehydro-Diels-Alder (HDDA) reaction or (ii) bimolecular Diels-Alder reaction between one of its alkynes with furan. The relative rates of these initial events are such that two of the substrates react essentially in only one of modes (i) or (ii). The third is intermediate in behavior; its bifurcation is dependent on the concentration of the furan reactant. These results teach, more generally, principles relevant to the design of efficient HDDA-based reaction cascades.

Diversity-Oriented Approaches to Polycyclics and Bioinspired Molecules via the Diels-Alder Strategy: Green Chemistry, Synthetic Economy, and Beyond

We describe diverse approaches to various dienes and their utilization in the Diels-Alder reaction to produce a variety of polycycles. The dienes covered here are prepared by simple alkylation reaction or via the Claisen rearrangement or by enyne metathesis of alkyne or enyne building blocks. Here, we have also included the Diels-Alder chemistry of dendralenes, a higher analog of cross-conjugated dienes. The present article is inclusive of o-xylylene derivatives that are generated in situ starting with benzosultine or benzosulfone derivatives. The Diels-Alder reaction of these dienes with various dienophiles gave diverse polycyclic systems and biologically important targets.

Straightforward synthesis of 5-bromopenta-2,4-diynenitrile and its reactivity towards terminal alkynes: a direct access to diene and benzofulvene scaffolds

The high-yielding synthesis of 5-bromopenta-2,4-diynenitrile (BrC5 N) was achieved for the first time. Its reactivity with triisopropylsilylacetylene and triisopropylsilylbutadiyne in the presence of copper and palladium as co-catalysts and diisopropylamine was evaluated. It revealed an unprecedented cascade reaction leading to a diene in one case and to a benzofulvene in the other case, with a unique structure. Both of them were characterized by X-ray crystallography, among other techniques. The mechanism of the reaction leading to the diene was investigated experimentally. Theoretical calculations at the DFT level suggest that the mechanism leading to the benzofulvene relies on a hexa-dehydro Diels-Alder (HDDA)-type of mechanism. This work constitutes an example of an unanticipated reactivity leading to an important increase of chemical complexity.

Pyridynes and indolynes as building blocks for functionalized heterocycles and natural products

This feature article showcases the use of pyridynes and indolynes to construct functionalized heterocycles and complex natural products.