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DFT calculations at M062X/6‐31+G(d,p) and M062X/Def2SVP levels of theory were utilized to explore the mechanism and cycloisomerization of 15‐membered triazaenediyne macrocycles in gas phase and Toluene. Trans triazaenediyne conducted concerted reactions to produce a trans diastereoselective tetracyclic fused structure passing through key cascade reactions starting by an Ene reaction that produced an allene intermediate (ΔG#=37.37 kcal/mol), and following a Diels‐Alder reaction which converted to trans‐tetracyclic ring overcoming a 34.10 kcal/mol barrier. In catalytic Rh (I), two steps reactions converted reagent to a diastereoselective trans‐cyclohexadiene tetracyclic by vanquishing barriers of 30.75 and 22.66 kcal/mol. [ABSTRACT FROM AUTHOR]

The cycloisomerization of alkyne‐tethered N‐benzoyloxycarbamates to 2‐(3H)oxazolones is described. Two catalytic systems are tailored for intramolecular 5‐exo‐alkyne carboxyamidation and concomitant alkene isomerization. PtCl2/CO (5 mol%, toluene, 100 °C) promotes both carboxyamidation and alkene isomerization but has a limited substrate scope. On the other hand, FeCl3 (5 mol%, CH3CN, 100 °C) promotes carboxyamidation effectively but a cocatalyst is required for the exocyclic alkene isomerization. Thus, a two‐step one‐pot protocol has been developed for a broader reaction scope, which involves FeCl3‐catalyzed carboxyamidation and base‐induced alkene isomerization. Crossover experiments suggest that these reactions proceed mainly through a mechanism involving acylnitrenoid intermediates rather than carbenoid intermediates. [ABSTRACT FROM AUTHOR]

A series of hitherto unknown 5,14‐diphenylbenzo[j]naphtho[2,1,8‐def][2,7]phenanthrolines, containing a 5‐azatetracene and a 2‐azapyrene subunit, were prepared by combination of Pd‐catalyzed cross‐coupling reactions with a one‐pot Povarov/cycloisomerization reaction. In the final key step four new bonds are formed in one step. The synthetic approach allows for a high degree of diversification of the heterocyclic core structure. The optical and electrochemical properties were studied experimentally and by DFT/TD‐DFT and NICS calculations. Due to the presence of the 2‐azapyrene subunit, the typical electronic nature and characteristics of the 5‐azatetracene moiety are lost and the compounds are electronically and optically more related to 2‐azapyrenes. [ABSTRACT FROM AUTHOR]

Tuning the photophysical properties of π‐conjugated oligomers by functionalization of skeleton, to achieve an optically and electronically advantageous building block for organic semiconductor materials is a vital yet challenging task. In this work, a series of structurally well‐defined polyaryl‐functionalized α‐oligofurans, in which aryl groups are introduced precisely into each of the furan units, are rapidly and efficiently synthesized by de novo metal‐free synthesis of α‐bi(arylfuran) monomers for the first time. This new synthetic strategy nicely circumvents the cumbersome substituent introduction process in the later stage by the preinstallation of the desired aryl groups in the starting material. The characterization of α‐oligo(arylfuran)s demonstrates that photoelectric properties of coplanar α‐oligo(arylfuran)s can be tuned through varying aryl groups with different electrical properties. These novel α‐oligo(arylfuran)s have good hole transport capacity and can function as hole‐transporting layers in organic light‐emitting diodes, which is indicative of significant breakthrough in the application of α‐oligofurans materials in OLEDs. And our findings offer an avenue for the ingenious use of α‐oligo(arylfuran)s as p‐type organic semiconductors for OLEDs. [ABSTRACT FROM AUTHOR]

Cascade reactions provide a straightforward access to many valuable compounds and reduce considerably the number of steps of a synthetic sequence. Among the domino and multicomponent processes that involve alkynes, the cascade reaction between alkynoic acids and C-, N-, O- and S-aminonucleophiles stands out as a particularly powerful tool for the one-pot construction of libraries of nitrogen-containing heterocyclic compounds with scaffold diversity and molecular complexity. This reaction, based on an initial metal-catalyzed cycloisomerization that generates an alkylidene lactone intermediate, was originally catalyzed by gold(I) catalysts, along with silver salts or Brönsted acid additives, but other alternative metal catalysts have emerged in the last decade as well as different reaction media. This review examines the existing literature on the topic of metal-catalyzed cascade reactions of acetylenic acids and dinucleophiles and discusses aspects concerning substrate/catalyst ratio for every catalyst system, nature of the aminonucleophile involved and substrate scope. In addition, alternative solvents are also considered, and an insight into the pathway of the reaction and possible intermediates is also provided. [ABSTRACT FROM AUTHOR]

The treatment of N-tosylpropargyl amines 1 with 1,3-dicarbonyl compounds 2 in the presence of AuBr3 (5 mol%) and AgOTf (15 mol%) afforded poly-substituted furans 3 in good-to-high yields via the gold-catalyzed cleavage of the sp3 carbon–nitrogen bond.

Benzofuranones are attractive synthetic targets in medicinal and natural products. The cycloisomerization of o‐alkynyl phenol to benzofuran‐3(2H)‐one is reported here using gold(I) and Selectfluor. This benzofuranone can also be obtained from benzofuran without a gold catalyst. This study presents two appealing synthetic methods for benzofuran‐3(2H)‐ones that use readily available starting materials, have high chemoselectivity and operate under very mild conditions. [ABSTRACT FROM AUTHOR]

Herein, an I2‐catalyzed unprecedented cycloisomerization of ynamides is developed, furnishing various functionalized bis(indole) derivatives in generally good to excellent yields with wide substrate scope and excellent atom‐economy. This protocol not only represents the first molecular‐iodine‐catalyzed tandem complex alkyne cycloisomerizations, but also constitutes the first chemoselective cycloisomerization of tryptamine‐ynamides involving distinctively different C(sp3)−C(sp3) bond cleavage and rearrangement. Moreover, chiral tetrahydropyridine frameworks containing two stereocenters are obtained with moderate to excellent diastereoselectivities and excellent enantioselectivities. Meanwhile, cycloisomerization and aromatization of ynamides produce pyrrolyl indoles with high efficiency enabled by I2. Additionally, control experiments and theoretical calculations reveal that this reaction probably undergoes a tandem 5‐exo‐dig cyclization/rearrangement process. [ABSTRACT FROM AUTHOR]

Substituted pyrroles are prevalent aromatic ring systems encountered in numerous natural products and bioactive molecules. Herein, a novel ynamide scaffold has been designed, synthesized and subsequently applied to an unprecedented gold‐catalyzed and para‐toluenesulfonic acid (PTSA)‐assisted cascade cycloisomerization reaction for the synthesis of β‐pyrrole substituted α,β‐unsaturated ketones. Notably, the reaction is stereoselective, offering E‐isomer as major product with low to moderate yield. [ABSTRACT FROM AUTHOR]

This review focuses on the carbocyclization reactions of alkyne-containing substrates catalyzed by Brønsted acids. The electrophilic activation occurs either by direct protonation of the alkyne or by formation of an intermediate cation that further reacts with the alkyne to give a key vinyl cation intermediate. Specific focus is placed on the selectivity of the various methodologies described herein and on mechanistic rationales. 1 Introduction 2 Brønsted Acid Catalyzed Alkyne Carbocyclization Involving C(sp) Reaction Partners 2.1 Brønsted Acid Catalyzed Cyclization of Diynes 2.2 Brønsted Acid Catalyzed Cyclization of Allenynes 3 Brønsted Acid Catalyzed Alkyne Carbocyclization Involving C(sp2) Reaction Partners 3.1 Brønsted Acid Catalyzed Cycloisomerization of Enynes 3.1.1 Initial Protonation of the Alkene Function 3.1.2 Initial Protonation of the Alkyne Function 3.1.3 A Conia-Ene Reaction 3.2 Electrophile-Induced Carbocyclization Involving Other C(sp2)-Based Reaction Partners 3.2.1 Iminium Electrophiles 3.2.2 Oxonium Electrophiles 3.2.3 Isocyanate and Thioisocyanate Electrophiles 3.3 Brønsted Acid Catalyzed Cycloisomerization of Aryl-alkynes 3.3.1 Reactions Using Excess or Stoichiometric Amounts of Brønsted Acid 3.3.2 Reactions Using Substoichiometric Amounts of Brønsted Acid 3.3.3 Heteroatom-Substituted Alkynes 3.3.4 Enantioselective Brønsted Acid Catalyzed Cycloisomerization of Aryl-alkynes 4 Brønsted Acid Catalyzed Alkyne Carbocyclization Involving C(sp3) Reaction Partners: Alkyne-alkane Cycloisomerization 5 Conclusion and Outlook [ABSTRACT FROM AUTHOR]

Herein, we describe an Au-catalyzed cycloisomerization process of a nitroalkyne that was projected to construct the central 2,2-disubstituted pseudoindoxyl core of the natural product Austamide. Our intended strategy for forging the pseudoindoxyl core is based on the nitroalkyne cycloisomerization followed by a subsequent intramolecular [3 + 2] cycloaddition. However, the [3 + 2] cycloaddition occurred in an undesired manner, ultimately leading to a complex hexacyclic scaffold. • Austamide. • Pseudoindoxyl natural product. • Gold catalysis. • Nitroalkyne cycloisomerization. • Intramolecular [3 + 2] cycloaddition. [ABSTRACT FROM AUTHOR]

Rh(I)-catalyzed cycloisomerizations of bicyclo[1.1.0]butanes provide a fruitful approach to cyclopropane-fused heterocycles. Products and stereochemical outcome are highly dependent on catalyst. The triphenylphosphine (PPh3) ligand provides pyrrolidines, placing substituents anti to the cyclopropyl group. The 1,2-bis(diphenylphosphino)ethane (dppe) ligand yields azepanes with substituents syn to the cyclopropyl group. In this work, quantum mechanical DFT calculations pinpoint a reversal of regio- and diastereoselectivity, suggesting a concerted (double) C−C bond cleavage and rhodium carbenoid formation, driven by strain-release. The ligand-influenced cleavage step determines the regioselectivity of carbometalation and product formation, and suggests new applications of bicyclobutanes. Ligands alter the regioselectivity of Rh(I)- catalyzed cycloisomerizations of bicyclobutanes. Here, mechanistic studies performed by the authors pinpoint a concerted C−C bond cleavage and carbenoid formation responsible for the divergence. [ABSTRACT FROM AUTHOR]

A new approach for the synthesis of benzofurans starting from non-aromatic precursors is reported. The reaction involves sequential DBU-mediated cycloisomerization for furanylation followed by benzene ring construction via oxidative aromatization in the presence of Oxone®. Atom- and pot-economy, simple reaction conditions, the straightforward preparation of starting materials and access to diverse substituted benzofurans are the major advantages of this reaction. [ABSTRACT FROM AUTHOR]

Sodium sulfinate addition to (E)‐β‐chlorovinyl ketones followed by the aerobic cycloisomerization of alkenyl sulfone intermediates has been established for the synthesis of 3‐sulfonyl furans. The optimal cycloisomerization catalyst system was identified to be Cu(OTf)2, where molecular oxygen acts as a terminal oxidant. With three points of structural variation in both β‐chlorovinyl ketones and sodium sulfinates, the current aerobic cycloisomerization strategy allows the synthesis of highly functionalized and substituted furan derivatives. [ABSTRACT FROM AUTHOR]

The present article presents a personalized Account on the synthesis of nitrogen heterocycles by combination of regioselective Pd-catalyzed cross-coupling reactions of polyhalogenated heterocycles, i.e., Suzuki–Miyaura and Sonogashira reactions, with acid-mediated cycloisomerization reactions. In many cases, the products constitute new heterocyclic core structures and show interesting optical and electronic properties. 1 Introduction 2 Acridines 3 Phenathridines 4 Azapyrenes 5 Thienoquinolines 6 Pyrrolonaphthyridines 7 6-Aza-ullazines 8 Conclusions [ABSTRACT FROM AUTHOR]

We report here the generation of α,3‐dehydrotoluenes, a relatively rare subset of reactive intermediates of the dehydroaromatic family, from isolable allenynes. The substructure motif in the allenyne substrates is distinct from, and complementary to, those found in Myers‐Saito/Schmittel‐type cycloisomerizations. The reactions reported here give rise to product profiles that provide insight about the electronic nature (i.e. diradical vs. zwitterion vs. cyclic allene) of the particular isomeric DHT(s) that is(are) produced under different reaction conditions differing most significantly in the polarity of the reaction solvent. One example also revealed previously unobserved carbene‐like reactivity of the DHT. [ABSTRACT FROM AUTHOR]

A transition metal‐free method for the cycloisomerization of propargylic amides to oxazoles was developed. The reaction utilizes in situ generated hydrogen chloride in hexafluoroisopropanol (HFIP). With the aid of Design of Experiments optimization a wide range of substrates was transformed into the desired oxazoles. The method allows product formation without side reactions eliminating the need for extensive work‐up and purification. [ABSTRACT FROM AUTHOR]

Two bis-(propyl-imidazolium)-napthalenediimide (NDI) salts were prepared and used as N-heterocyclic carbene (NHC) precursors for the preparation of dimetallic complexes of rhodium and iridium. Infrared spectroelectrochemical studies indicate that the metals are sensitive to changes in the electronic state of the NDI moiety. The catalytic behavior of the rhodium and iridium complexes was tested in the cycloisomerization of alkynoic acids, where the complexes showed effective redox-switching properties. [ABSTRACT FROM AUTHOR]

A series of 1,4‐thiazepanes were synthesized using gold‐catalysis from 1,3‐aminothioethers having two or three carbon stereocenters. The process consists in a 7‐exo‐dig cyclization with C−S bond formation and concomitant allyl S‐to‐C migration, which occurs intramolecularly as demonstrated by a cross‐over experiment. X‐ray crystal‐structure analysis of one product confirmed the expected stereochemistry. Two examples of reactions, azide‐alkyne cycloaddition (CuAAC) and N‐alkylation, illustrate the potential post‐functionalization of the synthesized 1,4‐thiazepanes. Finally, the N‐propargyl 1,4‐thiazepanes underwent a sulfur‐assisted 1,6‐enyne cycloisomerization, which led through a 6‐exo‐dig cyclization to N,S‐heterobicyclo[4.3.1] systems with a bridgehead double bond. [ABSTRACT FROM AUTHOR]

Cycloisomerizations are powerful skeletal rearrangements that allow the construction of complex molecular architectures in an atom‐economic way. We present here an unusual type of cyclopropyl enyne cycloisomerization that couples the process of a cycloisomerization with the activation of a C−C bond in cyclopropanes. A set of substituted non‐canonical tricyclic cyclobutanes were synthesized under mild conditions using [(Ph3P)2Fe(CO)(NO)]BF4 as catalyst in good to excellent yields with high levels of stereocontrol. [ABSTRACT FROM AUTHOR]

A modular approach to a wide range of 1‐cyano‐3‐(hetero)arylindolizines through aldol‐cyclopropanation‐oxidative cycloisomerization is described where DDQ was utilized for the regioselective synthesis of the pyrrole units from cyclopropyl pyridines for the first time. A key to success is regioselective oxidation of benzylic position by DDQ, which enabled us to access to one regioisomer out of two possible ones. Homo‐dimerization at the C2, C5, or C7 sites of indolizines in the presence of DDQ has been discovered for the first time as well. [ABSTRACT FROM AUTHOR]

It is reported herein that by exploiting the commonly shared bicyclic decahydroquinoline motif, a gold‐catalyzed enamide–alkyne cycloisomerization reaction is developed to access tricyclic cores in a simple way. These tricyclic cores further serve as an advanced platform for the divergent enantioselective collective total syntheses of five Lycopodium alkaloids, belonging to three different structural types, in a concise and protecting‐group‐free fashion. The key transformations in the second phase include: 1) a transannular reductive Heck cyclization for installation of the azepane ring in fawcettidine, fawcettimine, and lycoposerramine Q; 2) a domino Mukaiyama hydration/Grob fragmentation process for construction of the ten‐membered lactam system in phlegmariurine B; 3) a Fukuyama one‐pot protocol for the construction of the 2‐pyridone motif in lycoposerramine R. The newly developed strategy is expected to pave the way for the synthesis of other structurally related Lycopodium alkaloids. [ABSTRACT FROM AUTHOR]

Gold catalysis has witnessed immense evolution in recent years, yet it still requires the use of activators to render the common [AuCl(L)] complexes catalytically active. Herein, the H-bonding donor properties of hexafluoroisopropanol (HFIP) are utilized for Au–Cl bond activation and the ancillary ligand and counteranion effects on cycloisomerization reactions are showcased in HFIP as solvent. [ABSTRACT FROM AUTHOR]

An efficient gold(I)-catalyzed redox cycloisomerization/nucleophilic addition/reduction reaction of o-nitroalkynes with various H-phosphorus oxides is established. Through the intramolecular redox cyclization of o-nitroalkynes and subsequent intermolecular nucleophilic addition/reduction with no external reactant, a variety of arylphosphoryl and alkylphosphoryl indolin-3-ones with high functional-group compatibility are obtained in moderate to good yields. Mechanistic studies suggest that phosphorus nucleophiles mediate the cleavage of the N–O bond as a reductant. [ABSTRACT FROM AUTHOR]

The azepinone scaffolds are an important framework of pharmaceutically interesting compounds and many bioactive natural products. This review mainly focused on the metal‐catalyzed approaches towards the synthesis of azepinones (2001–2021). Mainly it covers cycloisomerization and metal carbenoid insertion. It also involves transformations through cycloaddition reactions, metathesis reactions, microwave reactions and direct C−H/C−C/C−N activation reactions. The applications, substrate scope, limitations and plausible mechanisms of these transformations have also been discussed. [ABSTRACT FROM AUTHOR]

The confinement of a catalytic site is an efficient strategy to control a reaction and modulate its selectivity. In the present work, a new class of structurally simple and easily accessible bulky tri‐(ortho‐biaryl)phosphine ligands were accessed, and their gold(I) complexes synthesized. Their X‐ray diffraction analysis and the comparative evaluation of their VBur% and G steric parameters against a series of gold complexes commonly employed in catalysis demonstrated their confined nature. Despite their notable steric congestion, these complexes exhibited remarkable catalytic activities and unusual selectivities, both in nature and level, that make them unique in the field of synthetic homogeneous gold catalysis. [ABSTRACT FROM AUTHOR]

Synthetic routes leading to [6,6,6]‐tricyclic compounds were reported using metal catalysts from ene‐yne‐aldehydes in our group. Based on this result, the synthesis of various [6,6,6]‐tricyclic compounds is reported using transition metal‐catalyzed cyclization from enyne‐carbonyl substrates containing H, Me, Ph, OMe, CO2Me, and CO2i‐Pr groups. Tricyclic compounds containing the CO2Me group would be the prospective precursors of the natural product Lepenine. [ABSTRACT FROM AUTHOR]

Dehydrogenative cycloisomerization/arylation sequence of heteroatom nucleophile‐tethered unactivated alkynes provides a facile and powerful approach to C−C bond formation between the generated heterocycles and unfunctionalized arenes. Here, we describe a hypervalent iodine(III)‐catalyzed synthesis of oxazoles concomitant with the introduction of aryl groups into side chain from N‐propargyl carboxamides and arenes, representing first C(sp3)−C(sp2) bond formation by the catalytic dehydrogenative cycloisomerization/arylation reaction in exo‐dig modes. [ABSTRACT FROM AUTHOR]

Pd0 catalysts featuring phosphorus‐based monodentate ligands can detour the reactivity of carbonyl‐tethered alkylidenecyclopropanes (ACPs) from standard (3+2) cycloadditions towards tandem cycloisomerization/cross‐coupling processes. This new reactivity lies on the formation of key π‐allyl oxapalladacyclic intermediates, which are subsequently trapped with external nucleophilic partners, instead of undergoing canonical C−O reductive eliminations. Importantly, the use of imine‐tethered ACP's is also feasible. Therefore, the method provides a straightforward and stereoselective entry to a wide variety of highly functionalized cyclic alcohols and amines. [ABSTRACT FROM AUTHOR]

In the area of drug discovery, nitrogen/oxygen-based heterocycles have been utilized as key intermediates and have great attention in organic chemistry. A literature survey reveals that in the past two decades propargyl alcohols and propargyl amines contributed significantly to the synthesis of such bioactive heterocycles. Propargyl alcohols and propargyl amine precursors are alkyne-based compounds and can be easily converted to propargylic carbocations. The nucleophilic nature of the triple bond and electrophilic nature of furnished carbocations from propargyl alcohols and propargyl amines makes these intermediates highly reactive and useful for the synthesis of various heterocycles by cycloaddition, intermolecular/intramolecular-cyclization, cycloisomerization, cyclo-condensation reactions. Here in this review, we are going to summarize important and useful methodologies for the development of nitrogen and oxygen-based heterocycles using propargyl alcohols and propargyl amines. [ABSTRACT FROM AUTHOR]

Herein, we report the integration of simple linear regressions with gold(I) catalysis to interrogate the influence of phosphine structure on metal-catalyzed organic transformations. We demonstrate that observed product ratios in [4 + 3]/[4 + 2] cycloisomerization processes are influenced by both steric and electronic properties of the phosphine, which can be represented by the Au-Cl distance. In contrast, the observed selectivity of a similar [2 + 3]/[2 + 2] cycloisomerization is governed by L/B1, a steric parameter. Using this correlation, we were able to accurately predict the selectivity of a previously untested, Buchwald-type ligand to enhance selectivity for the same transformation. This ligand found further utility in increasing the selectivity of a previously reported gold-catalyzed cycloisomerization/arylation of 1,6-enynes by ~1 kcal/mol.

In comparison to mononuclear gold Lewis acid catalysts, digold complexes and dual‐gold catalysis have illustrated a distinct and powerful potential for the activation of carbon‐carbon multiple bonds. Herein, this concept is pushed further by designing novel tetranuclear gold(I) dicationic complexes structurally supported by strongly stabilizing constraint diphosphinoferrocenyl ligands and attractive closed‐shell Au⋅⋅⋅Au aurophilic interactions. The use of a molecularly‐defined tetranuclear dicationic aurophilic gold(I) precatalyst for the selectivity‐challenging cycloisomerization of low‐substituted 1,6‐enynes favors the formation with high selectivity of strained azabicyclo[4.1.0]hept‐4‐enes – even in the complete absence of activating/orienting substituents on alkyne and olefin reactive functions. This selectivity is not achieved by the reported phosphine‐ and carbene‐stabilized mono‐ and dinuclear cationic gold(I) complexes, including the ones formed from the same ligands. More importantly this selectivity differs also from nanoparticles and heterogeneous gold catalysts reported to date. DFT studies correlated to experimental mechanistic investigations support an unprecedented "cluster‐like" reactivity from polynuclear cooperation at the origin of this peculiar selectivity where the aurophilic interactions preexist, and pre‐organize, gold cluster reactive intermediates. [ABSTRACT FROM AUTHOR]

The synthesis of the ABCD tetracyclic partial structures of the fungal indole diterpenes janthitrem B, JBIR‐137, and shearinine G is reported. The route starts from 5‐formylated indoline that is coupled to a dihydropyran moiety, followed by Prins cyclization. A diene was obtained that was oxygenated in a divergent manner. The hydroxylated tetracyclic western half of janthitrem B was obtained in eight steps and 10 % overall yield. We also share our experience with alternative approaches passing via alkynylated precursors. This includes the gold‐catalyzed cycloisomerization of a 6‐ethynyl‐5‐prenylindoline. [ABSTRACT FROM AUTHOR]

Abstract. We report herein the synthesis of [7,5]‐fused bicyclic acetals, also named furooxepines through a gold(I)‐catalyzed domino reaction. During this transformation, two molecules of homopropargyl alcohol react together, in a sequence including an intramolecular hydroalkoxylation, condensation, a 1,6‐enyne cycloisomerization, acetalization and an isomerization. This gold(I)‐catalyzed domino reaction allow the formation of three bonds, two heterocycles and a tetrasubstituted carbon stereocenters in a one‐step operation with 100% atom economy. Post‐functionalizations allow the formation of tetrahydrofurans. [ABSTRACT FROM AUTHOR]

We describe the divergent construction of two nitrogen‐containing polycyclic systems by gold‐catalyzed cycloisomerizations. The gold‐catalyzed cascade hydroamination and 7‐endo‐dig‐selective cycloisomerization of nitrogen‐tethered 1,8‐diynes yielded indolo[1,7‐ab]benzazepines in one pot. In contrast, when Johnphos‐coordinated gold catalyst was used, the same 1,8‐diynes were transformed into indolo[1,2‐a]quinolines by 6‐endo‐dig‐selective cycloisomerization along with rearrangement of the substituent on one of the alkynes. The reaction of a naphthalene‐tethered substrate provided a helically chiral aza[6]helicene. [ABSTRACT FROM AUTHOR]

Functionalization, namely the introduction of side groups onto the molecular scaffold of a helicene, may have either the purpose of modifying the electronic properties of the parent helicene, e.g., by adding electron-withdrawing or electron-donating groups, or the scope of providing the helicene with a "handle", which can be reacted to bind the molecule to another molecule or to a solid structure, such as a carbon or metal surface, or again to allow for complexation of the helicene with metal ions. The possible approaches are two-fold: the synthesis of the helicene can be performed using starting materials that already contain a side group, or the side group can be introduced after the synthesis of the parent helicene. As azahelicenes are helicenes bearing one or more nitrogen atom(s) in the molecular framework, parent azahelicenes can be functionalized on carbon atoms by exploiting the presence of the electron-withdrawing nitrogen atom. Moreover, they can be transformed into quaternary salts, whose properties are quite different from those of the parent azahelicenes in terms of the solubility and electronic properties. This review aims to provide a survey of the different synthetic methods available to attain this fascinating class of compounds. [ABSTRACT FROM AUTHOR]

The first chiral helicene‐NHC gold(I) complexes efficient in enantioselective catalysis were prepared. The L‐shaped chiral ligand is composed of an imidazo[1,5‐a]pyridin‐3‐ylidene (IPy) scaffold laterally substituted by a configurationally stable [5]‐helicenoid unit. The chiral information was introduced in a key post‐functionalization step of a NHC‐gold(I) complex bearing a symmetrical anionic fluoreno[5]helicene substituent, leading to a racemic mixture of complexes featuring three correlated elements of chirality, namely central, axial and helical chirality. After HPLC enantiomeric resolution, X‐ray crystallography and theoretical calculations enabled structural and stereochemical characterization of these configurationally stable NHC‐gold(I) complexes. The high potential in asymmetric catalysis is demonstrated in the benchmark cycloisomerization of N‐tethered 1,6‐enynes with up to 95 : 5 er. [ABSTRACT FROM AUTHOR]

A catalytic cycloisomerization of enyne diesters derived from 2‐propargyloxyarylaldehydes is described. The cycloisomerization, catalyzed by 10 mol% In(OTf)3, provides access to 2H‐chromenes bearing diethyl 2‐(hetero)arylidene malonates at 3‐position. This enyne metathesis‐type reaction is also useful for the synthesis of thia‐, aza‐ and quinoline analogs of the 3‐substituted 2H‐chromenes. DDQ mediated oxidative C−N bond formation and further synthetic manipulation enables the conversion of 3‐substituted 2H‐chromene into chromene‐fused molecular scaffold. Pd(0)‐catalyzed intramolecular Heck reaction on suitably substituted 2H‐chromene provides indene‐based molecular scaffold. [ABSTRACT FROM AUTHOR]

An efficient protocol for the synthesis of 3‐ethynyl‐2‐aryl‐bicyclo[3.1.0]hexenes through a gold‐catalyzed enyne cyclization was established in this work. Depending on the specific substitution pattern, unusual cationic rearrangements of the corresponding gold carbene were revealed, leading to either cyclohexadienes or 5,9‐methanobenzo[8]annulene derivatives, respectively. [ABSTRACT FROM AUTHOR]

Cycloisomerization of N-tethered indole- and dihydropyrrole-arylpropargyl substrates into complex polycycles was investigated by using gold(I) catalysis. Enantioselectivities up to 93% ee were obtained in the asymmetric version of this process. DFT calculations rationalized the reactivity observed for the formation of such complex molecular frameworks. [ABSTRACT FROM AUTHOR]

A Ag(I)/PPh3-catalyzed chelation-controlled cycloisomerization of tryptamine-ynamide was developed to access the spiro[indole-3,4′-piperidine] scaffold in a racemic and diastereoselective manner. The diastereoselective products were achieved by a chiron approach. Density functional theory (DFT) calculations indicated that strong non-covalent effects between the substrate and catalyst/ligand complex stabilized the spiroindoleninium intermediate via cation-π–π interactions. [ABSTRACT FROM AUTHOR]

A series of inherently chiral (di)thiahelicenes/dithiophenohelicenes containing 5, 6, 9 or 11 (hetero)cycles in their helical backbone with thiophene subunit(s) embedded in it or annulated to its periphery were uniformly synthesized by intramolecular alkyne [2+2+2] cycloisomerization. Their structural, chemical and physicochemical properties such as crystal packing, oxidation, barrier to racemization, spontaneous thin film transformation or electrochemical behavior were studied and compared with the respective properties of the parent all‐carbon helicenes and dibenzohelicenes. [ABSTRACT FROM AUTHOR]

In recent years, the area of heterocycles with the employment of transition metal catalyst has attracted a huge attention. Greater levels of molecular complexity and better functional group compatibilities are obtained from easily available starting substrates under mild reaction conditions by developing novel approaches. For this purpose, the transition metal-catalyzed cycloisomerization reactions are introduced in molecular rearrangement steps. The catalytic methods have important advantages as compared to other approaches in the formation of heterocycles bearing new substituents. The syntheses of five-membered N-heterocycles employing copper catalysts have been described in this review article. [ABSTRACT FROM AUTHOR]

Thermally induced cycloisomerization reactions of 1,6‐allenynes gives α‐methylene‐γ‐lactams via intramolecular Alder‐ene reactions. The mechanism is supported by computational and deuterium labelling studies. This thermal, non‐radical method enables the discovery of a hitherto unknown route that proceeds via a divergent mechanism distinct from the previous [2+2] cycloisomerization manifold. [ABSTRACT FROM AUTHOR]

The transition‐metal‐catalyzed cyclization of bissilylethynylated N,N'‐dihydrotetraazapentacene (TIPS‐TAP‐H2) into bissilylated cyclopenta[fg,qr]pentacenes is reported. Depending on the catalyst either none, one or two silyl groups migrate and change their positions in the formed five‐membered rings. The optoelectronic properties are quite similar, whereas the packing motifs differ dramatically. Control experiments and quantum chemical calculations were performed to investigate the mechanism of the reaction and the selectivity of the silyl shift. [ABSTRACT FROM AUTHOR]

gem‐Hydrogenation of propargyl alcohol derivatives with [CpXRu(MeCN)3]PF6 (CpX=substituted cyclopentadienyl) as catalysts affords cationic pianostool ruthenium carbene complexes which are so electrophilic that they attack a tethered olefin to furnish cyclopentene products; cyclopropanation or metathesis do not compete with this novel transformation. If the transient carbenes carry appropriate propargylic substituents, however, they engage in ([2,3]‐sigmatropic) rearrangements to give enol esters (carbonates, carbamates, sulfonates) or alkenyl halides. Both pathways are unprecedented in the vast hydrogenation literature. The proposed mechanistic scenarios are in line with labeling experiments and spectroscopic data; most notably, PHIP NMR spectroscopy (PHIP=parahydrogen induced polarization) provides compelling evidence that the reactions are indeed triggered by highly unorthodox gem‐hydrogenation events. [ABSTRACT FROM AUTHOR]

A novel domino cycloisomerization of 1,3-dien-5-ynes for the synthesis of 7H-benzo[7]annulenes is reported. The noticeable feature of this domino reaction involves the assembly of the fused bicyclic motifs through a transamidation/5-exo-trig cyclization/8π-electrocyclization sequence in a single step. Finally, mechanistic investigations were conducted experimentally and supported by DFT calculations. [ABSTRACT FROM AUTHOR]

The construction of axially chiral N-heterobiaryls is of great interest as a result of their occurrence in organocatalysts, chiral ligands, natural products, and biologically active molecules. Despite remarkable achievements in this area, strategies for the preparation of new classes of axially chiral N-heterobiaryls remain to be further explored. Herein, we report the enantioselective synthesis of axially chiral arylquinolizones through an intramolecular atroposelective cycloisomerization. The reaction proceeds via the Brønsted acid-enhanced dearomatization of pyridine by a copper catalyst that allows for the formation of the desired products in excellent yields and enantioselectivities. The utility of this methodology is illustrated by a synthesis on gram scale production and transformation of the products into chiral thiourea catalysts. Mechanistic studies demonstrate that Brønsted acid plays a significant role in promoting the reactivity of the reaction, while both the steric and electronic effects of aryl substituents in substrate play a role in controlling the stereoselectivity. Methods to assemble axially chiral biaryl molecules with nitrogen-containing heterocycles can have implications in ligand development for homogeneous catalysis. Here the authors report enantioselective syntheses of axially chiral arylquinolizones, via an intramolecular atroposelective cycloisomerization. [ABSTRACT FROM AUTHOR]

Multitasking single-catalyst systems that allow multiple chemical transformations within a single reaction vessel are important for the development of eco-compatible chemistry. Here, we have developed a rhodium-catalyzed system that transforms 1-(allyloxy)-2-(cyclopropylmethyl)benzene derivatives to 2-ethyl-3-isopropylbenzofurans via double isomerization/cycloisomerization/aromatization. [ABSTRACT FROM AUTHOR]

The cascade cyclization reactions of 1,5‐bisallenes grant access to a great variety of products by precisely tuning the catalyst system and/or the reagents involved. Herein, we present our findings that 1,5‐bisallenes react with two molecules of dimethyl acetylenedicarboxylate to afford, in a completely diastereoselective manner, cis‐3,4‐arylvinyl pyrrolidines and cyclopentanes. DFT calculations have been used to postulate a mechanism for the developed reaction which encompasses a [2+2+2] cycloaddition reaction of the two alkynes and the external double bond of one allene, followed by a cycloisomerization involving the internal double bond of the second allene. [ABSTRACT FROM AUTHOR]