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The total synthesis of cryptotrione (1) was enabled by substrate‐controlled diastereoselective construction of the bicyclo[3.1.0]hexene framework through platinum‐catalyzed enyne cycloisomerization and Lewis acid induced polyene cyclization to construct the abietane‐type tricyclic diterpene skeleton. The stereogenic tertiary carbon center in the side chain was installed in a diastereodivergent manner by conjugate addition reactions. [ABSTRACT FROM AUTHOR]

Phenylfuropyridone natural products from fungi exhibit a range of antibacterial and cytotoxicity activities, and can potentiate azole antifungal compounds. We elucidated the biosynthetic pathway of compounds in the citridone family through heterologous reconstitution of the pfp pathway. We demonstrate that multiple members of this family can be accessed from a reactive ortho‐quinone methide (o‐QM) intermediate through electrocyclization, cycloisomerization, or conjugate addition. Formation of the quaternary carbon center in citridone B is catalyzed by an epoxide‐forming P450 enzyme, followed by carbon skeletal rearrangement. Our results showcase how nature harvests the reactivities of an o‐QM intermediate to biosynthesize complex natural products. [ABSTRACT FROM AUTHOR]

Summary of main observation and conclusion: Allenes are a class of very important compounds and the development of straightforward, efficient, and highly enantioselective synthetic strategies for allenes have attracted extensive interests. Along this line, it is well known that aryl‐substituted allenes may be readily racemized, thus, difficult to prepare in high ee. Herein, an efficient palladium‐catalyzed nucleophilic allenylation of malonates with racemic 4‐aryl‐2,3‐butadienyl carbonates has been developed. The selectivity issue of mono‐ vs. bis‐allenylation with 2‐non‐substituted malonates has been addressed. By utilizing (R)‐(–)‐DTBM‐SEGPHOS (5,5'‐bis[di(3,5‐di‐t‐butyl‐4‐methoxyphenyl)phosphino]‐4,4'‐bi‐1,3‐benzodioxole) as a chiral ligand, various aryl‐substituted allenes and bisallenes have been prepared with good to excellent yields with high chemoselectivity and enantioselectivity under mild reaction conditions. Au‐catalyzed cycloisomerization and APK (allenic Pauson–Khand) reactions affording optically active mono‐ and bicyclic products have been demonstrated. [ABSTRACT FROM AUTHOR]

Summary of main observation and conclusion: Allenes are a class of very important compounds and the development of straightforward, efficient, and highly enantioselective synthetic strategies for allenes have attracted extensive interests. Along this line, it is well known that aryl‐substituted allenes may be readily racemized, thus, difficult to prepare in high ee. Herein, an efficient palladium‐catalyzed nucleophilic allenylation of malonates with racemic 4‐aryl‐2,3‐butadienyl carbonates has been developed. The selectivity issue of mono‐ vs. bis‐allenylation with 2‐non‐substituted malonates has been addressed. By utilizing (R)‐(–)‐DTBM‐SEGPHOS (5,5'‐bis[di(3,5‐di‐t‐butyl‐4‐methoxyphenyl)phosphino]‐4,4'‐bi‐1,3‐benzodioxole) as a chiral ligand, various aryl‐substituted allenes and bisallenes have been prepared with good to excellent yields with high chemoselectivity and enantioselectivity under mild reaction conditions. Au‐catalyzed cycloisomerization and APK (allenic Pauson–Khand) reactions affording optically active mono‐ and bicyclic products have been demonstrated. [ABSTRACT FROM AUTHOR]

The rhodium-catalyzed cycloisomerization of ester-tethered 1,6-diynes bearing a cyclopropanol moiety produced tetralone/exocyclic diene hybrid molecules with thermodynamically unfavorable alkene geometry. The results of control experiments and density functional theory calculations suggest that the ester tether plays an important role in the efficiency of E/Z isomerization processes. [ABSTRACT FROM AUTHOR]

A modular approach for the construction of β- and γ-lactam fused dihydropyrazinones from the readily available Ugi adducts has been described. The sequential construction of rings through base-mediated cycloisomerization followed by acid-mediated cyclization yielded β-lactam fused dihydropyrazinones. However, the Ugi-derived dihydropyrazinones afforded γ-lactam fused dihydropyrazinones under base-mediated cycloisomerization. The regioselectivity in the cycloisomerization reactions is explained on the basis of ring-strain. Substrate scope, limitations and mechanistic investigations through DFT-calculations have been explored. [ABSTRACT FROM AUTHOR]

Reactions between pyridine-functionalized alkynes and an Fe(II) precursor supported by 2,5,8-trithia[9](2,9)-1,10-phenanthrolinophane afforded the first Fe(II)-indolizine and -indolizinone complexes. Structural analysis and theoretical calculations revealed the existence of unconventional "non-vinylidene" pathways and challenged the generality of vinylidene intermediacy in Fe(II)-induced alkyne transformations. [ABSTRACT FROM AUTHOR]

The catalytic activity of cationic NHC‐ZnII and NHC‐AlIII (NHC=N‐heterocyclic carbene) complexes in reactions that require the electrophilic activation of soft C−C π bonds has been studied. The former proved able to act as a soft π‐Lewis acid in a variety of transformations. The benefit of the bulky IPr NHC ligand was demonstrated by comparison with simple ZnX2 salts. The tested NHC‐AlIII catalyst is not able to activate C−C π bonds but simple AlX2+ ions were found potent in some cases. [ABSTRACT FROM AUTHOR]

Triene 6π electrocyclization, wherein a conjugated triene undergoes a concerted stereospecific cycloisomerization to a cyclohexadiene, is a reaction of great historical and practical significance. In order to circumvent limitations imposed by the normally harsh reaction conditions, chemists have long sought to develop catalytic variants based upon the activating power of metal–alkene coordination. Herein, we demonstrate the first successful implementation of such a strategy by utilizing [(C5H5)Ru(NCMe)3]PF6 as a precatalyst for the disrotatory 6π electrocyclization of highly substituted trienes that are resistant to thermal cyclization. Mechanistic and computational studies implicate hexahapto transition‐metal coordination as responsible for lowering the energetic barrier to ring closure. This work establishes a foundation for the development of new catalysts for stereoselective electrocyclizations. [ABSTRACT FROM AUTHOR]

In contrast to well‐established symmetrical pincer complexes, non‐symmetrical metal pincers with the loss of C2v symmetry are much less studied. In this work, mesoionic NHTs (NHTs = N‐heterocyclic thiones), which can be viewed as the sulfur adducts of mesoionic carbenes, are incorporated into pincer complexes for the first time. Two symmetrical and non‐symmetrical phenylene‐bridged bis (NHT) compounds 3a/3b were synthesized as proligands via a "cycloaddition‐alkylation‐thionation" reaction sequence. In a case to access bis (NHT) compound 3c, N‐dealkylation reactions occurred. The carbene NMR signals of NHTs are only partially correlated to the π‐accepting abilities of carbenes, which is different from Bertrand's carbene‐phosphinidene system. The structural analysis of 3a/3b indicates that they possess C‐S partial double bonds. 3a and 3b served as the precursors to access two aryl anion‐linked [SCS/S′] pincer complexes 6a/6b. An external base proved to be essential for this cyclopalladation process. The catalytic properties of 6a/6b in the cycloisomerizations of alkynoic acids have been examined. Finally, non‐symmetrical complex 6a shows superior catalytic performance in such transformations contrasting to its symmetrical counterpart 6b. [ABSTRACT FROM AUTHOR]

A convenient three‐step synthesis of dibenzo[a,j]acridines is reported which relies on regioselective Pd‐catalyzed cross‐coupling reactions of 2,3,5,6‐tetrachloropyridine and subsequent Brønsted acid mediated cycloisomerization. Products are obtained in good yields and the method is broadly applicable. For selected dibenzo[a,j]acridines, photophysical and electrochemical properties were studied. [ABSTRACT FROM AUTHOR]

An overview of the recent literature on palladium‐catalyzed intramolecular Alder‐ene (IMAE) reaction of a variety of 1,n‐unsaturated systems is presented. The reaction which was first reported by Trost and Lautens provided an efficient alternative to the thermal or Lewis acid catalyzed cycloisomerizations involving ene‐type reaction. The IMAE cyclization of enynes and dienes has emerged as an important area and found significant applications in building up of complex molecular architectures and in the synthesis of several bioactive natural products. Since highly impactful reviews on this subject have covered the literature till 2015, this article focuses on summarizing the works subsequent to 2015. [ABSTRACT FROM AUTHOR]

Di‐tert‐butylated‐bis(phosphino)ferrocene ligands bearing phosphino substituents R (R=phenyl, cyclohexyl, iso‐propyl, mesityl, or furyl) allow tuning the selective formation of Au(I) halide complexes. Thus, dinuclear linear two‐coordinate, but also rare mononuclear trigonal three‐coordinate and tetrahedral four‐coordinate complexes were formed upon tuning of the conditions. Both Au(I) chloride and rarer Au(I) iodide complexes were synthesized, and their X‐ray diffraction analysis are reported. The significance of the control of structure and nuclearity in Au(I) complexes is further illustrated herein by its strong effect on the efficiency and selectivity of gold‐catalysed cycloisomerization. Cationic linear digold(I) bis(dicyclohexylphosphino) ferrocenes outperform other catalysts in the demanding regioselective cycloisomerization of enyne sulphonamides into cyclohexadienes. Conversely, tetrahedral and trigonal cationic monogold(I) complexes were found incompetent for enyne cycloaddition. We used the two‐coordinate linear electron‐rich Au(I) complex 2 b (R=Cy) to extend the scope of selective intramolecular cycloaddition of different 1,6‐enyne sulfonylamines with high activity and excellent selectivity to the endo cyclohexadiene products. [ABSTRACT FROM AUTHOR]

The reactivity of 7‐alkynycycloheptatrienes tethered to an aryl group under π‐acid catalysis has been studied. A variety of useful cyclic products were synthesized via Au(I)‐catalyzed skeletal reorganization, Cu(II)‐catalyzed hydroarylation, or Brønsted acid‐catalyzed tandem hydroarylation/Friedel‐Crafts reaction. We also report a rare type of skeletal reorganization involving the 1,3‐acetonide tether in the presence of a univalent cationic Ga(I)+ complex. [ABSTRACT FROM AUTHOR]

Herein, we developed an enantioselective addition of aliphatic aldehydes to 2-hydroxychalcone promoted by cooperative organocatalysts, giving access to hybrid flavonoids in excellent enantioselectivities. This reaction took advantage of cycloisomerization of 2-hydroxychalcone to form a transient flavylium under the irradiation of 24 W CFL, which was trapped by the in situ generated chiral enamine intermediate. The synergistic action of chiral phosphoric acid secured the excellent outcome of this reaction by ion-pairing with the transient flavylium. [ABSTRACT FROM AUTHOR]

An efficient synthetic method to prepare architecturally challenging spiro tetrahydro‐3H‐dicyclopenta[a,b]naphthalene derivatives from Au(I)‐catalyzed intramolecular sequential cycloisomerization of linear 1,3,9‐ene‐allene‐ynes is described. The proposed cycloisomerization pathway involves an initial gold‐catalyzed 1,3‐ene‐allene cycloisomerization, followed by a formal [4+2] cycloaddition to give fused spirocarbocyclic products containing a quaternary centre. [ABSTRACT FROM AUTHOR]

The metal‐promoted nucleophilic addition of sulfur ylides to π‐systems is a well‐established reactivity. However, the driving force of such transformations, elimination of a sulfide moiety, entails stoichiometric byproducts making them unfavorable in terms of atom economy. In this work, a new take on sulfur ylide chemistry is reported, an atom‐economical gold(I)‐catalyzed synthesis of dihydrobenzo[b]thiepines. The reaction proceeds under mild conditions at room temperature. [ABSTRACT FROM AUTHOR]

P‐pyrrole phosphines (R2Ppyr), in which a pyrrole group is directly bonded to the phosphorus atom, act as monodentate k‐P ligands towards gold(I) center to afford either neutral or cationic mononuclear complexes as well as neutral dinuclear complexes. All of these new gold(I) complexes have been structurally characterized and their first uses in catalysis have demonstrated their effectiveness as precatalysts for the enyne cycloisomerization reactions. [ABSTRACT FROM AUTHOR]

Preparative reactions that occur efficiently under dilute, buffered, aqueous conditions in the presence of biomolecules find application in ligation, peptide synthesis, and polynucleotide synthesis and sequencing. However, the identification of functional groups or reagents that are mutually reactive with one another, but unreactive with biopolymers and water, is challenging. Shown here are cobalt catalysts that react with alkenes under dilute, aqueous, buffered conditions and promote efficient cycloisomerization and formal Friedel–Crafts reactions. The constraining conditions of bioorthogonal chemistry are beneficial for reaction efficiency as superior conversion at low catalyst concentration is obtained and competent rates in dilute conditions are maintained. Efficiency at high dilution in the presence of buffer and nucleobases suggests that these reaction conditions may find broad application. [ABSTRACT FROM AUTHOR]

An efficient synthesis of 2,5-disubstituted furans directly from alkynyl ketones has been developed via tandem gold(I)-catalyzed isomerization of alkynyl ketones to allenyl ketones and cycloisomerization. The key to the success of this chemistry is the use of a biphenyl-2-ylphosphine ligand featuring a critical remote tertiary amino group. [ABSTRACT FROM AUTHOR]

Herein, we report a tandem cycloisomerization/nucleophilic addition/cyclization of 2-amino chalcone with bifunctional nucleophiles driven by visible light. This cascade process is realized by the irradiation of a blue LED at room temperature, which provides a concise route to structurally diverse benzo[d][1,3]oxazocine scaffolds. Mechanistic studies show that the reaction is initiated with the E to Z isomerization of a C–C double bond upon the irradiation of visible light, followed by cyclization/rearomatization to generate a transient quinolinium intermediate, which is trapped by the nucleophile and cyclized to produce the polycyclic benzo[d][1,3]oxazocine. [ABSTRACT FROM AUTHOR]

A series of expanded‐ring NHC gold complexes of the formula (NaphtDHD−Ar)Au−X (NaphtDHD=4,5‐dihydro‐1H‐naphtho[1,8‐ef][1,3]diazocin‐3(2H)‐ylidene; Ar: Mes=2,4,6‐trimethylphenyl, Dipp=2,6‐diisopropylphenyl or Xyl=2,6‐dimethylphenyl; X=Cl, NCCH3, NTf2) have been synthesized, including the first gold(I) triflimidate complex (5) stabilized by an eight‐membered NHC ligand. The new organogold compounds have been characterized by mass spectrometry, IR spectroscopy, and 1H and 13C NMR spectroscopy. The structural geometries of 3 b–c and 5 have been unequivocally established by crystallographic analysis revealing broad N‐C‐N angles (>121°) and high buried volume values (46–54%). The first catalytic studies were carried out on the cycloisomerization of 1,6‐enynes, obtaining full conversions (0.5 mol% catalyst loading) and excellent endo/exo selectivity (up to 99:1), and on the gold‐catalyzed phenol synthesis. Lastly, the (NaphtDHD‐Dipp)Au+ NTf2− species was subjected to a kinetic experiment in the cyclization of a N‐propargyl carboxamide to evaluate the efficiency of the pre‐formed catalyst (5) and the in situ activated gold complex (3 b+AgNTf2). [ABSTRACT FROM AUTHOR]

Polycyclic compounds having biological activities can be modified by employing different substituents. Substituent-dependent generation of tricyclic frameworks by the rhodium-catalyzed cycloisomerization of homopropargyl allene-alkynes was investigated by using density functional theory calculations. A mechanistic study revealed that all the three reactions, A (Z = Me, X = tBu), B (Z = Me, X = Me), and C (Z = H, X = Me), underwent initial oxidative cyclization, from which the divergent cycloisomerization was triggered when different substituents were employed. It was found from our calculations that reactions A and B underwent alkyne insertion into the Rh–C(sp2) bond and resulted in a key metal carbenoid intermediate. From the metal carbenoid species, reaction A bearing a more hindered tert-butyl group prefers 1,2-hydrogen migration to produce a 6/5/4 tricyclic product, while reaction B bearing a less hindered methyl group prefers 1,2-carbon migration leading to ring expansion and producing a 6/5/5 tricyclic product. The origins of the distinct selectivity were disclosed, which are beneficial for the development of novel related reactions. For reaction C (Z=H, Z=H or Z=H, Z=Me), the β-hydrogen elimination was found to be favored over the alkyne insertion into the Rh–C(sp2) bond and thus the monocyclic product is delivered. The E/Z effects involved in the above two cases were also discussed. [ABSTRACT FROM AUTHOR]

A novel gold‐catalyzed cycloisomerization of 1,6‐diynes was achieved, providing an atom‐economic approach to a diverse set of bicyclo[2.2.1]hept‐5‐en‐2‐ones in moderate to good yields. With unsymmetrical starting materials with two different internal alkynyl substituents, to some extent, the regioselectivity could be controlled by both electronic and steric factors. This unprecedented reactivity pattern may inspire new and unconventional strategies for the preparation of bridged ring systems. [ABSTRACT FROM AUTHOR]

We report the synthesis of PEGylated imidazolium (bromide and tetrafluoroborate) and tris-imidazolium (bromide) salts containing triazole linkers, and their use as stabilizers for the preparation of water-soluble gold nanoparticles by reduction of tetrachloroauric acid with sodium borohydride. The nanomaterials have been characterized. The X-Ray Photoelectron Spectroscopy (XPS) indicated the presence of Au(I) and Au(0) species, the oxidized form being more abundant in the nanomaterial derived from the tris-imidazolium bromide. The catalytic activity of these gold nanoparticles has been proved in the A3 coupling between carbonyl compounds, terminal alkynes and amines to afford propargylamines, and in the cycloisomerization of γ-alkynoic acids to enol lactones. The nanomaterial derived from the PEG-tagged tris-imidazolium bromide provided the best performance and it has been recycled in both reactions (up to four and six runs) taking advantage of its solubility properties. [ABSTRACT FROM AUTHOR]

This article presents a new strategy for achieving regiocontrol over the endo versus exo modes of cycloisomerizations of epoxide‐containing alcohols, which leads to the formation of five‐ or six‐membered cyclic ethers. Unlike traditional methods relying on achiral reagents or enzymes, this approach utilizes chiral phosphoric acids to catalyze the regiodivergent selective formations of either tetrahydrofuran‐ or tetrahydropyran‐containing products. By using methyl ester of epoxide‐containing antibiotic mupirocin as the substrate, it is demonstrated that catalytic chiral phosphoric acids (R)‐TCYP and (S)‐TIPSY could be used to achieve the selective formation of either the six‐membered endo product (95:5 r.r.) or the five‐membered exo product (77:23 r.r.), correspondingly. This cyclization was found to be unselective under the standard conditions involving various achiral acids, bases, or buffers. The subsequent mechanistic studies using state‐of‐the‐art quantum chemical solutions provided the description of the potential energy surface, which is fully consistent with the experimental observations. Based on these results, highly detailed reaction paths are obtained and a concerted and highly synchronous mechanism is proposed for the formation of both exo and endo products. [ABSTRACT FROM AUTHOR]

Metal-catalyzed allene cycloisomerizations provide rapid entry into five-membered carbocyclic frameworks, a common motif in natural products and pharmaceuticals. While both Au(I) and Pd(0)-catalyzed allene cycloisomerizations give 5- endo - dig cyclization, Pd prefers the syn diastereomer in contrast to the anti isomer observed with Au. The change in stereoselectivity is proposed to arise from buildup of A1,3 strain during the key carbopalladation step to furnish the cycloisomerized products in moderate to good dr with yields comparable to Au(I) catalysts. [ABSTRACT FROM AUTHOR]

This chapter discusses the reactions of 4-phosphorylated 5-hydroxypenta-2,3-dienoates with protected or unprotected hydroxy group involving 5-endo-trig cyclizations. Reactions with electrophiles produce mixtures of the 2,5-dihydro-1,2-oxaphosphole-5-carboxylates and the 5-phosphoryl-furan-2(5H)-ones by competitive electrophilic cyclization due to the neighboring phosphonate (phosphine oxide) and the carboxylate groups participation. 4-Phosphorylated 5-hydroxypenta-2,3-dienoates were smoothly converted into the corresponding 4-phosphoryl-2,5-dihydrofuran-2-carboxylates by using 5 mol% of a silver salt as a catalyst in the 5-endo-trig cycloisomerization reaction. [ABSTRACT FROM AUTHOR]

Divergent reaction pathways of β‐chlorovinyl ketones have been established to provide stereodefined alkylidenecyclopent‐2‐enones and 2‐alkenylfuran derivatives. The switch of the reaction pathways was made possible by the microwave‐assisted thermal Nazarov cyclization reaction of β‐chlorovinyl ketones, whereas the soft α‐vinyl enolization of β‐chlorovinyl ketones allowed the generation of allenyl ketone intermediates that in turn smoothly underwent cycloisomerization to the 2‐alkenyl furan derivatives in good to excellent yields. [ABSTRACT FROM AUTHOR]

A synthetic method to prepare cycloalkyl‐ and (hetero)aryl‐fused bicyclo[3.3.1]nonane derivatives from gold(I)‐catalyzed cyclopropanation/[5+3] cycloaddition of 1,4,9‐ and 1,4,10‐allenenynes is described. The suggested double cycloisomerization mechanism delineates the first example of a stepwise [5+3] cycloaddition pathway in gold catalysis, a mode of reactivity that is also sparse in organic chemistry. Experimental and Density Functional Theory (DFT) computational studies based on a proposed gold carbenoid species provides insight into this unique selectivity that leads to the assembly of the architecturally challenging bridged carbocyclic motif. [ABSTRACT FROM AUTHOR]

The iridium‐catalyzed cycloisomerization of various alkynoic acids was successfully developed, and a series of five‐, six‐, and especially seven‐membered unsaturated lactones were constructed with moderate yields and excellent regioselectivities (up to 68% yield, >99:1 rr). In addition, the indole compound can be easily prepared with 75% yield through this efficient synthetic methodology. Moreover, a plausible mechanism for this Ir‐catalyzed cycloisomerization of alkynoic acids was proposed. [ABSTRACT FROM AUTHOR]

The complexes CpCoL2 (Cp = C5 H5 ; L = CO or CH2 =CH2) mediate the cycloisomerizations of α,δ,ω-enynenes containing allylic ether linkages to 3-(oxacyclopentyl or cycloalkyl)furans via the intermediacy of isolable CpCo- η 4-dienes. A suggested mechanism comprises initial complexation of the triple bond and one of the double bonds, then oxidative coupling to a cobalt-2-cyclopentene, terminal double bond insertion to assemble a cobalta-4-cycloheptene, β-hydride elimination, and reductive elimination to furnish a CpCo- η 4-diene. When possible, the cascade continues through cobalt-mediated hydride shifts and dissociation of the aromatic furan ring. The outcome of a deuterium labeling experiment supports this hypothesis. The reaction exhibits variable stereoselectivity with a preference for the trans -product (or, when arrested, its syn -Me CpCo- η 4-diene precursor), but is completely regioselective in cases in which the two alkyne substituents are differentiated electronically by the presence or absence of an embedded oxygen. Regioselectivity is also attained by steric discrimination or blocking one of the two possible β-hydride elimination pathways. When furan formation is obviated by such regiocontrol, the sequence terminates in a stable CpCo- η 4-diene complex. The conversion of the cyclohexane-fused substrate methylidene-2-[5-(2-propenyloxy)-3-pentynyl]cyclohexane into mainly 1-[(1 R *,3a S *,7a S *)-7a-methyloctahydroinden-1-yl]-1-ethanone demonstrates the potential utility of the method in complex synthesis. [ABSTRACT FROM AUTHOR]

An iodine promoted cascade cycloisomerization of 1-en-6,11-diynes is presented for the easy preparation of tetrahydrobenzo[f]isoquinolines. This developed reaction system is identified as having good functional-group applicability and can be scaled up to gram quantities. In this transformation, two new cyclic frameworks and one carbonyl group are formed with four new bonds constructed. Additionally, the resulting iodo-substituted compounds could be further derived through simple elimination reactions. [ABSTRACT FROM AUTHOR]

The combined use of gold as transition metal catalyst and N‐heterocyclic carbene (NHC) as organic catalyst in the same solution for relay catalytic reactions was disclosed. The ynamide substrate was activated by gold catalyst to form unsaturated ketimine intermediate that subsequently reacted with the enals (via azolium enolate intermediate generated with NHC) effectively to form bicyclic lactam products with excellent diastereo‐ and enantio‐selectivities. The gold and NHC coordination and dissociation can be dynamic and tunable events, and thus allow the co‐existence of both active metal and carbene organic catalysts in appreciable concentrations, for the dual catalytic reaction to proceed. [ABSTRACT FROM AUTHOR]

Abstract: Supported Pd nanoparticles catalyzed the reductive cycloisomerization of diynes to give 1,3‐dienes, and subsequent intermolecular [4+2] cycloaddition with dienophiles led to cyclohexene derivatives. Various diynes and dienophiles participated in the one‐pot reaction to give the corresponding products in good to high yields. Al2O3‐supported Pd catalysts were highly tolerant toward air and reusable. [ABSTRACT FROM AUTHOR]

The first total synthesis of nominal harziane diterpenoid 1 is disclosed, whose spectral characteristics did not match those of the reported natural product. Stereochemical analysis and subsequent synthesis of the epimeric tertiary alcohol led to reassignment of configuration of the natural product as shown for 2. At the heart of the synthesis is an enyne cycloisomerization that sets a key quaternary stereocenter within a cyclobutane with high diastereocontrol. The route features strategies for the synthesis of the highly congested 6‐5‐7‐4 carbon skeleton characteristic of the caged harziane diterpenoids. [ABSTRACT FROM AUTHOR]

N‐Fluorobenzenesulfonimide (NFSI), one of the most popular fluorinating agents in organic synthesis, was used as a ligand of silver triflate (AgOTf) to assist the trapping of the persistent spiroindoleninium intermediates by forming a weak noncovalent interaction between the substrate and metal ligand. The AgOTf/NFSI catalytic system was identified in the cycloisomerization of tryptamine‐ynamide to spiro[indoline‐3,4′‐piperidine] derivatives. The substrate tolerances were demonstrated by various synthetic substrates and commercially available nucleophiles. A cation‐π‐π model was proposed to elucidate the mechanism of action based on the experimental observations and density functional theory (DFT) calculations. [ABSTRACT FROM AUTHOR]

Polarity inversion is the hallmark of N-heterocyclic carbene (NHC) organocatalysis, with the generation and reaction of acyl anion equivalents known for more than 70 years. In contrast, polarity inversion through 1,4-addition of NHCs to conjugate acceptors was first applied in a catalytic reaction in 2006. This sub-field of NHC-organocatalysis has developed steadily over the subsequent years, enabling novel coupling reactions, enantioselective cycloisomerizations, polymerizations, and other reactions. In this review, this emerging area of NHC-organocatalysis is discussed with comprehensive coverage. In addition, notes regarding the use of other Lewis base catalysts for related reactions, and comments regarding NHC selection for this type of catalysis, are provided. Polarity inversion through 1,4-addition of N-heterocyclic carbenes (NHCs) to conjugate acceptors was first reported in 2006 and has subsequently evolved as an active subfield of NHC-organocatalysis. In this review, this emerging area of NHC-organocatalysis is discussed with comprehensive coverage. [ABSTRACT FROM AUTHOR]

The use of Pd nanoparticles (Pd-NPs) to realize several important organic reactions allows efficient catalysis with low metal loading (<1000 ppm), hence providing a greener catalytic system. However, to be truly green Pd-NPs need to be synthesized in a sustainable manner and be able to react in aqueous media in order to avoid the use of organic solvents. Here we describe an original and eco-friendly synthesis of Pd-NPs (using benign reactants and simple conditions) perfectly stable in water. Remarkably, this synthesis allows for control over their size and morphology by simply tuning the pH of the stabilizer. We then evaluate the catalytic efficiency of these Pd-NPs on six different model reactions (Suzuki Miyaura, Sonogashira, Heck, nitrophenol reduction and pentynoic cycloisomerization) in aqueous media. We show that the stabilizer structure influences the activity owing to its ability to promote the mass transfer of the organic substrates towards the NP surface in the aqueous environment. Finally, catalytic evaluations show that our nano-catalysts prepared in an eco-friendly manner are among the best catalysts described so far in the literature in each case, with high turnover frequencies reached with a low loading of palladium. [ABSTRACT FROM AUTHOR]

Experimental and theoretical explorations were performed on the pathways of the cascade cycloisomerizations of tryptamine-N-ethynylpropiolamide substrates. The methodology provided a common strategy to access either indolizino[8,7-b]indoles or indolo[2,3-a]quinolizines in a switchable fashion. [ABSTRACT FROM AUTHOR]

Herein, we report an efficient method for the synthesis of the pyrido[3,2- c ]coumarin scaffold, one of the privileged heterocycle-fused coumarin scaffolds, via a AgNO3-catalyzed cycloisomerization of 4-(propynylamino)coumarins obtained from diverse 4-hydroxycoumarins. This concise method affords pyrido[3,2- c ]coumarin analogues bearing diverse substituents on the benzene or pyridine ring in moderate to good yields. Moreover, this methodology was extended to the facile synthesis of polyneomarline C, a natural pyrido[3,2- c ]coumarin derivative isolated from the Chinese herbal medicine Polyalthia nemoralis A. DC., in three steps and in 26% overall yield from the known 4-hydroxycoumarin. [ABSTRACT FROM AUTHOR]

The synthesis of 10 water‐soluble β‐cyclodextrin‐tagged NHC‐gold(I) complexes is described. Key steps are nucleophilic substitutions, as well as, copper‐(CuAAC)‐ and ruthenium‐(RuAAC)‐catalyzed azide alkyne cycloadditions. Whereas the CuAAC reliably affords 1,4‐disubstituted 1,2,3‐triazoles, the regioselectivity of the RuAAC depends on the structure of the coupling partners. Permethylated cyclodextrin‐tagged NHC‐gold(I) complexes are soluble both in water and in organic solvents. They show excellent catalytic activity and recyclability in cyclization reactions of functionalized allenes and alkynes in bulk water. The enantioselective cycloisomerization of γ‐ and δ‐hydroxyallenes could be achieved with up to 38 % ee. Thus, it is possible to take advantage of the chirality of the cyclodextrin moiety for enantioselective gold‐catalyzed transformations. [ABSTRACT FROM AUTHOR]

Surface organic ligands play a critical role in stabilizing atomically precise metal nanoclusters in solutions. However, it is still challenging to prepare highly robust ligated metal nanoclusters that are surface‐active for liquid‐phase catalysis without any pre‐treatment. Now, an N‐heterocyclic carbene‐stabilized Au25 nanocluster with high thermal and air stabilities is presented as a homogenous catalyst for cycloisomerization of alkynyl amines to indoles. The nanocluster, characterized as [Au25(iPr2‐bimy)10Br7]2+ (iPr2‐bimy=1,3‐diisopropylbenzimidazolin‐2‐ylidene) (1), was synthesized by direct reduction of AuSMe2Cl and iPr2‐bimyAuBr with NaBH4 in one pot. X‐ray crystallization analysis revealed that the cluster comprises two centered Au13 icosahedra sharing a vertex. Cluster 1 is highly stable and can survive in solution at 80 °C for 12 h, which is superior to Au25 nanoclusters passivated with phosphines or thiols. DFT computations reveal the origins of both electronic and thermal stability of 1 and point to the probable catalytic sites. This work provides new insights into the bonding capability of N‐heterocyclic carbene to Au in a cluster, and offers an opportunity to probe the catalytic mechanism at the atomic level. [ABSTRACT FROM AUTHOR]

[Display omitted] This study demonstrates a novel method for synthesizing rare heterocyclic compounds, namely, cyclic isoureas using alkenoic ureas via cobalt-catalyzed metal-catalyzed hydrogen atom transfer/radical polar crossover (MHAT/RPC). This method successfully produces five-membered rings along with the rare six- and seven-membered cyclic isoureas. Density functional theory (DFT) calculations were performed to elucidate the reaction mechanism focusing on cyclization. [ABSTRACT FROM AUTHOR]

(Difluoromethylated cyclopropane represents an important motif, which is widely found in bioactive and functional molecules. Despite significant progress in modern chemistry, the atom-economic and enantioselective synthesis of difluoromethylated cyclopropanes is still challenging. Herein, an Rh 2 (II)-catalyzed asymmetric enyne cycloisomerization is described to construct chiral difluoromethylated cyclopropane derivatives with up to 99% yield and 99% ee in low catalyst loading (0.2 mol%), which can be easily transformed into highly functionalized difluoromethylated cyclopropanes with vicinal all-carbon quaternary stereocenters by ozonolysis. Mechanistic studies and the crystal structures of alkyne-dirhodium complexes reveal that the cooperative weak hydrogen bondings between the substrates and the dirhodium catalyst may play key roles in this reaction.)., (© 2023 The Authors. Advanced Science published by Wiley-VCH GmbH.)

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