Site-selective synthesis of indanyl-substituted indole derivatives via 1,3-dithiane induced Nazarov cyclization

We report an efficient site-selective synthetic method to C2 and C3 indanyl-substituted indole derivatives via 1,3-dithianyl induced Nazarov-type cyclization. In particular, C2-selective indanyl-substituted indoles were directly obtained by a BF3·Et2O-promoted sequence of intramolecular C3-C2 migration and Nazarov-type cyclization process.

Anion-accelerated asymmetric Nazarov cyclization: access to vicinal all-carbon quaternary stereocenters

We have developed a catalytic asymmetric Nazarov cyclization that results in the formation of two contiguous all-carbon quaternary stereocenters in high yield with excellent levels of asymmetric induction. This method requires no catalyst recognition elements in the starting materials that are simple diketoesters. Geometrically pure E or Z isomers of the starting material lead to diastereomerically pure products with high enantioselectivity because the species that undergoes cyclization is a rhodium enolate that is configurationally stable.

Oxa-Nazarov Cyclization-Michael Addition Sequence for the Rapid Construction of Dihydrofuranones

The Nazarov cyclization has been established as a powerful tool in constructing cyclopentenone skeletons. In sharp contrast, oxa-Nazarov cyclization that affords dihydrofuranones, a new type of product, has been less explored. In this work, we report the I2-catalyzed oxa-Nazarov cyclization-Michael addition cascade between vinyl α-diketones and enones. The protocol allows access to a range of functionalized dihydrofuranones with good to high yields, and diverse further transformations on the products have been achieved. Furthermore, the mechanistic studies reveal that the 1,2-hydride shift occurs simultaneously during the dihydrofuranone formation.

Divergent Reactivity of Triaryldivinyl Ketones: Competing 4π and Putative 6π Electrocyclization Pathways

This work describes an acid-promoted cyclization of triaryldivinyl ketones containing a thiophene moiety in the α-position. Two cyclization pathways are accessible: one a 4π-Nazarov cyclization and the other we propose to proceed through a 6π electrocyclic mechanism. The relative proportion of products from these divergent pathways is affected by reaction conditions and steric bulk in the substrate. We present experimental and computational evidence that when using HCl in dioxane, the 4π-conrotatory electrocyclization is more favorable, whereas GaCl₃ in methylene chloride shifts the chemoselectivity toward a putative 6π-disrotatory electrocyclization. DFT calculations suggest that a complex interplay between kinetic and thermodynamic factors is implicated in the chemodivergent behavior.

Acid-Promoted Iso-Nazarov Cyclization of Conjugated trans-Dienones and Dienals for the Synthesis of 2-Cyclopentenones

The acid-promoted cyclization of all-trans linearly conjugated dienones and dienals constitutes a synthetic strategy for the construction of 2-cyclopentenones.

Total Syntheses of Calyciphylline A-Type Alkaloids (-)-10-Deoxydaphnipaxianine A, (+)-Daphlongamine E and (+)-Calyciphylline R via Late-Stage Divinyl Carbinol Rearrangements

Among the famous Daphniphyllum alkaloids family, the calyciphylline A-type subfamily has triggered particular interest from the organic synthesis community in recent years. Here, we report divergent total syntheses of three calyciphylline A-type alkaloids, namely, (-)-10-deoxydaphnipaxianine A, (+)-daphlongamine E, and (+)-calyciphylline R. Our work highlights an efficient, divergent strategy via late-stage divinyl carbinol rearrangements, including an unprecedented oxidative Nazarov electrocyclization using an unfunctionalized tertiary divinyl carbinol and an unusual allylic alcohol rearrangement. A highly efficient "donor-acceptor" platinum catalyst was used for a critical nitrile hydration step. Moreover, the power of selective amide reductions has also been showcased by novel and classic tactics.

One-Pot Assembly and Synthetic Applications of Geminal Acyl/Alkoxy Tetrasubstituted Allenes

Polysubstituted allenes are useful synthetic intermediates in many applications, offering structural complexity, modularity, and their axial chirality in further transformations. While acyl and alkoxy-substituted allenes are known, there are currently few examples of allenes containing both functionalities and no reports of geminally substituted acyl/alkoxy allenes being isolated and characterized. Herein, we report the synthesis of tetrasubstituted allenes featuring a novel geminal acyl/alkoxy substitution. These unique "push-pull" allenes are bench-stable and exhibit interesting reactivity in several applications.

Titanium-Mediated aza-Nazarov Annulation for the Synthesis of N-Fused Tricycles: A General Method to Access Lamellarin Analogues

Fused heterocycles with nitrogen incorporation are of particular bioactive use and high importance in many research fields, especially isoquinoline-based [6/6/5] tricycles. Here, we report a unique strategy to access multifunctional N-fused tricycles from α,β-unsaturated isoquinoline ketone and sulfonamide under mild reaction conditions. The methodology features wide substrate tolerance, and a set of N-fused heteroarenes including quinoline, phthalazine, quinazoline, quinoxaline, and benzothiazole cores are furnished efficiently. Moreover, the protocol is easy to scale up to synthesize lamellarin analogues, and the amide group of the product is also easy to transfer to other functional groups.

Iodolium salts as halogen-bond donor catalysts in the Nazarov cyclization: the molecular oxygen enigma

Nazarov cyclizations of activated precurosrs are achieved under iodolium catalysis, provided that oxygen is present for catalyst activation and turnover.

Selective 1,4-arylsulfonation of 1,3-enynes via photoredox/nickel dual catalysis

A photoredox/nickel-catalyzed selective 1,4-arylsulfonation of 1,3-enynes to access structurally diverse sulfone-containing allenes has been established with low catalyst loading.

Alkynedicobalt mediated vinylogous Nazarov reactions

The SnCl₄ mediated reactions of cross conjugated aryl enynone-Co₂(CO)₆ and dienynone-Co₂(CO)₆ complexes afford benzocycloheptynone complexes or cycloheptenynone complexes in a thermal vinylogous Nazarov process. The rate of ring closure is strongly dependent on the ketone α-substituent.

Room Temperature Allenation of Terminal Alkynes with Aldehydes

A gold-catalyzed room temperature allenation of terminal alkynes (ATA) with aldehydes affording 1,3-disubstituted allenes with diverse functional groups has been developed by identifying a gold(I) catalyst and an amine. The practicality of this reaction has been demonstrated by a ten gram-scale synthesis and the synthetic potentials have been demonstrated via various transformations and formal total synthesis of (-)-centrolobine. Mechanistic studies revealed that the gold catalyst, the aldehyde effect, the fluoroalkyl hydroxyl solvent (TFE or HFIP) and the structure of amine are vital in this room temperature ATA reaction.

Cascade 8π Electrocyclization/Benzannulation to Access Highly Substituted Phenylpyridines

A cascade 8π electrocyclization/benzannulation reaction was developed to obtain the synthetically important highly substituted phenyl-pyridines. This method shows great potential in the rapid and inexpensive application of the scalable and operationally simple production of accessible substrates. On the basis of the resulting phenyl-pyridine products, a new Ru catalyst and bidentate ligand were designed and prepared, further demonstrating its high practicability.

Rh-Catalyzed cyclization of 2,3-allenoic acids in the presence of 2,3-allenols

Herein we report a [Cp*RhCl₂]₂-catalyzed coupling cyclization of two different classes of allenes with 2,3-allenoic acids affording 2(5H)-furanone skeletons of products and 2,3-allenols forming a conjugated (E)-enal or enone functionality to the β-position of the 2(5H)-furanones. These products are important building blocks for the syntheses of potentially bioactive compounds. The reaction proceeded via the nucleometalation, insertion, and stereodefined 1,4-H delivery carried by rhodium.

Polymerization of Allenes by Using an Iron(II) β-Diketiminate Pre-Catalyst to Generate High Mn Polymers

Herein, we report an iron(II)-catalyzed polymerization of arylallenes. This reaction proceeds rapidly at room temperature in the presence of a hydride co-catalyst to generate polymers of weight up to Mn =189 000 Da. We have determined the polymer structure and chain length for a range of monomers through a combination of NMR, differential scanning calorimetry (DSC) and gel permeation chromatography (GPC) analysis. Mechanistically, we postulate that the co-catalyst does not react to form an iron(II) hydride in situ, but instead the chain growth is proceeding via a reactive Fe(III) species. We have also performed kinetic and isotopic experiments to further our understanding. The formation of a highly unusual 1,3-substituted cyclobutane side-product is also investigated.

A palladium-catalyzed approach to allenic aromatic ethers and first total synthesis of terricollene A

A palladium-catalyzed C-O bond formation reaction between phenols and allenylic carbonates to give 2,3-allenic aromatic ethers with decent to excellent yields under mild reaction conditions has been described. A variety of synthetically useful functional groups are tolerated and the synthetic utility of this method has been demonstrated through a series of transformations of the allene moiety. By applying this reaction as the key step, the total syntheses of naturally occurring allenic aromatic ethers, eucalyptene and terricollene A (first synthesis; 4.5 g gram scale), have been accomplished.

Recent progress towards the transition-metal-catalyzed Nazarov cyclization of alkynes via metal carbenes

In recent years, transition-metal-catalyzed tandem cyclization reactions of alkynes, especially those involving a metal carbene intermediate, have received worthwhile interest, as this type of reaction does not require the use of risky and potentially explosive diazo compounds as starting materials for carbene generation. A significant and general strategy for the stereospecific synthesis of 5-membered cycles is Nazarov cyclization based on the 4π-conrotatory electrocyclization of a conjugated pentadienyl cation to afford a cyclopentenyl cation. In this review, we introduce an overview of recent advances in the transition-metal-catalyzed Nazarov cyclization of alkynes via a metal carbene intermediate, and categorize these reactions according to the structure of the metal carbene. Our aim is to accelerate advancements in this enchanting area of research.

Decarboxylation-triggered homo-Nazarov cyclization of cyclic enol carbonates catalyzed by rhenium complex

Decarboxylative homo-Nazarov cyclization catalyzed by a Lewis acid was achieved using a cyclic enol carbonate bearing a cyclopropane moiety as a substrate. Various substrates were converted into the corresponding multi-substituted cyclohexenones in good yields via decarboxylation, followed by 6-membered ring formation involving cyclopropane-ring-opening.

Selectivity for Alkynyl or Allenyl Imidamides and Imidates in Copper-Catalyzed Reactions of Terminal 1,3-Diynes and Azides

Copper-catalyzed reactions of terminal 1,3-diynes with electron-deficient azides to generate either 3-alkynyl or 2,3-dienyl imidamides and imidates are described. The selectivity depends on the diyne substituents and the nucleophile that reacts with the ketenimide intermediate generated from the corresponding triazole precursor. Reactions of 1,3-diynes containing a propargylic acetate afford [3]cumulenyl imidamides, while reactions using methanol as the trapping agent selectively generate 2,3-dienyl imidates. Five-membered heterocycles were obtained from 1,3-diynes containing a homopropargylic hydroxyl or amine substituent.

Visible light mediated functionalization of allenes

This review summarizes the visible light mediated strategies for the functionalization of allenes.

Advances in organocatalytic asymmetric reactions of vinylindoles: powerful access to enantioenriched indole derivatives

This review summarizes advances in vinylindole-based organocatalytic asymmetric reactions since 2008 and includes the applications of some methodologies in the total synthesis of natural products, points out remaining challenges in this research area.

Acid-catalyzed allenylation of pyrazolones with propargyl alcohols

A TsOH-catalyzed allenylation of pyrazolones with propargylic alcohols has been developed. The established reaction system is well tolerated by a wide scope of pyrazolones and propargylic alcohols. The process has the salient features of operational simplicity, facile scale-up and high yield. In particular, the integration of the pharmaceutical-related pyrazolone skeleton and the allenyl group into a single molecule not only enriches the structural diversity of the pyrazolone scaffold, but potentially also contributes to a broader spectrum of biological activity. Furthermore, it is easy to synthesize 3aa in gram-scale with the yield and efficiency basically maintained, making the practical application of this process more prominent.

Pentannulation of N-heterocycles by a tandem gold-catalyzed [3,3]-rearrangement/Nazarov reaction of propargyl ester derivatives: a computational study on the crucial role of the nitrogen atom

The tandem gold(I)-catalyzed rearrangement/Nazarov reaction of enynyl acetates in which the double bond is embedded in a piperidine ring was computationally and experimentally studied. The theoretical calculations predict that the position of the propargylic acetate substituent has a great impact on the reactivity. In contrast to our previous successful cyclization of the 2-substituted substrates, where the nitrogen favors the formation of the cyclized final product, the substitution at position 3 was computed to have a deleterious effect on the electronic properties of the molecules, increasing the activation barriers of the Nazarov reaction. The sluggish reactivity of 3-substituted piperidines predicted by the calculations was further confirmed by the results obtained with some designed substrates.

Asymmetric Nazarov Cyclizations of Unactivated Dienones by Hydrogen-Bond-Donor/Lewis Acid Co-Catalyzed, Enantioselective Proton-Transfer

We report an enantioselective Nazarov cyclization catalyzed by chiral hydrogen-bond-donors in concert with silyl Lewis acids. The developed transformation provides access to tri-substituted cyclopentenones in high levels of enantioselectivity (up to 95% e.e.) from a variety of simple unactivated dienones. Kinetic and mechanistic studies are consistent with a reversible 4π-electrocyclization C-C bond-forming step followed by rate- and enantio-determining proton-transfer as the mode of catalysis.

Denitrative imino-diaza-Nazarov cyclization: synthesis of pyrazoles

An iodine-catalyzed denitrative imino-diaza-Nazarov cyclization (DIDAN) methodology has been developed for the synthesis of pyrazoles with high to excellent yields by using α-nitroacetophenone derivatives and in situ generated hydrazones. The key transformation of this oxidative 4π-electrocyclization proceeds through an enamine-iminium ion intermediate. This rapid one-pot DIDAN protocol results in the selective generation of C-C and C-N bonds and cleavage of a C-N bond.

Biomimetic 2-Imino-Nazarov Cyclizations via Eneallene Aziridination

Amidoallyl cations are appealing three-carbon synthons for the preparation of complex amine-containing carbocycles; however, methods to generate and utilize these reactive species are limited and underexplored compared to those for oxallyl cations. Here we disclose a bioinspired strain-driven ring opening of bicyclic methyleneaziridines to 2-amidopentadienyl cation intermediates that readily engage in Nazarov cyclizations. Advantages of this strategy include ease of generation and improved reactivity compared to 3-pentadienyl cations, control over the ultimate position of the alkene, the potential for high dr between vicinal stereocenters, and the ability to further elaborate the products to fully substituted aminocyclopentanes. Experimental and computational studies support a dual role for the Rh₂L_n complex as both a nitrene transfer catalyst and a Lewis acid promoter, insight that provides a framework for the future development of asymmetric 2-imino-Nazarov cyclizations.

Lewis Acid-Mediated Cyclization of Allenyl Aryl Ketones

The cyclization of a series of nonheterocyclic allenyl aryl ketones was examined using boron trifluoride etherate and indium triflate to mediate the reaction. Yields with BF₃ were low in most instances due mainly to competitive destruction of the substrates. With In(OTf)₃, there was less decomposition, and the yields of the cyclized product were much higher, but only for substrates with electron-donating substituents. Cyclization did not occur without those substituents. A computational study using the ωB97X-D/6-311+G(2d,p)//ωB97X-D/6-31+G(d,p) method confirmed better stability of the σ-complexed substrate by indium(III) and that meta-substituents on the phenyl ring of the substrate significantly influenced the activation barrier of the cyclization, whereas the effect of para-substituents was almost negligible. The computational results supported the idea that the cyclization is a 4π-electrocyclization and not a 5-endo-dig ring closure as had been proposed in the literature.

Discovery of novel bacterial queuine salvage enzymes and pathways in human pathogens

Queuosine (Q) is a complex tRNA modification widespread in eukaryotes and bacteria that contributes to the efficiency and accuracy of protein synthesis. Eukaryotes are not capable of Q synthesis and rely on salvage of the queuine base (q) as a Q precursor. While many bacteria are capable of Q de novo synthesis, salvage of the prokaryotic Q precursors preQ0 and preQ1 also occurs. With the exception of Escherichia coli YhhQ, shown to transport preQ0 and preQ1, the enzymes and transporters involved in Q salvage and recycling have not been well described. We discovered and characterized 2 Q salvage pathways present in many pathogenic and commensal bacteria. The first, found in the intracellular pathogen Chlamydia trachomatis, uses YhhQ and tRNA guanine transglycosylase (TGT) homologs that have changed substrate specificities to directly salvage q, mimicking the eukaryotic pathway. The second, found in bacteria from the gut flora such as Clostridioides difficile, salvages preQ1 from q through an unprecedented reaction catalyzed by a newly defined subgroup of the radical-SAM enzyme family. The source of q can be external through transport by members of the energy-coupling factor (ECF) family or internal through hydrolysis of Q by a dedicated nucleosidase. This work reinforces the concept that hosts and members of their associated microbiota compete for the salvage of Q precursors micronutrients.

Decarboxylative Nazarov Cyclization-Based Chirality Transfer for Asymmetric Synthesis of 2-Cyclopentenones

Asymmetric synthesis of 2-cyclopentenones was achieved by chirality transfer based on Lewis acid catalyzed decarboxylative Nazarov cyclization of optically active cyclic enol carbonates, which are prepared by silver-catalyzed carbon dioxide incorporation into optically pure propargyl alcohols. The stereochemistry at the 4,5-positions of the 2-cyclopentenones was cleanly constructed by reflecting the stereochemistry of the starting materials. This method could be applied to various substrates to obtain the corresponding products in high yields with highly efficient chirality transfer.

The LANCA three-component reaction to highly substituted β-ketoenamides - versatile intermediates for the synthesis of functionalized pyridine, pyrimidine, oxazole and quinoxaline derivatives

The LANCA three-component reaction of lithiated alkoxyallenes LA, nitriles N and carboxylic acids CA leads to β-ketoenamides KE in good to excellent yields. The scope of this reaction is very broad and almost all types of nitriles and carboxylic acids have successfully been used. The alkoxy group introduced via the allene component is also variable and hence the subsequent transformation of this substituent into a hydroxy group can be performed under different conditions. Enantiopure nitriles or carboxylic acids can also be employed leading to chiral KE with high enantiopurity and dinitriles or dicarboxylic acids also lead to the expected bis-β-ketoenamides. β-Ketoenamides incorporate a unique combination of functional groups and hence a manifold of subsequent reactions to highly substituted heterocyclic compounds is possible. An intramolecular aldol-type condensation reaction efficiently furnishes pyridin-4-ols PY that can be further modified by palladium-catalyzed reactions, e.g., to specifically substituted furopyridine derivatives. Condensations of β-ketoenamides with ammonium salts or with hydroxylamine hydrochloride afford pyrimidines PM or pyrimidine N-oxides PO with a highly flexible substitution pattern in good yields. The functional groups of these heterocycles also allow a variety of subsequent reactions to various pyrimidine derivatives. On the other hand, acid-labile alkoxy substituents such as a 2-(trimethylsilyl)ethoxy group are required for the conversion of β-ketoenamides into 5-acetyl-substituted oxazoles OX, again compounds with high potential for subsequent functional group transformations. For acid labile β-ketoenamides bearing bulky substituents the acid treatment leads to acylamido-substituted 1,2-diketones DK that could be converted into quinoxalines QU. All classes of heterocycles accessed through the key β-ketoenamides show a unique substitution pattern - not easily accomplishable by alternative methods - and therefore many subsequent reactions are possible.

Catalytic enantioselective construction of axial chirality in 1,3-disubstituted allenes

Metal-catalyzed enantioselective construction of the loosening axial allene chirality spreading over three carbon atoms using a chiral ligand is still a significant challenge. In the literature, steric effect of the substrates is the major strategy applied for such a purpose. Herein, we present a general palladium-catalyzed asymmetrization of readily available racemic 2,3-allenylic carbonates with different types of non-substituted and 2-substituted malonates using (R)-(−)-DTBM-SEGPHOS as the preferred ligand to afford 1,3-disubstituted chiral allenes with 90~96% ee. This protocol has been applied to the first enantioselective synthesis of natural product, (R)-traumatic lactone. Control experiments showed that in addition to the chiral ligand, conducting this transformation via Procedure C, which excludes the extensive prior coordination of the allene unit in the starting allene with Pd forming a species without the influence of the chiral ligand, is crucial for the observed high enantioselectivity.

Highly enantioselective synthesis of allenes has been relying, so far, on the steric hindrance of substrates. Here the authors achieve excellent stereocontrol in the synthesis of chiral allenes with a palladium-DTBM-SEGPHOS catalytic system in a non-substrate-dependent manner.