Divergent C-H Insertion-Cyclization Cascades of N-Allyl Ynamides

Intramolecular Cyclopropanation of Active Methylene Derivatives Based on FeCl2 or FeCl3-Promoted Radical-Polar Crossover Reactions

Radical-polar crossover reactions were studied for the intramolecular cyclopropanation of active methylene derivatives. In the presence of FeCl3 as a stoichiometric oxidant and K2HPO4 as a base, the dehydrogenative cyclopropanation of active methylenes proceeded through the FeCl3-promoted oxidative radical cyclization followed by the ionic cyclization to give the bicyclic cyclopropanes. The use of α-chloro-active methylenes leads the subcatalytic cyclopropanation involving two redox pathways. In the presence of K2HPO4, the redox cyclopropanation proceeded by using FeCl2 (20 mol%) in combination with ligand (20 mol%).

Structurally divergent enantioselective synthesis of benzofuran fused azocine derivatives and spiro-cyclopentanone benzofurans enabled by sequential catalysis

An important objective in organic synthesis and medicinal chemistry is the capacity to access structurally varied and complex molecules rapidly and affordably from easily available starting materials. Herein, a protocol for the structurally divergent synthesis of benzofuran fused azocine derivatives and spiro-cyclopentanone benzofurans has been developed via chiral bifunctional urea catalyzed reaction between aurone-derived α,β-unsaturated imine and ynone followed by switchable divergent annulation reactions by Lewis base catalysts (DBU and PPh3) with concomitant epimerization. The skeletally diversified products were formed in high yields with high diastereo- and enantioselectivities. Computational analysis with DFT and accurate DLPNO-CCSD(T) has been employed to gain deeper insights into mechanistic intricacies and investigate the role of chiral and Lewis base catalysts in skeletal diversity.

Base-Promoted Intramolecular Addition of Vinyl Cyclopropanecarboxamides to Access Conformationally Restricted Aza[3.1.0]bicycles

3-Azabicyclo[3.1.0]hexanes are common structural components in natural products and bioactive compounds. Traditionally, the metal-mediated cyclopropanation domino reaction of chain enzymes is the most commonly used strategy for the construction of this type of aza[3.1.0]bicycle derivative. In this study, a base-promoted intramolecular addition of alkenes used to deliver conformationally restricted highly substituted aza[3.1.0]bicycles is reported. This reaction was tailor-made for saturated aza[3.1.0] bicycle-containing fused bicyclic compounds that may be applied in the development of concise and divergent total syntheses of bioactive compounds.

Gold(I)-Catalyzed Benzylic C(sp3 )-H Functionalizations: Divergent Synthesis of Indole[a]- and [b]-Fused Polycycles

Phenyl azides substituted by an (alkylphenyl)ethynyl group facilitate benzylic sp³ (C-H) functionalization in the presence of a JohnPhosAu catalyst, resulting in indole-fused tetra- and pentacycles via divergent N- or C-cyclization. The chemoselectivity is influenced depending on the counter-anion, the electron density of the α-imino gold(I) carbene, and the alkyl groups stabilizing the benzylic carbocation originating from a 1,5-hydride shift. An isotopic labeling experiment demonstrates the involvement of an indolylgold(I) species resulting from a tautomerization that is much faster than the deauration. The formation of a benzylic sp³ (C-H) functionalization leading to an indole-fused seven-membered ring is also demonstrated.

Steric, Electronic and Conformational Synergistic Effects in the Gold(I)-catalyzed α-C-H Bond Functionalization of Tertiary Amines

Direct C-H bond functionalization is a useful strategy for the straightforward formation of C-C and C-Heteroatom bonds. In the present work, a unique approach for the challenging electrophilic Au-catalyzed α-C-H bond functionalization of tertiary amines is presented. Electronic, steric and conformational synergistic effects exerted by the use of a malonate unit in the substrate were key to the success of this transformation. This new reactivity was applied to the synthesis of tetrahydro-γ-carboline products which, under oxidative conditions, could be converted into valuable structural motifs found in bioactive alkaloid natural products.

Palladium-Catalyzed Synthesis of C-1 Deuterated Aldehydes from (Hetero) Arenes Mediated by C (sp2)-H Thianthrenation

A palladium-catalyzed deuterated formylation of aryl sulfonium salts is prepared conveniently from readily available arenes, which enables the expedient synthesis of a series of structurally diverse C-1 deuterated aldehydes with 96%-99% deuterium incorporation. The easy to handle and cost-effective DCOONa provides a deuterium source, which can be introduced onto the formyl units with excellent selectivity under the palladium-catalytic redox neutral conditions. This catalytic route can accomplish the direct late-stage C-H functionalization of bioactive molecules and natural product derivatives assisted by C (sp²)-H thianthrenation. Moreover, on the basis of this practical approach, several deuterated drugs and analogues could be prepared with excellent levels of deuterium incorporation.

Recent Advances in Practical Synthesis of C1 Deuterated Aromatic Aldehydes Enabled by Catalysis and Beyond

C 1 -selective deuteration of aromatic aldehydes is of great importance for isotopic labeling and for improving the characteristics of drug molecules. Due to the recent increase in the use of deuterated pharmacological drugs, there is a pressing need for synthetic procedures that are efficient to produce deuterated aromatic aldehyde analouges. Deuterium labeling approaches are typically used as an effective tool for researching pharmaceutical absorption, distribution, metabolism, and excretion (ADME). Furthermore, deuterium-labeled pharmaceuticals are intended to increase therapeutic effectiveness and reduce side effects by extending the half-life of drug response. In the last few years, several catalytic or non-catalytic methods have been developed to synthesize deuterated aromatic aldehydes. In this concern, we offer a brief overview of the various synthetic strategies and practical methods for the formyl-selective deuterium labeling of aromatic aldehydes using different deuterium sources.

Gold-Catalyzed Annulations with Nucleophilic Nitrenoids Enabled by Heteroatom-Substituted Alkynes

The combination of a nucleophilic nitrene equivalent, a triple bond and a π-acid catalyst has underpinned numerous efficient transformations for the preparation of azacycles. This personal account details our efforts in developing an annulation strategy. Adding a nucleophilic nitrenoid to an activated alkyne can generate carbenoid character that is then quenched by a cyclisation onto the nitrenoid substituent. The use and development of N-acyl and N-heterocyclic pyridinium-N-aminides as 1,3-N,O and 1,3-N,N-dipole equivalents is discussed in the context of oxazole and heterocycle-fused imidazole formation, respectively. The resulting processes are highly efficient, practically straightforward, and tolerate considerable structural and functional group variation. Our use of heteroatom-substituted alkynes as enabling tools for reaction discovery is discussed. The reactivity accessed from the strong donor-like properties of ynamides is complemented by that obtained from alkynyl thioethers, which are emerging as interesting substrates for π-acid catalysis.

Straightforward Synthesis of Indenes by Gold-Catalyzed Intramolecular Hydroalkylation of Ynamides

An original and straightforward entry to polysubstituted indenes from readily available ynamides is reported. Upon reaction with a N-heterocyclic carbene-gold complex under mild conditions, activated keteniminium ions are generated whose unique electrophilicity triggers a [1,5]-hydride shift and a subsequent cyclization. The presence of an endocyclic enamide in the densely functionalized resulting indenes was shown to be especially useful and versatile, offering a range of opportunities for their further postfunctionalization.

Media-Driven Pd-Catalyzed Reaction Cascades with 1,3-Diynamides Leading Selectively to Either Indoles or Quinolines

Divergent Pd-catalyzed reaction cascades with various 1,3-diynamides yielding either 2-amino-3-alkynylindoles or 2-amino-4-alkenylquinolines were established. Omitting or adding TBAF (tetrabutylammonium fluoride) to the reaction of N,N-(2-iodophenyl)(4-toluenesulfonyl)-1,3-diynamides with secondary or primary amines in the presence of KOH in THF and catalytic amounts of Pd(PPh₃ )₄ completely changed the outcome of the reaction. In the absence of TBAF, 2-amino-3-alkynylindoles were the sole products, while the presence of TBAF switched the product formation to 2-amino-4-alkenylquinolines. Deuterium labeling proceeded selectively at the C3 and C11 positions of the 2-amino-4-alkenylquinoline products and this suggests the unprecedented formation of [4]cumulenimines from 1,3-diynamides as reactive key intermediates.

A Radical-Initiated Fragmentary Rearrangement Cascade of Ene-Ynamides to [1,2]-Annulated Indoles via Site-Selective Cyclization

Straightforward access to [1,2]-annulated indoles, key substructures in natural products, is highly desirable yet challenging. Herein, a radical triggered fragmentary cyclization cascade reaction of ene-ynamides is presented, providing a rapid access into [1,2]-annulated indoles by an intermolecular radical addition, intramolecular cyclization, desulfonylative aryl migration, and site-selective C(sp²)-N cyclization sequence. DFT calculations support oxidation of N-centered radical species to cations prior to the C-N bond formation, followed by an unusual aza-Nazarov cyclization.

Ynamides as Three-Atom Components in Cycloadditions: An Unexplored Chemical Reaction Space

While 1,3-dipolar cycloadditions have appeared to be a fertile area for research, as attested by the numerous synthetic transformations and resulting applications that have been developed during the past 60 years, the use of neutral three-atom components (TACs) in (3+2) cycloadditions remains comparatively sparse. Neutral TACs, however, have great synthetic potential given that their reaction with a π system can produce zwitterionic cycloadducts that may be manipulated for further chemistry. We report herein that ynamides, a class of carbon π systems that has seen wide interest over the last two decades, can be used as neutral TACs in thermally induced intramolecular (3+2) cycloaddition reactions with alkynes to yield a variety of functionalized pyrroles. The transformation is proposed to occur in a stepwise manner via the intermediacy of a pyrrolium ylide, from which the electron-withdrawing group on the nitrogen atom undergoes an intramolecular 1,2-shift to produce the neutral pyrrole. This work demonstrates a yet unexplored facet of ynamide reactivity with great potential in heterocyclic chemistry.

Divergent Gold Catalysis: Unlocking Molecular Diversity through Catalyst Control

The catalyst-directed divergent synthesis, commonly termed as "divergent catalysis", has emerged as a promising technique as it allows chartering of structurally distinct products from common substrates simply by modulating the catalyst system. In this regard, gold complexes emerged as powerful catalysts as they offer unique reactivity profiles as compared to various other transition metal catalysts, primarily due to their salient electronic and geometrical features. Owing to the tunable soft π-acidic nature, gold catalysts not only evolved as superior contenders for catalyzing the reactions of alkynes, alkenes, and allenes but also, more intriguingly, have been found to provide divergent reaction pathways over other π-acid catalysts such as Ag, Pt, Pd, Rh, Cu, In, Sc, Hg, Zn, etc. The recent past has witnessed a renaissance in such examples wherein, by choosing gold catalysts over other transition metal catalysts or by fine-tuning the ligands, counteranions or oxidation states of the gold catalyst itself, a complete reactivity switch was observed. However, reviews documenting such examples are sporadic; as a result, most of the reports of this kind remained scattered in the literature, thereby hampering further development of this burgeoning field. By conceptualizing the idea of "Divergent Gold Catalysis (DGC)", this review aims to consolidate all such reports and provide a unified approach necessary to pave the way for further advancement of this exciting area. Based on the factors governing the divergence in product formation, an explicit classification of DGC has been provided. To gain a fundamental understanding of the divergence in observed reactivities and selectivities, the review is accompanied by mechanistic insights at appropriate places.

Copper-catalyzed alkynylation/annulation cascades of N-allyl ynamides: regioselective access to medium-sized N-heterocycles

A synthetic strategy based on sequential application of aza-Claisen rearrangement, C–H functionalization, C–N coupling and cyclization as key steps has been developed for the synthesis of 6-, 7-, 8-, and 9-membered rings N-heterocycles.

Synthesis of polysubstituted azetidines via cascade trifluoromethylation/cyclization of N-allyl ynamides

A cascade trifluoromethylation/cyclization of N-allyl sulfonylynamides is developed, providing a direct access to azetidine-fused tricyclic compounds at room temperature.

Asymmetric synthesis with ynamides: unique reaction control, chemical diversity and applications

Ynamides are among the most powerful building blocks in organic synthesis and have become invaluable starting materials for the construction of multifunctional compounds and challenging architectures that would be difficult to prepare otherwise. The rapidly growing popularity originates from the unique reactivity and ease of manipulation of the polarized ynamide triple bond, the advance of practical methods for making them, and the simplicity of storage and handling. These attractive features and the demonstration of numerous synthetic applications have spurred the development of intriguing asymmetric reaction strategies during the last decade. An impressive variety of chemo-, regio- and stereoselective carbon-carbon and carbon-heteroatom bond forming reactions with ynamides have been developed and now significantly enrich the toolbox of synthetic chemists. This review provides a comprehensive overview of asymmetric ynamide chemistry since 2010 with a focus on the general scope, current limitations, stereochemical reaction control and mechanistic aspects.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

Formal oxo- and aza-[3 + 2] reactions of α-enaminones and quinones: a double divergent process and the roles of chiral phosphoric acid and molecular sieves

A double divergent process has been developed for the reaction of α-enaminones with quinones through facile manipulation of catalyst and additive, leading to structurally completely different products. The two divergent processes, which involve formal aza- and oxo-[3 + 2] cycloaddition reactions, are mediated by chiral phosphoric acid and molecular sieves, respectively. While inclusion of phosphoric acid in the reaction switched the reaction pathway to favor the efficient formation of a wide range of N-substituted indoles, addition of 4 Å molecular sieves to the reaction switched the reaction pathway again, leading to enantioselective synthesis of 2,3-dihydrobenzofurans in excellent yields and enantioselectivities under mild conditions. Studies in this work suggest that the chiral phosphoric acid acts to lower the transition state energy and promote the formation of amide intermediate for the formal aza-[3 + 2] cycloaddition and the molecular sieves serve to facilitate proton transfer for oxo-[3 + 2] cycloaddition. The reactivity of α-enaminones is also disclosed in this work.

Deuteration of Formyl Groups via a Catalytic Radical H/D Exchange Approach

H/D exchange at formyl groups represents the straightforward approach to C-1 deuterated aldehydes. This transformation has been recently realized by transition metal and NHC carbene catalysis. Mechanistically, all these processes involve an ionic pathway. Herein we report a distinct photoredox catalytic, visible light mediated neutral radical approach. Selective control of highly reactive acyl radical in the energy barrier surmountable, reversible reaction enables driving the formation of deuterated products when an excess of D2O is employed. The power of the H/D exchange process has been demonstrated for not only aromatic aldehydes, but also aliphatic substrates, which have been difficult in transitional metal catalyzed H/D exchange reactions, and for selective late-stage deuterium incorporation into complex structures with uniformly high deuteration level (>90%).

Formyl-selective deuteration of aldehydes with D2O via synergistic organic and photoredox catalysis

Formyl-selective deuteration of aldehydes is of high interest for labeling purposes and for optimizing properties of drug candidates. Herein, we report a mild general method for formyl-selective deuterium labeling of aldehydes with D₂O, an inexpensive deuterium source, via a synergistic combination of light-driven, polyoxometalate-facilitated hydrogen atom transfer and thiol catalysis. This highly efficient, scalable reaction showed excellent deuterium incorporation, a broad substrate scope, and excellent functional group tolerance and selectivity and is therefore a practical method for late-stage modification of synthetic intermediates in medicinal chemistry and for generating libraries of deuterated compounds.

Formyl-selective deuteration of aldehydes with D₂O mediated by the synergistic combination of light-driven, polyoxometalate-facilitated HAT and thiol catalysis is reported.

Practical Synthesis of C-1 Deuterated Aldehydes Enabled by NHC Catalysis

The recent surge in applications of deuterated pharmaceutical agents has created an urgent demand for synthetic methods that efficiently generate deuterated building blocks. Here we show that N-heterocyclic carbenes (NHC) promote a reversible hydrogen-deuterium exchange (HDE) reaction with simple aldehydes, which leads to a practical approach to synthetically valuable C-1 deuterated aldehydes. The reactivity of the well-established NHC catalysed formation of Breslow intermediates from aldehydes is reengineered to overcome the overwhelmingly kinetically favorable, irreversible benzoin condensation reaction and achieve the critical reversibility to drive the formation of desired deuterated products when an excess of D₂O is employed. Notably, this operationally simple and cost-effective protocol serves as a general and truly practical approach to all types of 1-D-aldehydes including aryl, -alkyl and -alkenyl aldehydes and enables chemoselective late-stage deuterium incorporation into complex, native therapeutic agents and natural products with uniformly high levels (>95%) of deuterium incorporation for a total of 104 substrates tested.

Gold(iii)-catalyzed chemoselective annulations of anthranils with N-allylynamides for the synthesis of 3-azabicyclo[3.1.0]hexan-2-imines

We herein report the gold(iii)-catalyzed selective annulation of anthranils with N-allylynamides under mild conditions. By trapping the in situ-generated α-imino gold carbenes, 3-azabicyclo[3.1.0]hexan-2-imines were obtained in high synthetic efficiency. The reaction, which can be conducted in the gram scale, tolerates electron-rich and electron-deficient anthranils as well as a diverse set of functionalized ynamides (aryl- and alkyl-substituted terminal).

Annulation Cascades of N-Allyl- N-((2-bromoaryl)ethynyl)amides Involving C-H Functionalization

An annulation cascades of N-allyl- N-((2-bromoaryl)ethynyl)amides with terminal alkynes or 1,3-dicarbonyls involving C-H functionalization for producing 2,3-functionalized indoles has been first developed by means of Cu catalysis. The method is enabled by the formation of the ketenimine intermediates to deliver 2,3-disubstituted indoles through a sequence of aza-Claisen rearrangement, C-H functionalization, Ullmann C-N coupling, and cyclization.

Catalytic Enantioselective Ynamide Additions to Isatins: Concise Access to Oxindole Alkaloids

The highly enantioselective addition of terminal ynamides to a variety of isatins, catalyzed by a bisoxazolidine copper complex under mild, base-free reaction conditions, is described. The reaction is broad in scope, scalable, applicable to unprotected isatins, and provides efficient access to 3-hydroxyoxindoles carrying a tetrasubstituted chiral center with excellent yields and enantioselectivities. Synthetically versatile, multifunctional 3-hydroxyindolinones are obtained by hydration, partial hydrogenation, or hydroxyacyloxylation of the ynamide moiety at room temperature and exhaustive hydrogenation followed by reductive detosylation and spontaneous cyclization affords cinchonamidine alkaloids.

A Gold Carbene Manifold to Prepare Fused γ-Lactams by Oxidative Cyclisation of Ynamides

Gold-catalysed oxidative cyclisation reactions of ynamides offer great promise in γ-lactam synthesis but are limited by preferential over-oxidation to form α-keto imides. Evaluating the factors that might limit N-cyclisation pathways led to effective gold-catalysed conditions that allow access to different fused γ-lactams on changing the ynamide N-substituent and accommodate previously incompatible substitution patterns. New and efficient methods for the synthesis of functionalised 3-aryl indoles and cyclohepta[c]pyrrol-1-one derivatives are presented. These conditions illustrate the complementarity of gold catalysis to other metals.

Deoxygenative Deuteration of Carboxylic Acids with D2 O

We report a general, practical, and scalable means of preparing deuterated aldehydes from aromatic and aliphatic carboxylic acids with D₂ O as an inexpensive deuterium source. The use of Ph₃ P as an O-atom transfer reagent can facilitate the deoxygenation of aromatic acids, while Ph₂ POEt is a better O-atom transfer reagent for aliphatic acids. The highly precise deoxygenation of complex carboxylic acids makes this protocol promising for late-stage deoxygenative deuteration of natural product derivatives and pharmaceutical compounds.

Palladium/Rhodium Cooperative Catalysis for the Production of Aryl Aldehydes and Their Deuterated Analogues Using the Water-Gas Shift Reaction

A novel Pd/Rh dual-metallic cooperative catalytic process has been developed to effect the reductive carbonylation of aryl halides in moderate to good yield. In this reaction, water is the hydride source, and CO serves both as the carbonyl source and the terminal reductant through the water-gas shift reaction. The catalytic generation of the Rh hydride allows for the selective formation of highly hindered aryl aldehydes that are inaccessible through previously reported reductive carbonylation protocols. Moreover, aldehydes with deuterated formyl groups can be efficiently and selectively synthesized using D2 O as a cost-effective deuterium source without the need for presynthesizing the aldehyde.

Gold(I)-Catalyzed Formal Intramolecular Dehydro-Diels-Alder Reaction of Ynamide-ynes: Synthesis of Functionalized Benzo[ b]carbazoles

A gold-catalyzed cycloisomerization of ynamide-ynes via a formal dehydro-Diels-Alder reaction has been developed, providing an attractive route to diversely substituted benzo[ b]carbazoles. The reaction likely proceeds via regioselective attack of the pendant alkyne moiety to a keteniminium ion intermediate followed by benzannulation. The method offers several advantages such as high efficiency, mild reaction conditions, and wide functional group tolerance and serves as a highly useful complement to the thermal DDA reactions of ynamide-ynes.

Catalytic [1,3] O-to-C Rearrangement: Rapid Access to Bridged Bicyclic Systems

A catalytic [1,3] O-to-C rearrangement from enyne-ethers was developed for the rapid synthesis of diverse bridged bicyclic systems. In this reaction, a vinyl oxonium intermediate, generated in situ from enyne-ether, was the precursor for the [1,3] O-to-C rearrangement. This versatile protocol represents the first example of catalytic [1,3] O-to-C rearrangement based on ring-expansion strategy, enabling efficient access to bridged bicyclic scaffolds.

Metal-Free Cyclization of ortho-Nitroaryl Ynamides and Ynamines towards Spiropseudoindoxyls

An efficient metal-free cascade reaction between 1-dibromovinyl-2-nitro-substituted arenes and secondary amines results in the formation of polycyclic pseudoindoxyls in a single step. The reaction mechanism leading to these fused ring systems was investigated, and is believed to involve the initial formation of nitroarylated ynamines/ynamides. These intermediates cycloisomerize towards N-alkenyl-tethered 2-aminoisatogens via a carbene intermediate as demonstrated by QTAIM (quantum theory of atoms in molecules) and ELF (electron localization function) analysis. A subsequent intramolecular dipolar cycloaddition afforded the title compounds.

One-Pot Synthesis of Deuterated Aldehydes from Arylmethyl Halides

A facile, one-pot approach for synthesizing deuterated aldehydes from arylmethyl halides was developed using D₂O as the deuterium source. The efficient process is realized by a sequence of formation, H/D exchange, and oxidation of pyridinium salt intermediates. The mild and air-compatible reaction conditions enable efficient synthesis of diverse deuterated aldehydes with high deuterium incorporation.