Diastereoselective synthesis of functionalized spiroindolines via intramolecular ipso-iodocyclization/nucleophile addition cascade reactions of indole-tethered ynones

Herein, we describe a highly diastereoselective approach for synthesizing polyfunctionalized spiroindolines from indolyl-ynones involving an ipso-iodocyclization/nucleophile addition cascade. The developed strategy allows the formation of a spirocyclic core and the installation of two functional groups in a single operation. Also this strategy is accompanied by the generation of two C-C and one C-I bonds and two contiguous stereocenters.

Ag(I)-catalyzed dearomatizing spirocyclization/nucleophile addition cascade reactions of indole-tethered ynones

Ag(I)-catalyzed highly diastereoselective construction of divergent spiroindolines is disclosed herein. The approach proceeds via dearomatizing spirocyclization of indole-tethered ynones followed by C-nucleophile or hydride trapping. The established strategy is accompanied by the generation of two new C-C bonds and two contiguous stereocenters. This strategy features a broad range of (hetero)arenes as C-nucleophiles and excellent diastereoselectivity.

Electrosynthesis of Spiro-indolenines via Dearomative Arylation of Indoles in Batch and Continuous Flow

An electrosynthesis of spiro-indolenines in batch and continuous flow was achieved through dearomative arylation of indoles with good functional group compatibility. User-friendly undivided cells were used under catalyst- and oxidant-free conditions. Moreover, the use of a flow electrolysis cell gave high daily productivity and excellent scale-up potential under less supporting electrolyte and higher substrate concentration conditions.

Application of Metal-Free Dearomatization Reaction as a Sustainable Strategy to Direct Access Complex Cyclic Compounds

The highly efficient construction of complicated heterocyclic frameworks in an atom- and step-economic manner is still one of the cores of synthetic chemistry. Dearomatization reactions show the unique advantage for the construction of functionalized heterocycles and have attracted widespread attention over the past two decades. The metal-free approach has proved to be a green and sustainable paradigm for the synthesis of spirocyclic, polycyclic and heterocyclic scaffolds, which are widely present in natural products and bioactive molecules. In this review, the advances in the recent six years (2017-2023) in metal-free dearomatization reactions are highlighted. Emphasis is placed on developments in the field of organo-catalyzed dearomatization reactions, oxidative dearomatization reactions, Brønsted acid- or base-promoted dearomatization reactions, photoredox-catalyzed dearomatization reactions, and electrochemical oxidation dearomatization reactions.

Insights into Synergistic Effects of Counterion and Ligand on Diastereoselectivity Switch in Gold-Catalyzed Post-Ugi Ipso-Cyclization

The concept of diastereoselectivity switch in gold catalysis is investigated, which primarily depends on the effects of ligand and counterion. The origins of gold-catalyzed post-Ugi ipso-cyclization for the diastereoselective synthesis of spirocyclic pyrrol-2-one-dienone have been explored with density functional theory calculations. The reported mechanism emphasized the importance of the cooperation of ligand and counterion in diastereoselectivity switch, leading to the stereocontrolling transition states. Furthermore, the nonbonding interactions primarily between the catalyst and the substrate play a significant role in the cooperation of ligand and counterion. This work would be useful to further understand the reaction mechanism of gold-catalyzed cyclization and the effects of ligand and counterion.

Riboflavin Photocatalyzed Dearomative Spiro-Etherification of Naphthols

The dearomative spiro-etherification of naphthols is achieved using catalytic amounts of riboflavin tetracetate (RFTA) as a photosensitizer and molecular oxygen as a terminal oxidizing agent under blue light (440 nm) irradiation in the presence of acid. The presence of acid increases the photooxidation power of RFTA and facilitates the dearomatization reaction.

Nitro-Spirocyclization of Biaryl Ynones with tert-Butyl Nitrite: Access to NO2-Substituted Spiro[5,5]trienones

A metal/peroxide-free involved simple cascade 6-exo-trig spirocyclization of tert-butyl nitrite with biaryl ynones has been finished, which resulted in various NO₂-modified spiro[5,5]trienones with good regioselectivity/yields. A variety of scaled-up experiments, reduction/epoxidation operations, and mechanistic studies were performed to verify the merits and spirocyclization process of this radical system. Finally, the structure of the spirocycles was confirmed by single-crystal X-ray diffraction.

Synthesis of Spiro[indole-3,3'-pyrrolidine]-2'-(thi)ones

Spiro[indole-3,3'-pyrrolidine]-2'-ones were synthesized via one-pot chloroformylation-dearomatizing spirocyclization of tryptamine derivatives. Moreover, the "thio" equivalent spiro[indole-3,3'-pyrrolidine]-2'-thiones, for which the synthesis and properties were previously unreported, were synthesized. The procedures are easily implemented, have a broad scope, and are transition-metal-free and cheap. To demonstrate the utility of the synthetic methodology, the spiro[indole-3,3'-pyrrolidine]-2'-ones were converted into heterocyclic scaffolds, such as an optically active spiroindoline and spirooxindole.

PIFA-mediated selenylative spirocyclization of indolyl ynones: facile access to selenated spiro[cyclopentenone-1,3'-indoles]

A fast selenylative spirocyclization of indolyl ynones mediated by PIFA has been developed. This transformation was enabled by the reactive RSeOCOCF₃ species generated in situ from diselenides with PIFA, involving an electrophilic dearomative cascade cyclization. This protocol provides a facile and efficient method for the synthesis of selenated spiro[cyclopentenone-1,3'-indoles] and tolerates broad functional groups.

Synthesis of Spiro[5.5]trienones- and Spiro[4.5]trienones-Fused Selenocyanates via Electrophilic Selenocyanogen Cyclization and Dearomative Spirocyclization

A practical strategy for the synthesis of spiro[5.5]trienones-fused selenocyanates and spiro[4.5]trienones-fused selenocyanates through electrophilic selenocyanogen cyclization and dearomative spirocyclization is reported. This approach was conducted under mild conditions with broad substrate scope and good functional group tolerance. The utility of this procedure is exhibited in the late-stage functionalization of nature product and drug molecules.

Synthesis of spiroindolenine-cyclopentenedione skeletons and their chemical behaviours: the first example of a lactone-type spiroindolenine structure

A manageable, one-pot, and high-yield protocol for synthesising highly reactive spiroindolenine derivatives is reported. Spiroindolenines are furnished by a reaction between DCC (dicyclohexylcarbodiimide) and indole-3-butenoic acid derivatives. The protocol proposed here involves the construction of a carbon-carbon bond through intramolecular domino cyclisation. The reaction mechanism for spirocyclisation is discussed; both NMR and X-ray analysis were used to verify the structure of spiroindolenine. The applied strategy allowed the formation of spiroindolenine with a dione substructure, which is an unknown compound with a spirocyclic nucleus. Further reactions of spiroindolenines with di-amines, a primary amine, and alcohol have been reported, and new types of indole derivatives, such as indoloquinoxalines, where the spirocentre atom undergoes a nucleophilic attack, are yielded.

Selectivity, Speciation, and Substrate Control in the Gold-Catalyzed Coupling of Indoles and Alkynes

A convenient and mild protocol for the gold-catalyzed intermolecular coupling of substituted indoles with carbonyl-functionalized alkynes to give vinyl indoles is reported. This reaction affords 3-substituted indoles in high yield, and in contrast to the analogous reactions with simple alkynes which give bisindolemethanes, only a single indole is added to the alkyne. The protocol is robust and tolerates substitution at a range of positions of the indole and the use of ester-, amide-, and ketone-substituted alkynes. The use of 3-substituted indoles as substrates results in the introduction of the vinyl substituent at the 2-position of the ring. A combined experimental and computational mechanistic study has revealed that the gold catalyst has a greater affinity to the indole than the alkyne, despite the carbon–carbon bond formation step proceeding through an η²(π)-alkyne complex, which helps to explain the stark differences between the intra- and intermolecular variants of the reaction. This study also demonstrated that the addition of a second indole to the carbonyl-containing vinyl indole products is both kinetically and thermodynamically less favored than in the case of more simple alkynes, providing an explanation for the observed selectivity. Finally, a highly unusual gold-promoted alkyne dimerization reaction to form a substituted gold pyrylium salt has been identified and studied in detail.

Indole-ynones as Privileged Substrates for Radical Dearomatizing Spirocyclization Cascades

Indole-ynones have been established as general substrates for radical dearomatizing spirocyclization cascade reactions. Five distinct and varied synthetic protocols have been developed─cyanomethylation, sulfonylation, trifluoromethylation, stannylation and borylation─using a variety of radical generation modes, ranging from photoredox catalysis to traditional AIBN methods. The simple and easily prepared indole-ynones can be used to rapidly generate diverse, densely functionalized spirocycles and have the potential to become routinely used to explore radical reactivity. Experimental and computational investigations support the proposed radical cascade mechanism and suggest that other new methods are now primed for development.

Copper catalyzed dearomatization by Michael-type addition of indolyl ynones: divergent synthesis of functionalized spiroindoles and cyclopenta[c]quinolin-3-ones

Described herein is a copper-catalyzed multifunctionalization of indolyl ynones, allowing the synthesis of dihalogen-substituted spiroindoles. This Cu catalysis is also applicable to the synthesis of cyclopenta[c]quinolin-3-ones via decarbonylation.

Visible Light-Promoted Selenylative Spirocyclization of Biaryl Ynones toward the Formation of Selenated Spiro[5.5]trienones

A visible-light induced dearomative cascade cyclization of biaryl ynones with diselenides under photocatalyst and external additive-free conditions has been explored, giving a series of selenated spiro[5.5]trienones in moderate to good...

Chemoselective Synthesis of Sulfenylated Spiroindolenines from Indolyl-ynones via Organosulfenyl Chloride-Mediated Dearomatizing Spirocyclization

A metal-free sulfenylation/spirocyclization of indolyl-ynones realized by organosulfenyl chloride, generated in situ from the reaction of disulfides and PhICl₂, is presented. This cascade one-pot process enables a chemoselective synthesis of diverse sulfenylated spiroindolenines depending on the substituent pattern at the two-position of indolyl-ynones.

Metal-Free Sulfonylative Spirocyclization of Indolyl-ynones via Insertion of Sulfur Dioxide: Access to Sulfonated Spiro[cyclopentenone-1,3'-indoles]

A three-component sulfonylative spirocyclization of indolyl ynones with aryldiazonium salts and a sulfur dioxide surrogate of DABCO·(SO₂)₂ has been developed, providing a range of sulfonated spiro[cyclopentenone-1,3'-indoles] in moderate to good yields. This transformation was initiated by an in situ generated arylsulfonyl radical and proceeded efficiently under metal-free conditions, involving a radical-induced dearomative cascade cyclization accompanied by the insertion of sulfur dioxide. This protocol provides an efficient and convenient method to access sulfonated spiroindolenines, and tolerant various functional groups.

A Thiol-Mediated Three-Step Ring Expansion Cascade for the Conversion of Indoles into Functionalized Quinolines

An operationally simple, high yielding three-step cascade process is described for the direct conversion of indole-tethered ynones into functionalized quinolines. A single "multitasking" thiol reagent is used to promote a three-step dearomatizing spirocyclization, nucleophilic substitution, and one-atom ring expansion reaction cascade under remarkably mild conditions. In addition, a novel route to thio-oxindoles is described, which was discovered by serendipity.

Heteroarene-tethered Functionalized Alkyne Metamorphosis

Heteroarene-tethered functionalized alkynes are multipotent synthons in organic chemistry. This detailed Review described herein offers a thorough discussion of the metamorphosis of heteroarene-tethered functionalized alkynes, an area which has earned much attention over the past decade in the straightforward synthesis of architecturally complex heterocyclic scaffolds in atom and step economic manner. Depending upon the variety of functionalized alkynes, this Review is divided into multiple sections. Amongst the vast array of synthetic transformations covered, dearomatizing spirocyclizations and cascade spirocyclization/rearrangement are of great interest. Synthetic transformations involving the heteroarene-tethered functionalized alkynes with scope, challenges, limitations, mechanism, their application in the total synthesis of natural products and future perceptions are surveyed.

Gold-Catalyzed Spirocyclization Reactions of N-Propargyl Tryptamines and Tryptophans in Aqueous Media

N-Propargyl tryptamine and tryptophan derivatives that are readily available from both synthetic and biocatalytic approaches undergo gold-catalyzed dearomative cyclizations in aqueous media to the corresponding spirocyclic derivatives. In addition to the efficiency of the method, operating in aqueous media affords a selective entry to C2-unsubstituted spiroindolenines that have long remained unattainable by Au(I) catalysis. Moderate to excellent yields of the desired spirocyclic products bearing various substituents were obtained.

A dearomative ipso-iodocyclization/desymmetrization sequence leading to optically active tricyclic piperazine scaffolds

Dearomative ipso-iodocyclization of 4-(1-ethoxyethoxy)-N-propargylanilines leading to 1-azaspiro[4.5]deca-3,6,9-trien-8-ones has been developed. This reaction is characterized by the yield of fewer toxic byproducts and is conducted by a more user-friendly protocol compared to other reported methods. The synthetic application of these spiro products was demonstrated by transformation of the iodo component into other functionalities. The desymmetrization of the resulting cyclohexadienones was also achieved by the diastereoselective aza-Michael addition of internal amino acid moieties to yield tricyclic piperazine scaffolds with two newly formed contiguous chiral centers.

Copper-Catalyzed Trifluoromethylation of Ynones Coupled with Dearomatizing Spirocyclization of Indoles: Access to CF3-Containing Spiro[cyclopentane-1,3'-indole]

A one-pot protocol for the Cu(I)-catalyzed difunctionalization of indolyl ynones has been achieved via trifluoromethylation of alkyne followed by dearomatizing spirocyclization of indoles. This cascade process enables constructing diverse CF₃-containing spiro[cyclopentane-1,3'-indole] scaffolds in moderate to excellent yields which have challenging quaternary spirocyclic carbon and tetrasubstituted alkenes.

Visible-Light-Promoted Selenylative Spirocyclization of Indolyl-ynones toward the Formation of 3-Selenospiroindolenine Anticancer Agents

A metal-free and efficient visible-light-induced spirocyclization of indolyl-ynones with diselenides at room temperature under air atmosphere to prepare 3-selenospiroindolenines in moderate to good yields has been developed. The resulting products were tested for in vitro anticancer activity by MTT assay, and compounds 3 c and 3 e showed potent cancer cell-growth inhibition activities.

Direct Synthesis of Sulfonylated Spiro[indole-3,3'-pyrrolidines] by Silver-Mediated Sulfonylation of Acrylamides Coupled with Indole Dearomatization

A dearomative tandem spiro-cyclization reaction of N-[(1H-indol-3-yl)methyl]methacrylamide derivatives with sulfinate sodium in the presence of AgNO₃ and K₂CO₃ is reported, which produced sulfonylated spiro[indole-3,3'-pyrrolidines] in medium to excellent yields. The characteristics of this transformation contain good functional group tolerance and ease of operation.

Enantioselective Synthesis of Spiroindolines via Cascade Isomerization/Spirocyclization/Dearomatization Reaction

The spiroindoline skeleton featured with 2,7-diazaspiro[4.4]nonane exists in various structurally intricate and biologically active monoterpene indole alkaloids. A catalytic asymmetric cascade enamine isomerization/spirocyclization/dearomatization succession to construct the spiroindoline was developed, which employed the indolyl dihydropyridine as a substrate under catalysis of the chiral phosphoric acid. This cascade reaction provided various spiroindolines in both diastereoselective and enantionselective fashions.

Synthesis of 3-phosphinoylbenzofurans via electrophilic phosphination cyclization

An expedient strategy for the synthesis of 3-phosphinoylbenzofurans through a cascade electrophilic phosphination cyclization has been presented.

Synthesis of Cyclobutane-Fused Angular Tetracyclic Spiroindolines via Visible-Light-Promoted Intramolecular Dearomatization of Indole Derivatives

An intramolecular dearomatization of indole derivatives based on visible-light-promoted [2+2] cycloaddition was achieved via energy transfer mechanism. The highly strained cyclobutane-fused angular tetracyclic spiroindolines, which were typically unattainable under thermal conditions, could be directly accessed in high yields (up to 99%) with excellent diastereoselectivity (>20:1 dr) under mild conditions. The method was also compatible with diverse functional groups and amenable to flexible transformations. In addition, DFT calculations provided guidance on the rational design of substrates and deep understanding of the reaction pathways. This process constituted a rare example of indole functionalization by exploiting visible-light-induced reactivity at the excited states.

Controllable synthesis of 3-iodo-2H-quinolizin-2-ones and 1,3-diiodo-2H-quinolizin-2-onesviaelectrophilic cyclization of azacyclic ynones

An efficient approach was developed to divergently synthesize 3-ipdo-2H-quinolizin-2-ones and 1,3-diiodo-2H-quinolizin-2-ones from azacyclic ynones with high regioselectivity under metal-free, room temperature conditions in air.