Modular Synthesis of Polycyclic Alkaloid Scaffolds via an Enantioselective Dearomative Cascade

Enantioselective Organocatalyzed Cascade Dearomatizing Spirocycloaddition Reactions of Indole-Ynones

An intramolecular organocatalytic cascade dearomatizing spirocycloaddition reaction of indole-ynone compounds containing O-silyl-naphthol substituents has been developed with the use of a chiral bifunctional thiourea. This process was able to provide various structurally diverse polycyclic spiroindolines in high yields (up to 98%) with excellent stereoselectivities (>20:1 dr, up to 98% ee) involving the formation of carbonylvinylidene ortho-quinone methide intermediates.

Cascade hydroarylation/Diels-Alder cycloaddition of alkynylindoles with electron-deficient alkynes and alkenes

Herein, a novel cascade gold(I)-catalyzed hydroarylation of alkynylindoles and subsequent Diels-Alder cycloaddition with electron-deficient alkynes and alkenes is described. A variety of azepino-fused hydrocarbazoles and carbazoles were obtained in moderate to excellent yields. Key features of this methodology are low catalyst loadings, high regioselectivity, broad functional group tolerances, access to important heterocycles, and 100% atom economy.

Gold(I)-Catalyzed Cascade Cyclization of Alkynyl Indoles for the Stereoselective Construction of the Quaternary Carbon Center of Akuammiline Alkaloids

A gold-catalyzed cyclization reaction of alkynyl-indoles has been developed for the stereoselective construction of the quaternary carbon center of fused indolines. This reaction efficiently produces fused indolines via diastereoselective 6-endo-dig cyclization controlled by a bulky TIPS group, followed by nucleophilic attack of the carboxy group on the resulting imine. The lactone moiety of the fused indoline can be reductively cleaved to produce a tricyclic indoline, which could be useful for the synthesis of akuammiline alkaloids.

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.

Synthesis of spirocyclic 1,2-diamines by dearomatising intramolecular diamination of phenols

The stereocontrolled synthesis of complex spirotricyclic systems containing an embedded syn-1,2-diaminocyclohexane unit is reported, based upon a dearomatising oxidation of phenols bearing pendant ureas capable of acting as double nucleophiles. This complexity-generating transformation yields products with rich functionality suitable for application in the synthesis of potentially bioactive compounds.

Access to Azepino-Annulated Benzo[c]carbazoles Enabled by Gold-Catalyzed Hydroarylation of Alkynylindoles and Subsequent Oxidative Cyclization

Herein, we report a facile and efficient synthetic method to construct azepino[1,2-a]indoles through a novel gold(I)-catalyzed intramolecular hydroarylation of alkynylindoles. A wide range of functional groups can be well tolerated in this transformation, and the corresponding highly functionalized azepino[1,2-a]indole skeletons were obtained in moderate to excellent yields. Subsequent oxidation of the products gave the interesting and valuable polycyclic carbazoles, which were widely used as the key building blocks in materials science.

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.

Synthesis of medium-ring lactams and macrocyclic peptide mimetics via conjugate addition/ring expansion cascade reactions

A novel conjugate addition/ring expansion (CARE) cascade reaction sequence is reported that enables medium-sized ring and macrocyclic bis-lactams to be prepared from primary amines and cyclic imides. The reactions are simple to perform, generally high yielding, and very broad in scope, especially with respect to the primary amine component. CARE reactions can also be performed iteratively, enabling β-peptoid-based macrocyclic peptide mimetics to be ‘grown’ via well controlled, sequential 4-atom ring expansion reactions, with the incorporation of varied functionalised amines during each iteration.

A conjugate addition/ring expansion (CARE) cascade reaction sequence is reported that enables medium-sized ring and macrocyclic bis-lactams to be prepared from primary amines and cyclic imides.

Copper-catalyzed cascade reaction of tryptamines with diazo compounds to access hexahydropyrroloindoline derivatives

A Cu-catalyzed cyclopropanation/ring-opening/iminium cyclization of tryptamine derivatives with donor–acceptor diazo compounds is developed to furnish pyrroloindolines, creating three consecutive stereogenic centers.

Dinuclear Zinc Catalyzed Enantioselective Dearomatization [3+2] Annulation of 2-Nitrobenzofurans and 2-Nitrobenzothiophenes

The application of dinuclear zinc catalysts in a dearomatization reaction has been developed. Catalytic asymmetric dearomatization [3+2] annulations of 2-nitrobenzofurans or 2-nitrobenzothiophenes with CF₃ -containing N-unprotected isatin-derived azomethine ylides catalyzed by dinuclear zinc catalysts are realized with excellent diastereomer ratios (dr) of >20 : 1 and enantiomeric excess (ee) of up to 99 %. This protocol provides a practical, straightforward access to structurally diverse pyrrolidinyl spirooxindoles containing a 2,3-fused-dihydrobenzofuran (or dihydrobenzothiphene) moiety, and four contiguous stereocenters. Reactions can be performed on a gram scale. The absolute configuration of products is confirmed by X-ray single crystal structure analysis, and a possible mechanism is proposed.

Silver-Catalyzed Asymmetric Dearomatization of Electron-Deficient Heteroarenes via Interrupted Barton-Zard Reaction

Herein we report a catalytic asymmetric dearomatization reaction of electron-deficient heteroarenes with α-substituted isocyanoacetates through an interrupted Barton-Zard reaction. A range of optically active pyrrolo[3,4-b]indole derivatives was obtained in good yields (up to 97 %) with high stereoselectivities (up to >20:1 dr and 97 % ee), using a catalytic system consisting of a cinchona-derived amino-phosphine and silver oxide. This reaction features wide substrate scope and mild conditions, and provides a new strategy for developing asymmetric dearomatization reactions.

Total Synthesis of (+)-Alsmaphorazine C and Formal Synthesis of (+)-Strictamine: A Photo-Fries Approach

A bioinspired photo-Fries/imine capture cascade reaction was developed in continuous-flow mode, which facilitated the rapid construction of a series of diversely functionalized 2,7-heterocycle-fused tetrahydrocarbazoles, the ubiquitous core structures embedded in strychnos and akuammiline-type monoterpene indole alkaloids. The synthetic utility of this novel method has been preliminarily explored by the first total synthesis of (+)-alsmaphorazine C and formal synthesis of (+)-strictamine in a concise and efficient manner.

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.

Conjugated ynones in catalytic enantioselective reactions

Conjugated ynones are easily accessible feedstock and the existence of an alkyne bond endows ynones with different attractive reactivities, thus making them unique substrates for catalytic asymmetric reactions. Their compatibility under organocatalytic, metal-catalyzed as well as cooperative catalytic conditions has resulted in numerous enantioselective transformations. Importantly, conjugated ynones can act as nucleophiles or electrophiles, and serve as easily accessed synthons for different cyclization pathways. This review summarizes the recent literature examples of the catalytic reactions of conjugated ynones and related compounds such as alkyne conjugated α-ketoesters, and classifies these reaction types alongside mechanistic insights whenever possible. We aim to trigger more intensive research in the future to render the asymmetric transformation of ynones as a common and reliable tool for asymmetric synthesis.

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.

Catalytic asymmetric preparation of pyrroloindolines: strategies and applications to total synthesis

Pyrroloindolines are widely present in natural products. In this review, we summarize state-of-the-art of catalytic asymmetric synthesis of pyrroloindolines, as well as related applications to natural products total synthesis.

Recent Advances in Construction of Polycyclic Natural Product Scaffolds via One-Pot Reactions Involving Alkyne Annulation

Polycyclic scaffolds are omnipresent in natural products and drugs, and the synthetic strategies and methods toward construction of these scaffolds are of particular importance. Compared to simple cyclic ring systems, polycyclic scaffolds have higher structure complexity and diversity, making them suitable for charting broader chemical space, yet bringing challenges for the syntheses. In this review, we surveyed progress in the past decade on synthetic methods for polycyclic natural product scaffolds, in which the key steps are one-pot reactions involving intermolecular or intramolecular alkyne annulation. Synthetic strategies of selected polycyclic carbocycles and heterocycles with at least three fused, bridged, or spiro rings are discussed with emphasis on the synthetic efficiency and product diversity. Recent examples containing newly developed synthetic concepts or toolkits such as collective and divergent total synthesis, gold catalysis, C–H functionalization, and dearomative cyclization are highlighted. Finally, several “privileged synthetic strategies” for “privileged polycyclic scaffolds” are summarized, with discussion of remained challenges and future perspectives.

Asymmetric Dearomative Cascade Multiple Functionalizations of Activated N-Alkylpyridinium and N-Alkylquinolinium Salts

An enantioselective cascade reaction of N-alkylpyridinium and -quinolinium salts with o-hydroxybenzylideneacetones to access fused polyheterocycles through cross dienamine-mediated addition followed by trapping of the dearomatized enamine-type intermediates and aminal formation has been developed. A cascade assembly of N-benzyl-4-methylpyridinium salt and cyclic 2,4-dienones is further disclosed to give bridged frameworks via repetitive dearomatization and aromatization activation.