A Homo-Mannich Reaction Strategy Enables Collective Access to Ibophyllidine, Aspidosperma, Kopsia, and Melodinus Alkaloids

We report here a homo-Mannich reaction of cyclopropanol with an iminium ion, generated by an asymmetric allylic dearomatization of indole, to construct a tricyclic hydrocarbazole core, which is shared by a variety of monoterpenoid indole alkaloids across families. Through this approach, an all-carbon quaternary stereogenic center as well as an allyl and a ketone group were installed. Using this functionalized hydrocarbazole as the structural platform, D ring and E rings of different sizes (i.e., five-, six-, and seven-membered) were successively or simultaneously assembled, leading to a collective asymmetric synthesis of seven alkaloids belonging to the ibophyllidine, Aspidosperma, Kopsia, and Melodinus alkaloid families.

Photoredox-catalyzed diastereoselective dearomative prenylation and reverse-prenylation of electron-deficient indole derivatives

Prenylated and reverse-prenylated indolines are privileged scaffolds in numerous naturally occurring indole alkaloids with a broad spectrum of important biological properties. Development of straightforward and stereoselective methods to enable the synthesis of structurally diverse prenylated and reverse-prenylated indoline derivatives is highly desirable and challenging. In this context, the most direct approaches to achieve this goal generally rely on transition-metal-catalyzed dearomative allylic alkylation of electron-rich indoles. However, the electron-deficient indoles are much less explored, probably due to their diminished nucleophilicity. Herein, a photoredox-catalyzed tandem Giese radical addition/Ireland-Claisen rearrangement is disclosed. Diastereoselective dearomative prenylation and reverse-prenylation of electron-deficient indoles proceed smoothly under mild conditions. An array of tertiary α-silylamines as radical precursors is readily incorporated in 2,3-disubstituted indolines with high functional compatibility and excellent diastereoselectivity (>20:1 d.r.). The corresponding transformations of the secondary α-silylamines provide the biologically important lactam-fused indolines in one-pot synthesis. Subsequently, a plausible photoredox pathway is proposed based on control experiments. The preliminary bioactivity study reveals a potential anticancer property of these structurally appealing indolines.

Asymmetric Hydrogenation of Racemic Allylic Alcohols via an Isomerization-Dynamic Kinetic Resolution Cascade

Prochiral racemic allylic alcohols are converted to enantioenriched chiral alcohols bearing adjacent stereocenters catalyzed by a diamine diphosphine Ru complex in the presence of tBuOK. The protocol features a broad substrate scope (56 examples) and high diastereo- and enantioselectivities (up to >99:1 dr, >99% ee) and could be applied to the synthesis of enantioenriched chromane and indane compounds. Mechanistic studies suggest that the reaction proceeds via tBuOK-promoted allylic alcohol isomerization followed by Ru-catalyzed hydrogenative dynamic kinetic resolution.

Synthesis of C5-Allylindoles through an Iridium-Catalyzed Asymmetric Allylic Substitution/Oxidation Reaction Sequence of N-Alkyl Indolines

Iridium/Brønsted acid cooperative catalyzed asymmetric allylic substitution reactions at the C5 position of indolines have been reported for the first time. The highly efficient protocol allows rapid access to various C5-allylated products in good to high yields (48-97%) and enantioselectivities (82% to >99% ee) with wide functional group tolerance. The transformations allow not only the formation of C5-allylindoline derivatives but also the synthesis of C5-allylindole analogues in good yields and excellent stereoselectivities via an allylation/oxidation reaction sequence.

Rhodium-Catalyzed Chemodivergent Regio- and Enantioselective Allylic Alkylation of Indoles

The control of C3/N1 chemoselectivity in indole alkylation with the same electrophiles is still challenging. An Rh/bisoxazolinephosphane-catalyzed chemodivergent regio- and enantioselective allylic alkylation of indoles was developed. Chiral C3- and N1-allylindoles can be selectively obtained with high branched/linear ratio and up to 99 % ee by changing the counteranion of Rh, the allylic carbonate, the reaction temperature, and the ligand.

Palladium-catalyzed allylic alkylation dearomatization of β-naphthols and indoles with gem-difluorinated cyclopropanes

A palladium-catalyzed allylic alkylation dearomatization of β-naphthols and indoles with gem-difluorinated cyclopropanes has been developed for the first time.

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.

Allylic and Allenylic Dearomatization of Indoles Promoted by Graphene Oxide by Covalent Grafting Activation Mode

The site-selective allylative and allenylative dearomatization of indoles with alcohols was performed under carbocatalytic regime in the presence of graphene oxide (GO, 10 wt % loading) as the promoter. Metal-free conditions, absence of stoichiometric additive, environmentally friendly conditions (H₂ O/CH₃ CN, 55 °C, 6 h), broad substrate scope (33 examples, yield up to 92 %) and excellent site- and stereoselectivity characterize the present methodology. Moreover, a covalent activation model exerted by GO functionalities was corroborated by spectroscopic, experimental and computational evidences. Recovering and regeneration of the GO catalyst through simple acidic treatment was also documented.

Iridium-Catalyzed Enantioselective Intermolecular Indole C2-Allylation

The enantioselective intermolecular C2-allylation of 3-substituted indoles is reported for the first time. This directing group-free approach relies on a chiral Ir-(P, olefin) complex and Mg(ClO4 )2 Lewis acid catalyst system to promote allylic substitution, providing the C2-allylated products in typically high yields (40-99 %) and enantioselectivities (83-99 % ee) with excellent regiocontrol. Experimental studies and DFT calculations suggest that the reaction proceeds via direct C2-allylation, rather than C3-allylation followed by in situ migration. Steric congestion at the indole-C3 position and improved π-π stacking interactions have been identified as major contributors to the C2-selectivity.

Palladium(0)-Catalyzed Intermolecular Asymmetric Cascade Dearomatization Reaction of Indoles with Propargyl Carbonate

An intermolecular asymmetric cascade dearomatization reaction of indole derivatives with propargyl carbonate was developed. The challenges associated with both the chemoselectivity between the carbon and nitrogen nucleophile and the enantioselective control during the formation of an all-carbon quaternary stereogenic center were well addressed by a Pd catalytic system derived from the Feringa ligand. A series of enantioenriched multiply substituted fused indolenines were provided in good yields (71-86 %) with excellent enantioselectivity (91-96 % ee) and chemoselectivity (3/4>19:1 in most cases).

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.

Palladium-catalyzed N1-selective allylation of indoles with allylic alcohols promoted by titanium tetraisopropoxide

The N1-selective allylation of indoles with allylic alcohols has been accomplished by synergistic functions of palladium catalysts and titanium tetraisopropoxide.

Tryptophol and derivatives: natural occurrence and applications to the synthesis of bioactive compounds

This report presents some fundamental aspects related to the natural occurrence and bioactivity of tryptophol as well as the synthesis of tryptophols and their utilization for the preparation of naturally occurring alkaloid metabolites embedding the indole system.

Catalytic asymmetric dearomatization (CADA) reaction-enabled total synthesis of indole-based natural products

The recent enantioselective total syntheses of natural products enabled by catalytic asymmetric dearomatization reactions of indole derivatives are presented.

Iridium-Catalyzed Asymmetric Allylic Substitution Reactions

In this review, we summarize the origin and advancements of iridium-catalyzed asymmetric allylic substitution reactions during the past two decades. Since the first report in 1997, Ir-catalyzed asymmetric allylic substitution reactions have attracted intense attention due to their exceptionally high regio- and enantioselectivities. Ir-catalyzed asymmetric allylic substitution reactions have been significantly developed in recent years in many respects, including ligand development, mechanistic understanding, substrate scope, and application in the synthesis of complex functional molecules. In this review, an explicit outline of ligands, mechanism, scope of nucleophiles, and applications is presented.

A catalytic N-deacylative alkylation approach to hexahydropyrrolo[2,3-b]indole alkaloids

A versatile unprecedented strategy to diversely functionalized hexahydropyrrolo[2,3-b]indole alkaloids is described in high chemical yields. The synthesis features a key Pd(0)-catalyzed deacylative alkylation of N-acyl 3-substituted indoles using only 1 mol% of Pd(PPh3)4. The scope of this methodology is further defined in the asymmetric synthesis of pyrroloindolines using a diastereoselective approach.

A catalytic allylic cation-induced intermolecular allylation-semipinacol rearrangement

A catalytic intermolecular semipinacol rearrangement induced by allylic carbocations has been realized. This tandem reaction is highly efficient under the catalysis of ZnBr₂, generating a wide range of α-homoallyl substituted ketones which contain all-carbon quaternary centres in good to excellent yields (up to 98%) with moderate to high diastereoselectivities (up to >20 : 1). Synthetic application of this novel methodology in the construction of core structures of natural products is also reported.

Access to 3-Prenylated Oxindoles by α-Regioselective Prenylation: Application to the Synthesis of (±)-Debromoflustramine E

The development of a rapid, highly efficient, and one-pot synthesis of C3-α-prenylated oxindoles with simple reagents is described. The process is based on zinc-mediated α-regioselective prenylation of 3-acylidene-oxindole with commercially available prenyl bromide using inexpensive CeCl₃ as the catalyst. The new transformation tolerates a wide range of 3-acylidene-oxindoles, providing easy access to a variety of functionalized 3-prenylated oxindoles. The synthetic utility of the approach is verified by formal synthesis of the flustramine family alkaloid (±)-debromoflustramine E.

Palladium(0)-Catalyzed Intermolecular Asymmetric Allylic Dearomatization of Polycyclic Indoles

An intermolecular Pd-catalyzed allylic dearomatization reaction of polycyclic indoles with substituted allylic carbonates was realized in the presence of a newly synthesized chiral phosphoramidite ligand. Various polycyclic indoline and indolenine derivatives were successfully synthesized in excellent yields (up to 99%) with excellent enantioselectivity (up to 98% ee). The obtained products could undergo versatile transformations, increasing the application potential of the method in organic synthesis.

Direct C3-arylations of 2-indolylmethanols with tryptamines and tryptophols via an umpolung strategy

A Brønsted acid-catalyzed direct C3-arylation of 2-indolylmethanols with tryptamines and tryptophols has been established, leading to a series of potentially bioactive 2,3′-biindole derivatives with a broad substrate scope and generally good yields (38 examples, up to 96% yield).

Ir-Catalyzed reactions in natural product synthesis

This review highlights the recent applications of Ir-catalyzed reactions in the total synthesis of natural products.

Chemoselective N–H functionalization of indole derivatives via the Reissert-type reaction catalyzed by a chiral phosphoric acid

An asymmetric N-alkylation of indole derivatives via the Reissert-type reaction was realized in the presence of 10 mol% chiral phosphoric acid.

Total synthesis of natural productsviairidium catalysis

An overview of the highlights in total synthesis of natural products using iridium as a catalyst is given.

Total Synthesis of (-)-Geissoschizol through Ir-Catalyzed Allylic Amidation as the Key Step

The iridium-catalyzed, Zn(OTf)₂ promoted asymmetric allylic amidation of a secondary alcohol derived from tryptamine has been utilized to forge a tetrahydro-β-carboline. Based on this key step, a novel, facile, and enantioselective total synthesis of (-)-geissoschizol was achieved in 10 steps.

Applications of Iridium-Catalyzed Asymmetric Allylic Substitution Reactions in Target-Oriented Synthesis

Metal catalyzed allylic substitution is a cornerstone of organometallic and synthetic chemistry. Enantioselective versions have been developed with catalysts derived from transition metals, most notably molybdenum, nickel, ruthenium, rhodium, iridium, palladium, and copper. The palladium- and the iridium-catalyzed versions have turned out to be particularly versatile in organic synthesis because of the very broad scope of the nucleophile and great functional group compatibility. Assets of the iridium-catalyzed reaction are the formation of branched, chiral products from simple monosubstituted allylic substrates, high degrees of regio- and enantioselectivity, and use of modular, readily available chiral ligands. The possibility to use carbon, nitrogen, oxygen, and sulfur compounds as well as fluoride as nucleophiles allows a wide range of chiral building blocks to be prepared. Our Account begins with the presentation of fundamental reaction schemes and chiral ligands. We will focus our discussion on reactions promoted by phosphoramidite ligands, though numerous chiral ligands have been employed. The subsequent section presents a brief overview of reaction mechanism and experimental conditions. Two versions of the iridium-catalyzed allylic substitution have emerged. In type 1 reactions (introduced in 1997), linear allylic esters are commonly used as substrates under basic reaction conditions. In type 2 reactions (introduced in 2007), environmentally friendly branched allylic alcohols can be reacted under acidic conditions; occasionally, derivatives of allylic alcohols have also been applied. A unique feature of the type 2 reactions is that highly electrophilic allylic intermediates can be brought to reaction with weakly activated alkenes. The subsequent text is ordered according to the strategies followed to transform allylic substitution products to desired targets, most of which are natural products or drugs. Syntheses starting with an intermolecular allylic substitution are discussed first. Some fairly complex targets, for example, the potent nitric oxide inhibitor (-)-nyasol and the drug (-)-protrifenbute, have been synthesized via less than five steps from simple starting materials. Most targets discussed are cyclic compounds. Intermolecular allylic substitution with subsequent ring closing metathesis is a powerful strategy for their synthesis. Highlights are stereodivergent syntheses of Δ⁹-tetrahydrocannabinols (THC), wherein iridium- and organocatalysis are combined (dual catalysis). The combination of allylic alkylation with a Diels-Alder reaction was utilized to synthesize the ketide apiosporic acid and the drug fesoterodine (Toviaz). Sequential allylic amination, hydroboration and Suzuki-Miyaura coupling generates enones suitable for conjugate addition reactions; this strategy was employed in syntheses of a variety of alkaloids, for example, the poison frog alkaloid (+)-cis-195A (pumiliotoxin C). Intramolecular substitutions offer interesting possibilities to build up stereochemical complexity via short synthetic routes. For example, in diastereoselective cyclizations of chiral compounds, substrate control can be overruled by catalyst control in order to generate cis- and trans-isomers selectively from a given precursor. This approach was used to prepare a variety of piperidine and pyrrolidine alkaloids. Finally, complex polycyclic structures, including the structurally unusual indolosesquiterpenoid mycoleptodiscin A, have been generated diastereo- and enantioselectively from olefins by polyene cyclizations and from electron-rich arenes, such as indoles, in dearomatization reactions.