Enantioselective Total Syntheses of Leuconolam–Leuconoxine–Mersicarpine Group Monoterpene Indole Alkaloids

Construction of C2-indolyl-quaternary centers by branch-selective allylation: enabling concise total synthesis of the (±)-mersicarpine alkaloid

Herein we report a branch-selective allylation strategy for accessing C2-indolyl-all-carbon quaternary centers using allylboronic acids. This approach boasts broad functional group tolerance, scalability, and relies on easily accessible allyl alcohol precursors. Importantly, the C3-position of the indole remains free, offering a handle for further synthetic refinement. Mechanistic pathways, corroborated by density functional theory (DFT), suggest the involvement of an indolenine intermediate and a Zimmerman-Traxler-like transition state during allylboration. Demonstrating its efficacy, the method was applied to the total synthesis of the (±)-mersicarpine alkaloid and enabled formal synthesis of additional alkaloids, such as (±)-scholarisine G, (±)-melodinine E, and (±)-leuconoxine.

Iron-Mediated Photochemical Anti-Markovnikov Hydroazidation of Unactivated Olefins

Unactivated olefins are converted to alkyl azides with bench-stable NaN3 in the presence of FeCl3·6H2O under blue-light irradiation. The products are obtained with anti-Markovnikov selectivity, and the reaction can be performed under mild ambient conditions in the presence of air and moisture. The transformation displays broad functional group tolerance, which renders it suitable for functionalization of complex molecules. Mechanistic investigations are conducted to provide insight into the hydroazidation reaction and reveal the role of water from the iron hydrate as the H atom source.

Deciphering the quest in the divergent total synthesis of natural products

The divergent synthesis of natural products is rapidly developing towards achieving the goal of efficiency and economy in total synthesis. However, presently, the sustainable development of the synthesis of natural products does not permit the linear synthesis of a single target. In this case, divergent total synthesis is based on the identification of an advanced intermediate with structural features that can be mapped in more than two molecules. However, the identification of this intermediate and its scalable synthesis in enantiopure form are challenging. Herein, we present the details of the ingenious efforts by researchers in the last six years toward the divergent synthesis of two to as many as eight natural products initially via a single route, and then diverging from a common intermediate and further branching out toward several natural products. The planning and strategies adopted can serve as guidelines for the future development of efficient divergent routes aimed at achieving higher efficiency toward multiple targets, causing divergent synthesis to become an accepted common practice.

Electron Donor-Acceptor Complex Induced Fused Indoles with Hypervalent Iodine(III) Reagents

An operationally simple and efficient method for the cyclization of tertiary amines and hypervalent iodine reagents enabled by an EDA complex has been developed. A series of [1,2-α]indoles derivatives were obtained in good yields, including some key intermediates for the synthesis of biologically active molecules. In addition, this established strategy features a broad substrate scope and good functional group tolerance.

Catalytic I2-moist DMSO-mediated synthesis of valuable α-amidohydroxyketones and unsymmetrical gem-bisamides from benzimidates

We developed an efficient and straightforward I₂-catalyzed strategy for the synthesis of functionalized α-amidohydroxyketones and symmetrical and unsymmetrical bisamides using incipient benzimidate scaffolds as starting materials and moist-DMSO as a reagent and solvent. The developed method proceeds through chemoselective intermolecular N-C-bond formation of benzimidates and the α-C(sp³)-H bond of acetophenone moieties. The key advantages of these design approaches include broad substrate scope and moderate yields. High-resolution mass spectrometry of the reaction progress and labeling experiments provided suitable evidence regarding the possible mechanism. ¹H nuclear magnetic resonance titration revealed notable interaction between the synthesized α-amidohydroxyketones and some anions as well as biologically important molecules, which revealed a promising recognition property of these valuable motifs.

Diastereoselective Access to Triazolo[1,2-a]indolines via a Bio-Inspired Oxidative Cyclization of NH-Indoles

Establishing three-dimensional chemicals by using the C₂-C₃ π bond of indoles has always been a research hotspot in organic synthesis; however, employing the oxidative C₂-C₃ π bond of indoles to generate imine which would lead to the N₁-C₂ π bond cyclization under metal-free conditions is still rare. Here, we report a bio-inspired synthesis of triazolo[1,2-a]indolines by the oxidative cyclization between NH-indoles and azomethine imines with 3,3-dimethyldioxirane as the sole oxidant under metal-free and mild conditions. This finding represents an elegant instance of tri-functionalization of NH-indoles, which provides rapid access to a broad range of triazolo[1,2-a]indolines with tetrahydroisoquinolines in one single step. Up to 86% yield and above 20:1 dr value are observed. The radical mechanism and proton migration process have been speculated.

A short total synthesis of (±)-mersicarpine via visible light-induced cascade photooxygenation

A short total synthesis of the Kopsia alkaloid (±)-mersicarpine is presented. As the key step, a visible light-induced catalytic cascade photooxygenation was utilized, to convert a 3,3-disubstituted tetrahydrocarbazole intermediate, in one step, into a perhydropyrido[1,2-a]indole dione as the immediate precursor to the natural product. The synthesis of mersicarpine was achieved with an overall yield of 12% over 13 steps.

Total synthesis of (±)-mersicarpine following a 6-exo-trig radical cyclization

Described is a total synthesis of racemic mersicarpine from diethyl 4-oxopimelate. The synthetic route takes advantage of a 2-indolyl radical cyclization to construct the pyrido[1,2-a]indole scaffold bearing the all-carbon quaternary stereocenter.

Facile synthesis of the spiro-pyridoindolone scaffold via a gold-catalysed intramolecular alkynol cyclisation/hydroindolylation

A simple approach for the synthesis of pyridoindolone scaffolds with a spiroannulated tetrahydrofuran ring is described. The overall process comprises intramolecular sequential gold-catalysed 5-endo-dig alkynol cycloisomerization and subsequent addition of indole C2 to the in situ generated oxocarbenium cation.

Synthesis of indole derivatives as prevalent moieties present in selected alkaloids

Indoles are a significant heterocyclic system in natural products and drugs. They are important types of molecules and natural products and play a main role in cell biology. The application of indole derivatives as biologically active compounds for the treatment of cancer cells, microbes, and different types of disorders in the human body has attracted increasing attention in recent years. Indoles, both natural and synthetic, show various biologically vital properties. Owing to the importance of this significant ring system, the investigation of novel methods of synthesis have attracted the attention of the chemical community. In this review, we aim to highlight the construction of indoles as a moiety in selected alkaloids.

Total synthesis of the pseudoindoxyl class of natural products

The pseudoindoxyl sub-structural motif, amongst the large set of the indole class of alkaloids, represents a unique subset of the oxygenated indole class of the alkaloid family. A majority of this class of natural products contains complex bridged/polycyclic scaffolds with interesting biological profiles. They are thus attractive synthetic targets. Starting from 1963, twenty-eight natural products having the pseudoindoxyl scaffold have been isolated, among which the synthesis of 13 natural products has been accomplished. In this review, we highlight the completed as well as the formal total synthesis of the natural products with a spiro-pseudoindoxyl ring, with a focus on their development. The challenges and the future perspective based on the recent developments in the field will also be discussed. We strongly believe that this review will not only update but also attract the attention of researchers in dealing with the synthesis of pseudoindoxyl compounds.

Highly Enantioselective Iridium(I)-Catalyzed Hydrocarbonation of Alkenes: A Versatile Approach to Heterocyclic Systems Bearing Quaternary Stereocenters

We report a versatile, highly enantioselective intramolecular hydrocarbonation reaction that provides a direct access to heteropolycyclic systems bearing chiral quaternary carbon stereocenters. The method, which relies on an iridium(I)/bisphosphine chiral catalyst, is particularly efficient for the synthesis of five-, six- and seven-membered fused indole and pyrrole products, bearing one and two stereocenters, with enantiomeric excesses of up to >99 %. DFT computational studies allowed to obtain a detailed mechanistic profile and identify a cluster of weak non-covalent interactions as key factors to control the enantioselectivity.

Enantioselective Total Synthesis of (+)-Alstilobanine C, (+)-Undulifoline, and (-)-Alpneumine H

We report herein the enantioselective total synthesis of three monoterpene indole alkaloids, namely, (+)-alstilobanine C, (+)-undulifoline, and (-)-alpneumine H. The key features of our synthesis include: a) introduction of chirality via enantioselective deprotonation of a prochiral 4-substituted cyclohexanone; b) use of methoxymethyl (MOM) ether as both a hydroxyl protective group and a latent oxonium species for the formation of bridged oxepane and c) domino double reductive cyclization to build both the indole and the piperidine ring at the end of the synthesis. The synthesis confirmed the absolute configuration of these natural products assigned based on the biogenetic hypothesis.

Ultrathin Amorphous/Crystalline Heterophase Rh and Rh Alloy Nanosheets as Tandem Catalysts for Direct Indole Synthesis

Heterogeneous noble-metal-based catalysis plays an essential role in the production of fine chemicals. Rh-based catalysts are one of the most active candidates for indole synthesis. However, it is still highly desired to develop heterogeneous Rh-based catalysts with high activity and selectivity. In this work, a general, facile wet-chemical method is reported to synthesize ultrathin amorphous/crystalline heterophase Rh and Rh-based bimetallic alloy nanosheets (NSs), including RhCu, RhZn, and RhRu. Impressively, the amorphous/crystalline heterophase Rh NSs exhibit enhanced catalytic activity toward the direct synthesis of indole compared to the crystalline counterpart. Importantly, the obtained amorphous/crystalline heterophase RhCu alloy NSs can further enhance the selectivity to indole of >99.9% and the conversion is 100%. This work demonstrates the importance of phase engineering and metal alloying in the rational design and synthesis of tandem heterogeneous catalysts toward fine chemical synthesis.

A novel translation inhibitor, mersicarpine, inhibits S-phase progression and induces apoptosis in HL60 cells

Mersicarpine is an aspidosperma alkaloid isolated from the Kopsia genus of plants. Its intriguing structural features have attracted much attention in synthetic organic chemistry, but no biological activity has been reported. Here, we report the effects of mersicarpine on human leukemia cell line HL60. At concentrations above 30 µm, mersicarpine reversibly arrested cell cycle progression in S-phase. At higher concentrations, it induced not only production of reactive oxygen species, but also apoptosis. Macromolecular synthesis assay revealed that mersicarpine specifically inhibits protein synthesis. These results suggest that mersicarpine is a novel translation inhibitor that induces apoptosis.

Applications of Transition-Metal-Catalyzed Asymmetric Allylic Substitution in Total Synthesis of Natural Products: An Update

Metal-catalyzed asymmetric allylic substitution (AAS) reaction is one of the most synthetically useful reactions catalyzed by metal complexes for the formation of carbon-carbon and carbon-heteroatom bonds. It comprises the substitution of allylic substrates with a wide range of nucleophiles or S_N 2'-type allylic substitution, which results in the formation of the above-mentioned bonds with high levels of enantioselective induction. AAS reaction tolerates a broad range of functional groups, thus has been successfully applied in the asymmetric synthesis of a wide range of optically pure compounds. This reaction has been extensively used in the total synthesis of several complex molecules, especially natural products. In this review, we try to highlight the applications of metal (Pd, Ir, Mo, or Cu)-catalyzed AAS reaction in the total synthesis of the biologically active natural products, as a key step, updating the subject from 2003 till date.

The Allylic Alkylation of Ketone Enolates

The palladium-catalyzed allylic alkylation of non-stabilized ketone enolates was thought for a long time to be not as efficient as the analogous reactions of stabilized enolates, e. g. of malonates and β-ketoesters. The field has experienced a rapid development during the last two decades, with a range of new, highly efficient protocols evolved. In this review, the early developments as well as current methods and applications of palladium-catalyzed ketone enolate allylations will be discussed.

Application of Cp2TiCl-Promoted Radical-Induced Cyclization: An Expeditious Access to [a]-Annelated Indoles

An efficient and novel route for assembling pyrrolo/piperido[1,2-a]indoles is portrayed involving a radical-mediated reductive epoxide opening reaction of N-tethered epoxy-indoles that trigger facile intramolecular cyclization followed by an oxidative quenching step. Capitalizing on the operational simplicity of the method involving just two steps and use of an efficient C-C bond-forming reaction, this radical-based protocol enables the modular assembly of an important class of N-fused indole derivatives with versatile functional and structural diversity.

Benzimidates as gem-Diamidation and Amidoindolyzation Cascade Synthons with a Hydrated NiII Catalyst

We contributed a new benzimidate chemistry through moisture-insensitive Ni^II/Ni^II-Fe^III combo-catalysis for a simultaneous 2-3 bond-forming gem-diamidation and amidoindolyzation cascade reaction to construct symmetrical and unsymmetrical gem-(arylmethylene)amides and indolo(arylmethylene)amides, using emerging benzimidate synthons. The operational simplicity, mild nature, generality, and robustness of the strategy were validated through syntheses of a wide range of new molecules, labile sugar-based chiral compounds, and pharmaceuticals with high yields under the same reaction conditions.

Towards the Sarpagine-Ajmaline-Macroline Family of Indole Alkaloids: Enantioselective Synthesis of an N-Demethyl Alstolactone Diastereomer

the strategy involving the use of functionalized tetrahydro-6H-cycloocta[b]indol-6-one is reported as a key intermediate for synthesis of members of the sarpagine-ajmaline-macroline family of monoterpene indole alkaloids. The desired tricycle was synthesized through the following key steps: 1) Evans' syn-selective aldolization; 2) Liebeskind-Srogl cross-coupling using the phenylthiol ester of 3-chloropropanoic acid as a surrogate of acrylic thioester for the synthesis of 2,3-disubstituted indoles; and 3) ring-closing metathesis (RCM) for the formation of the eight-membered ring. An N-allylation followed by intramolecular 1,4-addition was planned for synthesis of the vobasine class of natural products. However, attempted cyclizations under a diverse set of conditions involving anionic, radical, and organopalladium/organonickel species failed to produce the bridged ring system. On the other hand, esterification of the pendant primary alcohol function with acetoacetic acid, followed by intramolecular Michael addition, afforded the desired tetracycle with excellent diastereoselectivity. Subsequent functional group manipulation and transannular cyclization of the amino alcohol afforded the N(1)-demethyl-3,5-diepi-alstolactone. We believe that the same synthetic route would afford the alstolactone should the amino alcohol with appropriate stereochemistry be used as the starting material.

Synthesis of 2,3-disubstituted indoles via a tandem reaction

A wide array of 2,3-disubstituted indoles were accessed in modest to good yields via a tandem reduction/condensation/fragmentation/cyclization sequence. Differential fragmentation made the reaction more complicated.

Pd-Catalyzed Aerobic Oxidative Heck Cross-Coupling for the Straightforward Construction of Indole δ-Lactams

The [6.5.6]-tricyclic indole δ-lactam represents a common key intermediate for the synthesis of a broad variety of structurally intriguing indole alkaloids. The development of a method for the versatile and straightforward construction of such structural motif is of great importance for potential synthetic applications. Herein, we present a co-ligand-prompted Pd-catalyzed 6-exo-trig intramolecular cyclization of indolyl amides via the aerobic oxidative Heck cross-coupling. The method provided a general and efficient way for the construction of [6.5.6]-tricyclic indole δ-lactams. A mechanistic study suggests that a Pd^(I)/Pd^(III) catalytic cycle should be responsible for effective coupling, which represents a mechanistically alternative pathway when compared with the Pd⁽⁰⁾/Pd^(II) cycle proposed for other related coupling reactions.

Direct Intermolecular Anti-Markovnikov Hydroazidation of Unactivated Olefins

We herein report a direct intermolecular anti-Markovnikov hydroazidation method for unactivated olefins, which is promoted by a catalytic amount of bench-stable benziodoxole at ambient temperature. This method facilitates previously difficult, direct addition of hydrazoic acid across a wide variety of unactivated olefins in both complex molecules and unfunctionalized commodity chemicals. It conveniently fills a synthetic chemistry gap of existing olefin hydroazidation procedures, and thereby provides a valuable tool for azido-group labeling in organic synthesis and chemical biology studies.

Enantioselective palladium-catalyzed allylic alkylation reactions in the synthesis of Aspidosperma and structurally related monoterpene indole alkaloids

Covering: up to the end of 2017 Enantioselective Pd-catalyzed allylic alkylations of prochiral enolates represent a powerful tool for the construction of all-carbon quaternary stereocenters. This review describes the emergence of such reactions as strategic linchpins that enable efficient, stereocontrolled syntheses of Aspidosperma and related monoterpene indole alkaloids.

Stereoselective Total Synthesis of Eburnane-Type Alkaloids Enabled by Conformation-Directed Cyclization and Rearrangement

Controlling the cis C20/C21 relative stereochemistry remains an unsolved issue in the synthesis of eburnane-type indole alkaloids. Provided herein is a simple solution to this problem by developing a unified and diastereoselective synthesis of four representative members of this class of natural products, namely, eburnamonine, larutensine, terengganensine B, and melokhanine E. The synthesis features the following key steps: a) an α-iminol rearrangement transforming the 3-hydroxyindolenine into spiroindolin-3-one, b) a highly diastereoselective conformation-directed cyclization leading to the melokhanine skeleton with the desired C20/C21 cis stereochemistry, and c) either an aza-pinacol or an unprecedented α-aminoketone rearrangement converting spiroindolinone back into the indole skeleton.

Unified enantioselective total syntheses of (−)-scholarisine G, (+)-melodinine E, (−)-leuconoxine and (−)-mersicarpine

A unified strategy enabled the enantioselective syntheses of (−)-scholarisine G, (+)-melodinine E, (−)-leuconoxine and (−)-mersicarpine from a common 2-alkylated indole bearing an all-carbon quaternary stereogenic center.

Heteroannulation of Arynes with α-Amino Imides: Synthesis of 2,2-Disubstituted Indolin-3-ones and Application to the Enantioselective Total Synthesis of (+)-Hinckdentine A

A novel heteroannulation reaction between α-amino imides and in situ generated arynes has been developed for the synthesis of 2,2-disubstituted indolin-3-ones. An enantioselective total synthesis of the marine alkaloid (+)-hinckdentine A was subsequently accomplished using this reaction as a key step. A catalytic enantioselective Michael addition of an α-aryl-α-isocyanoacetate to phenyl vinyl selenone was employed for the construction of the enantioenriched α-quaternary α-amino ester.

Metamorphosis of cycloalkenes for the divergent total synthesis of polycyclic indole alkaloids

This review summarizes the divergent synthesis of monoterpene indole alkaloids using cycloalkene as the turning point of structural diversity.