Enantioselective Total Syntheses of (−)-Rhazinilam, (−)-Leucomidine B, and (+)-Leuconodine F

Heterogeneous Photocatalytic Ring Expansion of Cyclic Ketones for the Construction of Medium-Sized Lactams

Construction of medium-sized ring compounds remains challenging in synthetic chemistry. Herein, we describe the synthesis of medium-sized lactams via a photoinduced ring expansion of benzo-fused cyclic ketones using graphitic carbon nitride (g-C3N4) as a photocatalyst. The ring expansion protocol provided an efficient access to 8-10-membered lactams in good yields and displayed good tolerance to a range of functional groups. The mechanism studies revealed that the photochemical reaction proceeds via an intermediary of a nitrogen radical, which is generated through an oxidative hydrogen atom transfer (HAT) process.

Hydrazone Activation in the Aminocatalytic Cascade Reaction for the Synthesis of Tetrahydroindolizines

In this Letter, we demonstrate the usefulness of hydrazone activation for the synthesis of biologically relevant tetrahydroindolizines. A pyrrol-derived hydrazone bearing a Michael acceptor moiety in the N-alkyl side chain has been designed with the aim of participating in the aminocatalytic cascade reaction leading to the annulation of the new six-membered heterocyclic scaffold. The application of (S)-(-)-α,α-diphenyl-2-pyrrolidinemethanol trimethylsilyl ether as the aminocatalyst allows for the iminium ion-enamine-mediated cascade to proceed in a fully stereoselective manner.

BiBr3 -Mediated Intramolecular Aza-Prins Cyclization of Aza-Achmatowicz Rearrangement Products: Asymmetric Total Synthesis of Suaveoline and Sarpagine Alkaloids

An intramolecular aza-Prins cyclization of aza-Achmatowicz rearrangement products was developed in which bismuth tribromide (BiBr3 ) plays a dual role as an efficient Lewis acid and source of the bromide nucleophile. This approach enables the facile construction of highly functionalized 9-azabicyclo[3.3.1]nonanes (9-ABNs), which are valuable synthetic building blocks and a powerful platform for the synthesis of a variety of alkaloid natural products and drug molecules. Suitable substrates for the aza-Prins cyclization include 1,1-disubstituted alkenes, 1,2-disubstituted alkenes, alkynes, and allenes, with good to excellent yields observed. Finally, we showcase the application of this new approach to the enantioselective total synthesis of six indole alkaloids: (-)-suaveoline (1), (-)-norsuaveoline (2), (-)-macrophylline (3), (+)-normacusine B (4), (+)-Na -methyl-16-epipericyclivine (5) and (+)-affinisine (6) in a total of 9-14 steps. This study significantly expands the synthetic utility of the aza-Achmatowicz rearrangement, and the strategy (aza-Achmatowicz/aza-Prins) is expected to be applicable to the total synthesis of other members of the big family of macroline and sarpagine indole alkaloids.

Brønsted Acid versus Phase-Transfer Catalysis in the Enantioselective Transannular Aminohalogenation of Enesultams

We have studied the enantioselective transannular aminohalogenation reaction of unsaturated medium-sized cyclic benzosulfonamides by using both chiral Brønsted acid and phase-transfer catalysis. Under optimized conditions, a variety of bicyclic adducts can be obtained with good yields and high enantioselectivities. The mechanism of the reaction was also studied by using computational tools; we observed that the reaction involves the participation of a conformer of the nine-membered cyclic substrate with planar chirality in which the stereochemical outcome is controlled by the relative reactivity of the two pseudorotational enantiomers when interacting with the chiral catalyst.

Enantioselective Total Synthesis of (-)-Limaspermidine and (-)-Kopsinine by a Nitroaryl Transfer Cascade Strategy

We report an intramolecular conjugate addition/Truce-Smiles/E1cb cascade of 2-nitrobenzenesulfonamide-functionalized cyclohexenones as a new entry to the core scaffold of monoterpene indole alkaloids. The method was applied to the asymmetric total synthesis of (-)-limaspermidine, (-)-kopsinilam, and (-)-kopsinine, as well as the framework of the kopsifoline alkaloids, thus highlighting its complementarity to existing approaches involving the use of indole-based starting materials or the interrupted Fischer indole synthesis. Furthermore, we show that the cascade tolerates various substituents on the nitroarene, opening the way to other natural products as well as non-natural analogues.

Rhazinilam-leuconolam family of natural products: a half century of total synthesis

Covering: 1972 to 2021The rhazinilam family of natural products exhibits a main structure with a stereogenic quaternary carbon and a tetrahydroindolizine core imbedded within a 9-membered macrocycle, imposing axial chirality. This unique architecture combined with their taxol-like antimitotic activities have attracted various attention, especially from synthetic chemists, notably in the past decade. The present review describes the known total and formal syntheses of the members of the rhazinilam family (rhazinilam, rhazinal, leuconolam and kopsiyunnanines), according to the strategy developed.

The application of C–H bond functionalization in the total syntheses of indole natural products

The recent advances in total synthesis of indole natural products focusing on the application of C–H bond functionalization are summarized.

Chemoselective [3 + 2] annulation of oxime acetate with 2-aryl-3-ethoxycarbonyl-pyrroline-4,5-dione: an entry to pyrrolo[2,3-b]pyrrole derivatives

A novel chemoselective [3 + 2] annulation reaction of easily accessible ketoxime acetate with 2-aryl-3-ethoxycarbonyl pyrroline-4,5-dione has been developed for the synthesis of unknown pyrrolo[2,3-b]pyrrole frameworks. This method involves copper-mediated N-O bond cleavage followed by the formation of carbon-carbon and carbon-nitrogen bonds. This operationally simple protocol provides broader functional group compatibility and good yields.

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.

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.

General and efficient synthesis of 1,2-dihydropyrrolo[3,4-b]indol-3-ones via a formal [3 + 2] cycloaddition initiated by C–H activation

A [Cp*RhCl₂]₂-catalyzed formal [3 + 2] cycloaddition involving a sequential coupling reaction initiated by C–H activation and aza-Michael addition has been developed for the general and efficient synthesis of 1,2-dihydropyrrolo[3,4-b]indol-3-ones.

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.

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.

Palladium-Catalyzed Enantioselective Desymmetrizing Aza-Wacker Reaction: Development and Application to the Total Synthesis of (-)-Mesembrane and (+)-Crinane

Reported is an unprecedented catalytic enantioselective desymmetrizing aza-Wacker reaction. In the presence of a catalytic amount of a newly developed Pd(CPA)₂ (MeCN)₂ catalyst (CPA=chiral phosphoric acid), a pyrox ligand, and molecular oxygen, cyclization of properly functionalized prochiral 3,3-disubstituted cyclohexa-1,4-dienes afforded enantioenriched cis-3a-substituted tetrahydroindoles in good yields with excellent enantioselectivities. A cooperative effect between the phosphoric acid and the pyrox ligand ensured efficient transformation. This reaction was tailor-made for Amaryllidaceae and Sceletium alkaloids as illustrated by its application in the development of the concise and divergent total synthesis of (-)-mesembrane and (+)-crinane.

Indium(iii)-catalyzed intramolecular dearomative cycloaddition ofN-sulfonylaziridines to indoles: facile synthesis of tetracyclic pyrroloindoline skeletons

A facile and versatile indium(iii)-catalyzed intramolecular dearomative cycloaddition ofN-sulfonylaziridines to indoles has been developed, selectively giving rigid tetracyclic pyrroloindoline skeletons in moderate to good yields.

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.

Regiodivergent condensation of 5-alkoxycarbonyl-1H-pyrrol-2,3-diones with cyclic ketazinones en route to spirocyclic scaffolds

The condensation of 5-alkoxycarbonyl-1H-pyrrolediones with cyclic ketazinones was systematically investigated. It was discovered that the regioselectivity of this reaction can be easily swapped between two alternative directions affording derivatives of partially hydrogenated indole or benzofurane. The control of this regioselectivity is efficiently governed by steric effects at the hydrazone moiety of the ketazinone reagent.

Short Enantioselective Total Synthesis of (-)-Rhazinilam Using a Gold(I)-Catalyzed Cyclization

(R)-(-)-Rhazinilam has been synthesized in nine steps and 20% overall yield. The key steps involve two metal-catalyzed processes: the enantioselective gold(I)-catalyzed cycloisomerization of an allene-functionalized pyrrole and the palladium-catalyzed hydrocarboxylation of a vinyl moiety with formate as a CO surrogate. This novel strategy represents the shortest and highest yielding enantioselective total synthesis of (-)-rhazinilam.

Stereoselective Synthesis of Quaternary Pyrrolidine-2,3-diones and β-Amino Acids

A facile, diastereoselective synthesis of highly substituted pyrrolidine-2,3-diones is reported, along with the one-step conversion of these heterocycles to novel β-amino acids and further functionalized derivatives. This method involves an unusually mild, one-pot, three-component cyclization/allylation followed by a Claisen rearrangement to provide unusual pyrrolidinone products that are densely functionalized and contain an all-carbon quaternary stereocenter. The reported reaction sequence is operationally simple, exquisitely diastereoselective, and provides gram-scale access to valuable heterocyclic scaffolds and β-amino acids not readily accessible via existing approaches.

Direct amidation of esters with nitroarenes

Esters are one of the most common functional groups in natural and synthetic products, and the one-step conversion of the ester group into other functional groups is an attractive strategy in organic synthesis. Direct amidation of esters is particularly appealing due to the omnipresence of the amide moiety in biomolecules, fine chemicals, and drug candidates. However, efficient methods for direct amidation of unactivated esters are still lacking. Here we report nickel-catalysed reductive coupling of unactivated esters with nitroarenes to furnish in one step a wide range of amides bearing functional groups relevant to the development of drugs and agrochemicals. The method has been used to expedite the syntheses of bio-active molecules and natural products, as well as their post-synthetic modifications. Preliminary mechanistic study indicates a reaction pathway distinct from conventional amidation methods using anilines as nitrogen sources. The work provides a novel and efficient method for amide synthesis.

Direct conversion of esters to amides, while attractive, is often limited to activated esters or highly nucleophilic amines. Here the authors report a nickel-catalysed reductive coupling between unactivated esters and nitroarenes, giving a direct route to aromatic amides.