Construction of Tetracyclic 3-Spirooxindole through Cross-Dehydrogenation of Pyridinium: Applications in Facile Synthesis of (±)-Corynoxine and (±)-Corynoxine B

A RhII-Catalyzed [4 + 3]-Cycloaddition via the Stereoselective Cyclopropanation of Vinyl Allenes En Route to Oxepino[b]indoles and Subsequent Elaboration to Spirooxindole Frameworks

Oxepino[b]indoles were obtained in good to excellent yields employing a [4 + 3]-cycloaddition initiated by a stereo- and regioselective, RhII-catalyzed cyclopropanation between a vinyl allene and diazooxindole to afford an intermediate cyclopropyl allene that engaged the oxindole carbonyl in a spontaneous hetero-[3,3]-rearrangement. A survey of functional group tolerance revealed a diverse array of substrates amenable to oxepino[b]indole formation. In addition to the intriguing architecture of the cycloadducts, exposure to either Brønsted acid or base enables the assembly of functionalized spirroxindoles via the unusual conversion of a 5-7 fused ring system to a 5-5 spirocycle.

Catalytic Enantioselective α-Ethynylation of Oxindoles: Total Synthesis of (-)-Corynoxine, (-)-Isorhynchophylline, (-)-Aspidospermidine, and (-)-Limaspermidine

The all-carbon quaternary stereogenic center of oxindoles is a crucial structural element of a broad spectrum of indole alkaloids, imparting these molecules with rigid three-dimensional configurations essential for their biological activities. Here, we present a catalytic asymmetric α-ethynylation reaction of oxindoles taking advantage of the catalysis of a spiropyrrolidine amide (SPA) triazolium. This transformation enables the enantioselective construction of the C3 quaternary carbon stereocenter of oxindoles while introducing a versatile ethynyl functionality. Employment of this methodology has been demonstrated in the divergent total synthesis of indole alkaloids (-)-corynoxine, (-)-isorhynchophylline, (-)-aspidospermidine, and (-)-limaspermidine, featuring a protecting group-dependent 1,6-Michael addition or an aminolysis/1,6-Michael addition sequence to generate two distinct types of spiro-indoles, tailored for different late-stage synthetic purposes.

Recent strategy for the synthesis of indole and indoline skeletons in natural products

Indole alkaloids are one of the most important classes of natural products found in nature, particularly in a wide variety of plants. These compounds have compact polycyclic systems with at least one nitrogen atom. Several of these alkaloids are bioactive and have raised hopes for the development of new drugs. Their biosynthesis involves tryptophan as an amino acid precursor, since the indole or indoline moiety is the main heterocycle of these natural products. However, in their quest to synthesize such complex architectures, chemists have developed several different strategies to produce this key heterocycle quickly and in an unnatural way. This review focuses on the recent total synthesis methods used to prepare the indole and indoline core of these important alkaloids. Novel and older methods that allow the rapid formation of this heterocycle are described as key steps in the total synthesis of these fascinating structures designed by Mother Nature.

Azine Dearomatization in Natural Product Total Synthesis

Since antiquity, alkaloid natural products have served as medicinal ingredients that still contribute as an inspiration for the development of novel therapeutics. For the synthetic chemist, much of the importance of natural products lies in their acting as a forcing-function for the invention of new synthetic strategies and tactics for molecular assembly. With this rich history in mind, it remains an important goal for chemists to build nitrogenous structures with greater efficiency, abiding by economies of synthesis. Nitrogenous aromatic feedstocks have been an intriguing starting point for the functionalization and construction of alkaloids for several decades, but recent advances in reaction design have opened new doors for leveraging their abundance in concise synthesis. Herein, advances in this area of synthetic ingenuity will be summarized with the aim of instructing chemists towards considering dearomatization as a strategic avenue for both target-oriented and diversity-oriented synthetic campaigns. Overall, syntheses are evaluated, compared, and contrasted to give a systematic overview of this continued area of research.

Access to Spiro-Quinazolines via an Acid-Catalyzed Ring-Opening of Isatins with N-Alkylureas

Herein, we report novel p-TSA·H2O-catalyzed ring-opening reactions of isatins with N-alkylureas, allowing access to spiro-quinazolines with excellent substrate scope and good yields. Introducing N,N-dialkylureas, 2,4-thiazolidinedione, or rhodamine into the reactions leads to a distinct set of three-component reactions, yielding innovative spiro-quinazolines incorporating sulfur atoms. Notably, the protocol achieves a superior level of atomic economy, with water as the sole byproduct.

NCS-Mediated Direct C(sp3)-H Oxygenation of 2-Methylindoles Using Water as the Oxygen Source

In continuation of our research interest in the green oxidation of indoles, we further explore the direct oxidation of 2-methylindoles to 2-formyl indoles promoted by NCS and associated with H2O as the oxygen source. This methodology was demonstrated to be a robust protocol consisting of chlorination, SN2', and oxidation processes, and presents a reasonably broad substrate scope and excellent functional group tolerance, thus enabling the preparation of high added-value versatile building blocks susceptible to further functionalization.

Metal free cross-dehydrogenative N-N coupling of primary amides with Lewis basic amines

Hydrazides, N-N containing structural motifs, are important due to their presence in a wide variety of biologically significant compounds. While the homo N-N coupling of two NH moieties to form the hydrazide N-N bond is well developed, the cross-dehydrogenative hetero N-N coupling remains very unevolved. Here we present an efficient intermolecular N-N cross-coupling of a series of primary benzamides with broad range of Lewis basic primary and secondary amines using PhI(OAc)2 as both a terminal oxidant and a cross-coupling mediator, without the need for metal catalysts, high temperatures, and inert atmospheres, and with substantial potential for use in the late-stage functionalization of drugs.

Unified Strategy Enables the Collective Syntheses of Structurally Diverse Indole Alkaloids

Natural products and their analogues are significant sources of therapeutic lead compounds. However, synthetic strategies for generating large collections of these molecules remain a significant challenge. The most difficult step in their synthesis is the design of a common intermediate that can be easily transformed into natural products belonging to different families. This study demonstrates the evolution of synthetic tactics designed to assemble the functionalized piperidines present in indole alkaloids from a common intermediate. More importantly, we also report a previously unknown Ir- and Er-catalyzed dehydrogenative spirocyclization reaction that enables direct access to spirocyclic oxindole alkaloids. As a practical application, the asymmetric total syntheses of 29 natural alkaloids belonging to different families were accomplished by following a uniform synthetic route. The proposed methodology extends the capability of the iridium-catalyzed dehydrogenative coupling reaction to the realm of indole-alkaloid synthesis and provides new opportunities for the efficient preparation of natural product-like molecules.

Total Synthesis of Tetracyclic Spirooxindole Alkaloids via a Double Oxidative Rearrangement/Cyclization Cascade

Skeleton rearrangement could rapidly transfer simple molecules to complex structures and has significant potential in the total synthesis of natural products. We developed a one-pot reaction cascade of double oxidative rearrangement of furan and indole followed by a nucleophilic cyclization that was successfully applied for the formal synthesis of rhynchophylline/isorhynchophylline and the first total synthesis of (±)-7(R)-geissoschizol oxindole/(±)-7(S)-geissoschizol oxindole. In addition, the geissoschizol oxindoles were revised to their C3 epimers, and the mechanism for the reversed stereochemistry through the retro-Mannich/Mannich cascade was proposed and supported by density functional theory calculations.

A Platform for the Synthesis of Corynantheine-Type Corynanthe Alkaloids

Corynantheine-type alkaloids are major components of the Mitragyna speciosa, also known as kratom, that exhibit unique pharmacological activity. However, no universal method to prepare these alkaloids has been reported. Disclosed herein is a catalytic, asymmetric platform that enables rapid access to corynantheine alkaloids. The first enantioselective total synthesis of (-)-corynantheidine pseudoindoxyl is described. The first asymmetric syntheses of (+)-corynoxine and (-)-corynoxine B were also achieved, along with enantioselective syntheses of (-)-corynantheidol and (-)-corynantheidine. Through this work, all series of corynantheine alkaloids including indole, spirooxindole, and pseudoindoxyl can now be accessed in the laboratory, enabling comprehensive biological investigation of kratom alkaloids to be undertaken.

Total Synthesis and Structural Plasticity of Kratom Pseudoindoxyl Metabolites

Mitragynine pseudoindoxyl, a kratom metabolite, has attracted increasing attention due to its favorable side effect profile as compared to conventional opioids. Herein, we describe the first enantioselective and scalable total synthesis of this natural product and its epimeric congener, speciogynine pseudoindoxyl. The characteristic spiro-5-5-6-tricyclic system of these alkaloids was formed through a protecting-group-free cascade relay process in which oxidized tryptamine and secologanin analogues were used. Furthermore, we discovered that mitragynine pseudoindoxyl acts not as a single molecular entity but as a dynamic ensemble of stereoisomers in protic environments; thus, it exhibits structural plasticity in biological systems. Accordingly, these synthetic, structural, and biological studies provide a basis for the planned design of mitragynine pseudoindoxyl analogues, which can guide the development of next-generation analgesics.

Discovery and Biomimetic Semisynthesis of Spirophyllines A-D from Uncaria rhynchophylla

Spirophyllines A-D (1-4), four new spirooxindole alkaloids all characterized by the spiro[pyrrolidin-3,3'-oxindole] core and a rare isoxazolidine ring, were isolated from Uncaria rhynchophylla. Their structures were determined by spectroscopic methods and confirmed by X-ray crystallography. Based on the biomimetic semisynthesis strategy, compounds 1-8 were synthesized in three steps via the key reactions of 1,3-dipolar cycloaddition and Krapcho decarboxylation from corynoxeine. Interestingly, compound 3 showed moderate inhibitory activity against the Kv1.5 potassium channel (IC50 = 9.1 μM).

Enantioselective construction of triaryl-substituted all-carbon quaternary stereocenters via organocatalytic arylation of oxindoles with azonaphthalenes

Azonaphthalenes have been verified as a class of effective arylation reagents in a variety of asymmetric transformations. Here a highly efficient approach to construct triaryl-substituted all-carbon quaternary stereocenters through chiral phosphoric acid-catalyzed enantioselective arylation of 3-aryl-2-oxindoles with azonaphthalenes is disclosed. This chemistry is scalable and displays excellent functional group tolerance, furnishing a series of 3,3-disubstituted 2-oxindole derivatives in good yields with excellent enantiocontrol. Preliminary mechanistic data suggest that the initially formed direct addition intermediate undergoes intramolecular annulation under acidic reaction conditions.

Rhynchines A-E: Cav3.1 Calcium Channel Blockers from Uncaria rhynchophylla

Rhynchines A-E (1-5), five new indole alkaloids with an unprecedented skeleton, were isolated from Uncaria rhynchophylla. The new skeleton was characterized by an indole moiety and a 2-oxa-8-azatricyclo[6,5,01,5,01,8]tridecane core, forming a unique 6/5/7/5/5 ring system. Their structures were determined by nuclear magnetic resonance, high-resolution electrospray ionization mass spectrometry, infrared spectroscopy, and calculated electronic circular dichroism (ECD) and were confirmed by X-ray crystallography. Interestingly, 1 and 2 showed strong inhibitory activities against the Cav3.1 calcium channel with IC50 values of 6.86 and 10.41 μM.

Enantioselective Construction of Spirooxindole-Fused Cyclopentanes

The present study reports an asymmetric organocatalytic cascade reaction of oxindole derivates with α,β-unsaturated aldehydes efficiently catalyzed by simple chiral secondary amine. Spirooxindole-fused cyclopentanes were produced in excellent isolated yields (up to 98%) with excellent enantiopurities (up to 99% ee) and moderate to high diastereoselectivities. The synthetic utility of the protocol was exemplified on a set of additional transformations of the corresponding spiro compounds. In addition, a study showing the promising biological activity of selected enantioenriched products was accomplished.

α-C(sp3)-H Arylation of Cyclic Carbonyl Compounds

α-C(sp3)-H arylation is an important type of C-H functionalization. Various biologically significant natural products, chemical intermediates, and drugs have been effectively prepared via C-H functionalization. Cyclic carbonyl compounds comprise of cyclic ketones, enones, lactones, and lactams. The α-C(sp3)-H arylation of these compounds have been exhibited high efficiency in forming C(sp3)-C(sp2) bonds, played a crucial role in organic synthesis, and attracted majority of interests from organic and medicinal communities. This review focused on the most significant advances including methods, mechanism, and applications in total synthesis of natural products in the field of α-C(sp3)-H arylations of cyclic carbonyl compounds in recent years.

Solvent-specific, DAST-mediated intramolecular Friedel-Crafts reaction: access to dibenzoxepine-fused spirooxindoles

A facile, DAST-mediated intramolecular cyclization of 3-hydroxy-3-(2-((3-methoxybenzyl)oxy)phenyl)indolin-2-one derivatives for the synthesis of spirooxindoles fused with dibenzoxepine moieties is described. The success of this reaction is highly dependent on the choice of solvent (promoted by DCM and 1,2-DCE) and the electronic nature of the pendant aromatic ring, which is favored by the presence of electron-donating substituents. The reaction is believed to proceed through an intramolecular Friedel-Crafts-type reaction. Various dibenzoxepine-fused spirooxindoles were successfully synthesized in up to 98% yield. This methodology provides libraries of structurally diverse and medicinally important small molecules that could aid in the search for new bioactive molecules.

Synthesis of Quaternary Spirooxindole 2H-Azirines under Batch and Continuous Flow Condition and Metal Assisted Umpolung Reactivity for the Ring-Opening Reaction

An efficient and new approach for the synthesis of spirooxindole 2H-azirines via intramolecular oxidative cyclization of 3-(amino(phenyl)methylene)-indolin-2-one derivatives in the presence of I₂ and Cs₂ CO₃ under batch/continuous flow is described. This method is mild and facile to synthesize a variety of spirooxindole 2H-azirines derivatives in gram-scale. Furthermore, we have synthesized spiroaziridine derivatives from spirooxindole 2H-azirines derivatives via addition of Grignard reagent. In addition, we discloses an metal assisted attack of Grignard nucleophile at N-centre rather than C- of the spirooxindole 2H-azirines, which concurrently underwent ring opening of transient aziridines to afford N-substituted Z-3-(aminophenyl)indolin-2-one. A plausible mechanism for azirination and ring-opening reaction is also presented.

Rapid Access to Benzimidazo[1,2-a]quinoline-Fused Isoxazoles via Pd(II)-Catalyzed Intramolecular Cross Dehydrogenative Coupling: Synthetic Versatility and Photophysical Studies

An efficient and atom-economical palladium-catalyzed intramolecular cross dehydrogenative coupling (CDC) reaction has been developed for the construction of highly π-conjugated benzimidazo[1,2-a]quinoline-fused isoxazole scaffolds using molecular oxygen as sole oxidant. The approach portrayed wide substrate scope with good functional group tolerance and depicted a useful tool for the generation of fluorescence active compounds with high quantum yield. Synthetic versatility of the method via Fe-catalyzed reductive isoxazole ring cleavage toward pyridine, pyrimidine, pyrazole fused heteropolycyclic compounds has been showcase.

Stereoselective synthesis and applications of spirocyclic oxindoles

This review summaries recent synthetic developments towards spirocyclic oxindoles and applications as valuable medicinal and synthetic targets.

Recent research progress of Uncaria spp. based on alkaloids: phytochemistry, pharmacology and structural chemistry

Medicinal plants are well-known in affording clinically useful agents, with rich medicinal values by combining with disease targets through various mechanisms. Plant secondary metabolites as lead compounds lay the foundation for the discovery and development of new drugs in disease treatment. Genus Uncaria from Rubiaceae family is a significant plant source of active alkaloids, with anti-hypertensive, sedative, anti-Alzheimer's disease, anti-drug addiction and anti-inflammatory effects. This review summarizes and discuss the research progress of Uncaria based on alkaloids in the past 15 years, mainly in the past 5 years, including biosynthesis, phytochemistry, pharmacology and structural chemistry. Among, focusing on representative compounds rhynchophylline and isorhynchophylline, the pharmacological activities surrounding the central nervous system and cardiovascular system are described in detail. On the basis of case studies, this article provides a brief overview of the synthesis and analogues of representative compounds types. In summary, this review provides an early basis for further searching for new targets and activities, discussing the mechanisms of pharmacological activity and studying the structure-activity relationships of active molecules.

E-Z isomerization of 3-benzylidene-indolin-2-ones using a microfluidic photo-reactor

Here, we report controlled E-Z isomeric motion of the functionalized 3-benzylidene-indolin-2-ones under various solvents, temperature, light sources, and most importantly effective enhancement of light irradiance in microfluidic photoreactor conditions. Stabilization of the E-Z isomeric motion is failed in batch process, which might be due to the exponential decay of light intensity, variable irradiation, low mixing, low heat exchange, low photon flux etc. This photo-μ-flow light driven motion is further extended to the establishment of a photostationary state under solar light irradiation.

The Chemistry of Kratom [Mitragyna speciosa]: Updated Characterization Data and Methods to Elucidate Indole and Oxindole Alkaloids

Two separate commercial products of kratom [Mitragyna speciosa (Korth.) Havil. Rubiaceae] were used to generate reference standards of its indole and oxindole alkaloids. While kratom has been studied for over a century, the characterization data in the literature for many of the alkaloids are either incomplete or inconsistent with modern standards. As such, full 1H and 13C NMR spectra, along with HRESIMS and ECD data, are reported for alkaloids 1-19. Of these, four new alkaloids (7, 11, 17, and 18) were characterized using 2D NMR data, and the absolute configurations of 7, 17, and 18 were established by comparison of experimental and calculated ECD spectra. The absolute configuration for the N(4)-oxide (11) was established by comparison of NMR and ECD spectra of its reduced product with those for compound 7. In total, 19 alkaloids were characterized, including the indole alkaloid mitragynine (1) and its diastereoisomers speciociliatine (2), speciogynine (3), and mitraciliatine (4); the indole alkaloid paynantheine (5) and its diastereoisomers isopaynantheine (6) and epiallo-isopaynantheine (7); the N(4)-oxides mitragynine-N(4)-oxide (8), speciociliatine-N(4)-oxide (9), isopaynantheine-N(4)-oxide (10), and epiallo-isopaynantheine-N(4)-oxide (11); the 9-hydroxylated oxindole alkaloids speciofoline (12), isorotundifoleine (13), and isospeciofoleine (14); and the 9-unsubstituted oxindoles corynoxine A (15), corynoxine B (16), 3-epirhynchophylline (17), 3-epicorynoxine B (18), and corynoxeine (19). With the ability to analyze the spectroscopic data of all of these compounds concomitantly, a decision tree was developed to differentiate these kratom alkaloids based on a few key chemical shifts in the 1H and/or 13C NMR spectra.

Retro-biosynthetic construction of corynanthe alkaloid skeletons from rhynchophylline alkaloids

Corynanthe alkaloids could be generated from rhynchophylline alkaloids in a retro-biosynthetic manner via a Wagner–Meerwein rearrangement.

Pyridinium salts: from synthesis to reactivity and applications

This review highlights the pyridinium salts in terms of their natural occurrence, synthesis, reactivity, biological properties, and diverse applications.

Concise Total Synthesis of (-)-Affinisine Oxindole, (+)-Isoalstonisine, (+)-Alstofoline, (-)-Macrogentine, (+)-Na -Demethylalstonisine, (-)-Alstonoxine A, and (+)-Alstonisine

A highly enantio- and diastereoselective strategy to access any member of the sarpagine/macroline family of oxindole alkaloids via internal asymmetric induction was developed from readily available d-(+)-tryptophan. At the center of this approach was the diastereospecific generation of the spiro[pyrrolidine-3,3'-oxindole] moiety at an early stage via a tert-butyl hypochlorite-promoted oxidative rearrangement of a chiral tetrahydro-β-carboline derivative. This key branching point determined the spatial configuration at the C-7 spiro center to be entirely 7R or 7S. Other key stereospecific processes were the asymmetric Pictet-Spengler reaction and Dieckmann cyclization, which were scalable to the 600 and 150 gram levels, respectively. Execution of this approach resulted in first enantiospecific total synthesis of (+)-isoalstonisine and (-)-macrogentine from the chitosenine series (7R), as well as (+)-alstonisine, (+)-alstofoline, (-)-alstonoxine A and (+)-Na -demethylalstonisine from the alstonisine series (7S).

Enantioselective Synthesis of Spiro[indolizidine-1,3'-oxindoles]

A three-step procedure for the enantioselective synthesis of spiro[indolizidine-1,3'-oxindoles], consisting of a stereoselective cyclocondensation reaction between (S)-tryptophanol and a prochiral or racemic δ-oxoester, bromination of the resulting oxazolopiperidone lactam, and a final stereoselective spirocyclization, is reported.

Construction of Spirooxindole Skeleton Through Intramolecular Dieckmann Cyclization

A highly efficient and direct approach was developed to construct the structurally diverse spirooxindole skeleton, which is an important basic motif in natural products. Both the 3,3'-pyrrolidonyl spirooxindoles and spiroindolin-2-one δ-lactones were smoothly obtained by the intramolecular Dieckmann cyclization of oxindoles in excellent yield under mild conditions.

Recent advances on the total synthesis of alkaloids in mainland China

Alkaloids are a large family of natural products that mostly contain basic nitrogen atoms. Because of their intriguing structures and important functions, they have long been popular targets for synthetic organic chemists. China's chemists have made significant progress in the area of alkaloid synthesis over past decades. In this article, selected total syntheses of alkaloids from research groups in mainland China during the period 2011–16 are highlighted.

Enantioselective synthesis of spirooxindole benzoquinolizines via organo-catalyzed cascade reactions

A Michael–Mannich–hemiaminalization–dehydration cascade reaction was developed for the construction of spirooxindole benzoquinolizine derivatives.

Transition-metal-free dehydrogenation coupling of pyridinium through a self-promoted hydride transfer process

An intramolecular dehydrogenation coupling between phenol and pyridinium through a unique substrate self-promoted hydride transfer process has been developed.

Cerium oxide-catalyzed multicomponent condensation approach to spirooxindoles in water

An efficient and facile green synthesis of spirooxindole derivatives bearing pyrano[2,3-c]pyrazole moiety has been achieved via a [Formula: see text]-NPs catalyzed four-component reaction in water. The protocol offers an environmentally benign and effective approach to highly functionalized and biologically interesting spiro[indoline-3,4[Formula: see text]-pyrano[2,3-c]pyrazole] derivatives. The synthesized compounds exhibit potent antioxidant and antibacterial activities.

Alstoscholarisines H-J, Indole Alkaloids from Alstonia scholaris: Structural Evaluation and Bioinspired Synthesis of Alstoscholarisine H

Alstoscholarisines H-J (1-3), new monoterpenoid indole alkaloids with an unprecedented skeleton created via the formation of a C-3/N-1 bond, were isolated from Alstonia scholaris. Their structures were established by extensive spectroscopic analyses and the assessment of single-crystal X-ray diffraction data. The total synthesis of alstoscholarisine H was achieved via the regioselective nucleophilic addition of pyridinium through a bioinspired iminium ion intermediate followed by Pictet-Spengler-like cyclization.