Arborisidine and Arbornamine, Two Monoterpenoid Indole Alkaloids with New Polycyclic Carbon-Nitrogen Skeletons Derived from a Common Pericine Precursor

An overview: total synthesis of arborisidine, and arbornamine

Arborisidine and Arbornamine are two monoterpenoid indole alkaloids that were isolated from the Malayan Kopsia arborea plant. This review provides valuable information about the total and formal syntheses of these alkaloids. The synthesis strategies discussed in this review, such as Pictet-Spengler cyclization, chemo- and stereoselective oxidative cyclization, Michael/Mannich cascade process, and intramolecular N-alkylation, can be useful for developing new methods to synthesize these and other similar compounds.

Asymmetric Total Synthesis of Alstrostine G Utilizing a Catalytic Asymmetric Desymmetrization Strategy

A highly effective enantioselective monobenzoylation of 1,3-diols has been developed for the synthesis of 1,1-disubstituted tetrahydro-β-carbolines. The chemistry has been successfully applied to the asymmetric total synthesis of (+)-alstrostine G, which also features a cascade Heck/hemiamination reaction enabling facile construction of the pivotal pentacyclic core.

Catalytic enantioselective nitrone cycloadditions enabling collective syntheses of indole alkaloids

Tetrahydro-β-carboline skeletons are prominent and ubiquitous in an extraordinary range of indole alkaloid natural products and pharmaceutical compounds. Powerful synthetic approaches for stereoselective synthesis of tetrahydro-β-carboline skeletons have immense impacts and have attracted enormous attention. Here, we outline a general chiral phosphoric acid catalyzed asymmetric 1,3-dipolar cycloaddition of 3,4-dihydro-β-carboline-2-oxide type nitrone that enables access to three types of chiral tetrahydro-β-carbolines bearing continuous multi-chiral centers and quaternary chiral centers. The method displays different endo/exo selectivity from traditional nitrone chemistry. The distinct power of this strategy has been illustrated by application to collective and enantiodivergent total syntheses of 40 tetrahydro-β-carboline-type indole alkaloid natural products with divergent stereochemistry and varied architectures.

A Formal Synthesis of (±)-Arborisidine

Herein, we report a formal synthesis of (±)-arborisidine via the creation of Jiao's intermediate with the critical caged structure. Starting from tryptamine, a Pictet-Spengler cyclization forged the piperidine ring, a Pd-catalyzed indole allylation and ring-closing metathesis protocol afforded a bridged aza-bicyclo[3.3.1]nonane moiety, and an intramolecular N-alkylation closed the final pyrrolidine ring. This study provides a new approach to the unique caged framework of arborisidine and relevant alkaloids.

Diastereocontrolled Construction of Spiroindolenines via Hexafluoroisopropanol-Promoted Dearomative Epoxide-Indole Cyclization

Herein, we report the discovery of the ipso-selective, dearomatizing spirocyclization of indole-tethered epoxides as a fundamentally new approach for constructing spiroindolenines equipped with three contiguous stereogenic centers under complete diastereocontrol (dr >99:1) and in high yields. Promoted by hexafluoroisopropanol, the protocol features a broad substrate scope, easy scale-up, and versatile transformations of the synthesized spiroindolenines.

Asymmetric Synthesis of Arboduridine

The first asymmetric total synthesis of the monoterpenoid indole alkaloid arboduridine has been accomplished. The tricyclic A/B/D ring system was constructed by an enantioselective Michael reaction followed by intramolecular nucleophilic addition. Intramolecular α-amination of a ketone forged the piperidine ring, while a Horner-Wadsworth-Emmons (HWE) reaction was used to form the pyrrolidine ring. A reduction cyclization cascade led to formation of the tetrahydrofuran ring.

Gilman reagent toward the synthesis of natural products

With the ever-increasing scope of organocuprates, a well-established Gilman reagent has been considered as an unprecedented synthetic tool in modern organic chemistry. The broad research profile of the Gilman reagent (R2CuLi in THF or Et2O) is owing to its propensity to carry out three kinds of reactions, i.e., epoxide ring opening reactions, 1,4-conjugate addition reactions, and SN2 reactions in a regioselective manner. This review examines the applications of Gilman reagent in the total synthesis of both abundant and scarce natural products of remarkable synthetic pharmaceutical profile reported since 2011. The presented insights will be of a vital roadmap to general organic synthesis and it will contribute to the development of new natural products and their analogues in future drug discovery.

Chemistry, bioactivity, biosynthesis, and total synthesis of stemmadenine alkaloids

Covering: up to July 2022Stemmadenine alkaloids are a restrictive sub-group of monoterpene indole alkaloids, represented by two congeners: stemmadenine and vallesamine. Their skeleton is defined by the cleavage of the C-3-C-7 bond of the Strychnos group's pentacyclic scaffold in monoterpene indole alkaloids. The parent alkaloid stemmadenine acts as a key intermediate in the biosynthesis of several major monoterpene indole alkaloid families, including regular Strychnos alkaloids, Aspidosperma alkaloids, and Iboga alkaloids. In this review, a complete coverage of the stemmadenine alkaloids, from the early reports till the present day at 2022, are presented, and their diverse biological activities are briefly described. Moreover, the biosynthetic proposal for stemmadenine and the proposed biogenetic conversion of stemmadenine-type alkaloids into vallesamine-type congeners are discussed in detail. Moreover, the successful synthetic strategies to access the strained stemmadenine scaffolds are fully reviewed.

Organocatalytic Enantioselective Pictet-Spengler Reaction of α-Ketoesters: Development and Application to the Total Synthesis of (+)-Alstratine A

We report herein an asymmetric Pictet-Spengler reaction of α-ketoesters. In the presence of a catalytic amount of simple alanine-derived squaramide and p-nitrobenzoic acid, reaction of tryptamines with methyl 2-oxoalkanoates afforded the corresponding 1-alkyl-1-methoxycarbonyl tetrahydro-β-carbolines (THBCs) in high yields and ee values. A primary kinetic isotope effect (KIE=4.5) using C2-deteurium-labelled tryptamine indicates that rearomatization through deprotonation of the pentahydro-β-carbolinium ion could be the rate- and enantioselectivity-determining step. A concise enantioselective total synthesis of (+)-alstratine A, a hexacyclic cagelike monoterpene indole alkaloid, featuring this reaction as a key step, was subsequently accomplished. Remeasurement of the [a]_D value of the natural product indicates that natural alstratine A is dextrorotatory rather than levorotatory as it was initially reported in the isolation paper.

Formal Synthesis of Arboridinine Enabled by a Double-Mannich Reaction

A formal synthesis of arboridinine has been achieved. In this synthesis, a double-Mannich reaction of the complex multisubstituted cyclohexanone was used to form the core skeleton of arboridinine.

Catalytic Enantioselective Pictet-Spengler Reaction of α-Ketoamides Catalyzed by a Single H-Bond Donor Organocatalyst

The asymmetric Pictet-Spengler reaction (PSR) with aldehydes is well known. However, PSR involving ketones as electrophilic partners is far-less developed. We report herein the first examples of catalytic enantioselective PSR of tryptamines with α-ketoamides. A new class of easily accessible prolyl-urea organocatalysts bearing a single H-bond donor function catalyzes the title reaction to afford 1,1-disubstituted tetrahydro-β-carbolines in excellent yields and enantioselectivities. The kinetic isotope effect using C2-deuterium-labelled tryptamine indicates that the rearomatization of the pentahydro-β-carbolinium ion intermediate might be the rate- and the enantioselectivity-determining step.

Total synthesis of monoterpenoid indole alkaloid (–)-arbophyllidine

The first asymmetric total synthesis of (–)-arbophyllidine, an unusual pentacyclic monoterpenoid indole alkaloid, has been achieved.

Merging dearomatization with redox-neutral C(sp3)–H functionalization via hydride transfer/cyclization: recent advances and perspectives

This review highlights the encouraging advances in hydride transfer-involved dearomatization reaction during the past decade, the content of which is categorized according to the hydride acceptors, namely vinylogous imines and quinone methides.

Divergent Synthesis of Skeletally Distinct Arboridinine and Arborisidine

A divergent synthesis of skeletally distinct arboridinine and arborisidine was achieved. The central divergent strategy was inspired by the divergent biosynthetic cyclization mode of arboridinine and arborisidine and their hidden topological connection. The branch point was reached through a Michael and Mannich cascade process. A site-selective intramolecular Mannich reaction was developed to construct the tetracyclic core of arboridinine, while a site-selective intramolecular α-amination of ketone was used to access the tetracyclic core of arborisidine. A strategic Peterson olefination through intramolecular nucleophile delivery was able to set up the exocyclic olefin of arboridinine.

Total Synthesis of (-)-Arborisidine

An asymmetric total synthesis of cage-like indole alkaloid arborisidine is presented. The new synthetic strategy features a catalytic parallel kinetic resolution based on ambident nucleophilicity (C3/N) of indole to set the absolute configurations of the two quaternary chiral centers, and a 5-exo-trig radical cyclization to form the bridged nitrogen-containing five-membered ring.

Synthesis of aza-quaternary centers via Pictet-Spengler reactions of ketonitrones

Despite the array of advances that have been made in Pictet-Spengler chemistry, particularly as it relates to the synthesis of β-carboline derivatives of both natural and designed origin, the ability to use such reactions to generate aza-quaternary centers remains limited. Herein, we report a simple procedure that enables the synthesis of a variety of such products by harnessing the distinct reactivity profiles of ketonitrones as activated by commercially available acyl chlorides. Notably, the reaction process is mild, fast, and high-yielding (54-97%) for a diverse collection of substrates, including some typically challenging ones, such as indole cores with electron-deficient substituents. In addition, by deploying an acyl bromide in combination with a thiourea promoter, a catalytic, asymmetric version has been established, leading to good levels of enantioselectivity (up to 83% ee) for several ketonitrones. Finally, the resultant N-O bonds within the products can also be functionalized in several unique ways, affording valuable complementarity to existing Pictet-Spengler variants based on the use of imines.

Recent Advances in the Synthesis of β-Carboline Alkaloids

β-Carboline alkaloids are a remarkable family of natural and synthetic indole-containing heterocyclic compounds and they are widely distributed in nature. Recently, these alkaloids have been in the focus of interest, thanks to their diverse biological activities. Their pharmacological activity makes them desirable as sedative, anxiolytic, hypnotic, anticonvulsant, antitumor, antiviral, antiparasitic or antimicrobial drug candidates. The growing potential inherent in them encourages many researchers to address the challenges of the synthesis of natural products containing complex β-carboline frameworks. In this review, we describe the recent developments in the synthesis of β-carboline alkaloids and closely related derivatives through selected examples from the last 5 years. The focus is on the key steps with improved procedures and synthetic approaches. Furthermore the pharmacological potential of the alkaloids is also highlighted.

Chemical Diversity and Bioactivities of Monoterpene Indole Alkaloids (MIAs) from Six Apocynaceae Genera

By the end of the twentieth century, the interest in natural compounds as probable sources of drugs has declined and was replaced by other strategies such as molecular target-based drug discovery. However, in the recent times, natural compounds regained their position as extremely important source drug leads. Indole-containing compounds are under clinical use which includes vinblastine and vincristine (anticancer), atevirdine (anti-HIV), yohimbine (erectile dysfunction), reserpine (antihypertension), ajmalicine (vascular disorders), ajmaline (anti-arrhythmic), vincamine (vasodilator), etc. Monoterpene Indole Alkaloids (MIAs) deserve the curiosity and attention of researchers due to their chemical diversity and biological activities. These compounds were considered as an impending source of drug-lead. In this review 444 compounds, were identified from six genera belonging to the family Apocynaceae, will be discussed. These genera (Alstonia, Rauvolfia, Kopsia, Ervatamia, and Tabernaemontana, and Rhazya) consist of 400 members and represent 20% of Apocynaceae species. Only 30 (7.5%) species were investigated, whereas the rest are promising to be investigated. Eleven bioactivities, including antibacterial, antifungal, anti-inflammatory and immunosuppressant activities, were reported. Whereas cytotoxic effect represents 47% of the reported activities. Convincingly, the genera selected in this review are a wealthy source for future anticancer drug lead.

Asymmetric Total Synthesis and Assignment of Absolute Configuration of Arbornamine

The asymmetric total synthesis of arbornamine was accomplished in 13 steps, leading to the assignment of its absolute configuration. The key features of the strategy include construction of the C16 quaternary carbon center by a highly diastereoselective Grignard reagent addition to N-tert-butanesulfinylimine, sequential site-selective amidation and N-alkylation to form the C and E rings, and [Ni(COD)₂]-mediated Michael addition to close the D ring.

Cytotoxic monoterpenoid indole alkaloids from the aerial part of Kopsia arborea

A phytochemical investigation on the 90% ethanol aqueous extract of the aerial part of Kopsia arborea led to the isolation of three new monoterpenoid indole alkaloids, kopsiarborines A-C (1-3). The new structures were elucidated by using spectroscopic data (NMR, IR, UV, and MS). The isolated alkaloids were tested in vitro for cytotoxic potentials against six human lung cancer cell lines (A549, ATCC, H446, H460, H292, and 95-D). As a result, alkaloids 1 and 2 exhibited significant cytotoxic activities against all the tested tumor cell lines with IC₅₀ values less than 20 µM.

The indole-based subincanadine alkaloids and their biogenetic congeners

The tryptamine-derived polycyclic bridged bioactive indole alkaloids subincanadines A-G were isolated in 2002 by Ohsaki and coworkers from the bark of the Brazilian medicinal plant Aspidosperma subincanum. Kobayashi proposed that subincanadines D-F could be biosynthetically resulting from stemmadenine via two different pathways and, furthermore, that the subincanadines A-C could be biogenetically resulting from subincanadines D and E. Kam and coworkers, in their focused efforts, isolated five indole alkaloids from Malaysian Kopsia arborea species, namely valparicine, apparicine, arboridinine, arborisidine, and arbornamine in combination with subincanadine E. On the basis of structural features, it has been proposed and proved in some examples that subincanadine E is a biogenetic precursor of these five different bioactive indole alkaloids bearing complex structural architectures. All important information on isolation, characterization, bioactivity, probable biogenetic pathways, and more specifically racemic and enantioselective total synthesis of subincanadine alkaloids and their biogenetic congeners are summarized in the present chapter. Special importance is given to the total synthesis and the synthetic strategies intended therein, comprising a set of main reactions.

A Cytotoxic Indole Characterized by Incorporation of a Unique Carbon-Nitrogen Skeleton and Two Pentacyclic Corynanthean Alkaloids Incorporating a Substituted Tetrahydrofuranone Ring from Kopsia arborea

Three new alkaloids were isolated from the bark extract of the Malayan Kopsia arborea, viz., arbophyllidine (1), an unusual pentacyclic, monoterpenoid indole characterized by an absence of oxygen atoms and incorporating a new carbon-nitrogen skeleton, and arbophyllinines A (2) and B (3), two pentacyclic corynanthean alkaloids incorporating a hydroxyethyl-substituted tetrahydrofuranone ring. The structures of the alkaloids were deduced based on analysis of the MS and NMR data and confirmed by X-ray diffraction analyses. The absolute configuration of arbophyllidine (1) was established based on experimental and calculated ECD data, while that of arbophyllinine A was based on X-ray diffraction analysis (Cu Kα). A reasonable biosynthetic route to arbophyllidine (1) from a pericine precursor is presented. Arbophyllidine (1) showed pronounced in vitro growth inhibitory activity against the HT-29 human cancer cell line with IC₅₀ 6.2 μM.

Total Synthesis of (+)-Arborisidine

The first total synthesis of arborisidine, a unique Kopsia indole alkaloid possessing a fully substituted cyclohexanone ring system with two quaternary carbons, has been achieved in seven steps in racemic format from tryptamine and in nine steps in asymmetric format from d-tryptophan methyl ester. Key elements of the design include a carefully orchestrated decyanation protocol to finalize the asymmetric formation of an aza-quaternary center that is challenging to access in optically active format via direct Pictet-Spengler cyclizations with tryptamine, a metal-promoted 6- endo-dig cyclization of an enyne to establish the second core quaternary center, and regiospecific functionalizations of the resultant complex diene to finalize the target structure. The distinct and efficient nature of the developed solution is highlighted by several unsuccessful approaches and unexpected rearrangements.

Progress in total synthesis of subincanadine alkaloids and their congeners

A concise account of isolation, characterization, bioactivity, plausible biogenetic pathways, and most importantly, total synthesis of structurally fascinating and biologically imperious indole-based subincanadine alkaloids and their biogenetic congeners are described in the present review with special emphasis on total synthesis and therein an involved set of key reactions.

Diastereoselective Synthesis of (±)-epi-Subincanadine C

Starting from indolylmaleimide, concise and efficient total synthesis of (±)-epi-subincanadine C was described via stereoselective Wittig olefination, base-induced selective mono-prenylation, regioselective Grignard reaction, diastereoselective Pictet-Spengler cyclization, regioselective oxidative carbon-carbon double-bond cleavage, one-pot reductions, and intramolecular cyclization pathway. An attempted synthesis of (±)-subincanadine C via diastereoselective Grignard addition to the α,β-unsaturated γ-lactam or diastereoselective reduction of a carbon-carbon double bond also resulted in yet another route to (±)-epi-subincanadine C.

Application of the Asymmetric Pictet-Spengler Reaction in the Total Synthesis of Natural Products and Relevant Biologically Active Compounds

Tetrahydroisoquinolines are the framework of numerous natural products predominantly alkaloids, an important and one of the most wide spread families of naturally occurring compounds in the plant kingdom. Tetrahydroisoquinolines are commonly constructed through an old reaction, the so-called Pictet−Spengler Reaction (PSR). In this reaction, a β-aryl ethylamine undergoes an acid mediated condensation with a suitable aldehyde or ketone, followed by ring closure. In this review, we aim to highlight the applications of the asymmetric variant of this old name reaction in the total synthesis of natural products, chiefly, alkaloids, which exhibit significant biological properties.

Short Synthesis of the Monoterpene Indole Alkaloid (±)-Arbornamine

The first total synthesis of the monoterpene indole alkaloid (±)-arbornamine (1) has been completed, which proceeds in only 6 steps and 31% overall yield from three readily available, known compounds. The synthesis features a cascade involving a Pictet-Spengler cyclization/intramolecular ammonolysis to create the tetracyclic core of arbornamine (1) in a single chemical operation. The subsequent elaboration of 5 into 1 was effected by a key reductive Heck reaction and global reduction.

Survey of natural products reported by Asian research groups in 2016

The new natural products reported in peer-reviewed articles in 2016 in journals with good reputations were reviewed and analyzed. The advances that Asian research groups made in the field of natural products chemistry in 2016 were summarized. Compounds with unique structural features and/or promising bioactivities originating from Asian natural sources were discussed based on structural classification.

Rauvomines A and B, Two Monoterpenoid Indole Alkaloids from Rauvolfia vomitoria

Two unusual normonoterpenoid indole alkaloids rauvomine A (1) and rauvomine B (2), together with two known compounds peraksine (3) and alstoyunine A (4), were isolated from the aerial parts of Rauvolfia vomitoria. The structures with absolute configurations of 1 and 2 were elucidated by spectroscopic analysis, single-crystal X-ray diffraction, and electronic circular dichroism (ECD) calculations. Compound 2 is a novel C₁₈ normonoterpenoid indole alkaloid with a substituted cyclopropane ring that forms an unusual 6/5/6/6/3/5 hexcyclic rearranged ring system. The plausible biogenetic pathways of 1 and 2 were proposed. Compound 2 exhibited significant anti-inflammatory activity.

Three novel indole alkaloids from Kopsia officinalis

Three new indole alkaloids, named kopsioffines A-C (1-3), possessing relatively novel ten-membered lactam ring, and one known compound (11,12-demethoxy-16-deoxypauciflorine, 4) were isolated from the leaves and stems of Kopsia officinalis. Their structures were elucidated by means of spectroscopic methods. The absolute configuration of 1 was determined by calculated electronic circular dichroism data. The hypothetical biosynthetic pathway of 1 was postulated. All these isolates were evaluated for their inhibitory effects on α-glucosidase. None of them showed activity with IC₅₀ far beyond 50μM.