Alkaloids of Kopsia

Development of a Non-Directed Petasis-Type Reaction by an Aromaticity-Disrupting Strategy

The Petasis-type reaction, which couples an imine and boronic acid, is an important tool for C-C bond formation in organic synthesis. However, the generality of this transformation has been limited by the requirement for a directing heteroatom to enable reactivity. Herein, we report the development of a non-directed Petasis-type reaction that allows for the coupling of trifluoroborate salts with α-hydroxyindoles. By disrupting aromaticity to generate a reactive iminium ion, in conjunction with using trifluoroborate nucleophiles, the method generates a new C-C bond without the need for a directing group. This reaction is operationally simple, providing α-functionalized indoles in up to 99 % yield using sp, sp² , and sp³ -hybridized trifluoroborate nucleophiles. Finally, this reaction is applied as a novel bioconjugation strategy to link biologically active molecules and toward the convergent synthesis of non-natural heterodimeric bisindole alkaloid analogs.

Divergent Total Synthesis of Four Kopsane Alkaloids: N-Carbomethoxy-10,22-dioxokopsane, Epikopsanol-10-lactam, 10,22-Dioxokopsane, and N-Methylkopsanone

We have achieved the divergent total synthesis of four kopsane alkaloids which share a complex heptacyclic caged ring system. Key transformations include an asymmetric Diels-Alder reaction to assemble the central bicyclo[2.2.2]octane moiety and the quaternary stereocenter at C20, a SmI₂ -mediated cascade reduction/aldol reaction to construct the five-membered ring and the quaternary stereocenter at C7, and a late-stage cascade reductive amination/cyclization to establish the highly strained caged ring system.

Samarium(ii) iodide-mediated reactions applied to natural product total synthesis

Natural product synthesis remains a field in which new synthetic methods and reagents are continually being evaluated. Due to the demanding structures and complex functionality of many natural products, only powerful and selective methods and reagents will be highlighted in this proceeding. Since its introduction by Henri Kagan, samarium(ii) iodide (SmI₂, Kagan's reagent) has found increasing use in chemical synthesis. Over the years, many reviews have been published on the application of SmI₂ in numerous reductive coupling procedures as well as in natural product total synthesis. This review highlights recent advances in SmI₂-mediated synthetic strategies, as applied in the total synthesis of natural products since 2004.

Natural product synthesis remains a field in which new synthetic methods and reagents are continually being evaluated.

Total Synthesis of (-)-Kopsifoline A and (+)-Kopsifoline E

We report the first total synthesis of (-)-kopsifoline A and (+)-kopsifoline E. Our synthetic strategy features a biogenetically inspired regioselective C17-functionalization of a versatile intermediate containing the pentacyclic core of aspidosperma alkaloids. The vinylogous urethane substructure of this intermediate affords (-)-kopsifoline D via C3-C21 bond formation under the Mitsunobu reaction conditions, while it enables selective C17-functionalization en route to (-)-kopsifoline A and (+)-kopsifoline E.

Kopsanone inhibits proliferation and migration of invasive colon cancer cells

Colorectal cancer (CRC) is the second leading cause of cancer-related death globally. In spite of the increasing knowledge on molecular characteristics of different cancer types including CRC, there is limitation in the development of an effective treatment. The present study aimed to verify the antitumor effect of kopsanone, an indole alkaloid. To achieve this, we treated human colon cancer cells (Caco-2 and HCT-116) with kopsanone and analyzed its effects on cell viability, cell-cell adhesion, and actin cytoskeleton organization. In addition, functional assays including micronuclei formation, colony formation, cell migration, and invasiveness were performed. We observed that kopsanone reduced viability and proliferation and induced micronuclei formation of HCT-116 cells. Also, kopsanone inhibited anchorage-dependent colony formation and modulated adherens junctions (AJs), thus increasing the localization of E-cadherin and β-catenin in the cytosol of the invasive cells. Finally, fluorescence assays showed that kopsanone decreased stress fibers formation and reduced migration but not invasion of HCT-116 cells. Taken together, these findings indicate that kopsanone reduces proliferation and migration of HCT-116 cells via modulation of AJs and can therefore be considered for future in vivo and clinical investigation as potential therapeutic agent for treatment of CRC.

4,21-Secovincanol, a Novel Immunosuppressive Monoterpenoid Indole Alkaloid from Kopsia Hainanensis

4,21-Secovincanol (1), a novel C-21/N-4 cleavage monoterpenoid indole alkaloid, along with four analogs (2-5), were obtained from the aerial parts of Kopsia hainanensis. Structurally, compound 1 might be a derivative of epivincanol (2) via C-21/N-4 cleavage. Their structures were confirmed by means of comprehensive spectroscopic data analysis and comparison with the reported data. All isolates significantly inhibited Con A-stimulated mice splenocytes proliferation at 10-40 μM in a dose-dependent manner in vitro. Especially, compound 3 exhibited potent activities comparable to positive control (Dexamethasone, DXM).

Enantioselective formation of quaternary carbon stereocenters in natural product synthesis: a recent update

Covering: 2013-2018 Natural products bearing quaternary carbon stereocenters have attracted tremendous interest from the synthetic community due to their diverse biological activities and fascinating molecular architectures. However, the construction of these molecules in an enantioselective fashion remains a long-standing challenge because of the lack of efficient asymmetric catalytic methods for installing these motifs. The rapid progress in the development of new-generation efficient chiral catalysts has opened the door for several asymmetric reactions, such as Michael addition, dearomative cyclization, polyene cyclization, α-arylation, cycloaddition, allylation, for the construction of quaternary carbon stereocenters in a highly enantioselective fashion. These asymmetric catalytic methods have greatly facilitated the synthesis of complex natural products with improved output and overall efficiency. In this concise review, we highlight the progress in the last six years in complex natural product synthesis, in which at least one quaternary carbon stereocenter has been constructed via asymmetric catalytic technologies, with particular emphasis on the analysis of the stereochemical model of each enantioselective transformation.

Total Syntheses of Lycopodium and Monoterpenoid Indole Alkaloids Based on Biosynthesis-Inspired Strategies

The merits of biogenetic considerations in the chemical syntheses of natural products have been emphasized by describing the total syntheses of Lycopodium alkaloids; lycodine, flabellidine, lycopodine, and flabelliformine, as well as monoterpenoid indole alkaloids; C-mavacurine, kopsiyunnanine K, koumine, and 11-methoxy-19R-hydroxygelselegine.

Total Syntheses of (-)-Deoxoapodine, (-)-Kopsifoline D, and (-)-Beninine

The total syntheses of Aspidosperma and Kopsia alkaloids (-)-deoxoapodine, (-)-kopsifoline D, and (-)-beninine are described through a domino deprotection-Michael addition-nucleophilic substitution protocol to assemble the core framework in efficient steps. Corey-Bakshi-Shibata reduction was employed to afford the enantioenriched intermediate for the total syntheses of the aforementioned alkaloids. The chirality was shown to completely transfer to the backbone using Johnson-Claisen rearrangement. The enantioselective total syntheses of (-)-kopsifoline D and (-)-beninine were accomplished for the first time. Our strategy opens up practical avenues for the total synthesis of structurally similar alkaloids.

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.

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.

Catalytic reductive [4 + 1]-cycloadditions of vinylidenes and dienes

Cycloaddition reactions provide direct and convergent routes to cycloalkanes, making them valuable targets for the development of synthetic methods. Whereas six-membered rings are readily accessible from Diels-Alder reactions, cycloadditions that generate five-membered rings are comparatively limited in scope. Here, we report that dinickel complexes catalyze [4 + 1]-cycloaddition reactions of 1,3-dienes. The C₁ partner is a vinylidene equivalent generated from the reductive activation of a 1,1-dichloroalkene in the presence of stoichiometric zinc. Intermolecular and intramolecular variants of the reaction are described, and high levels of asymmetric induction are achieved in the intramolecular cycloadditions using a C ₂-symmetric chiral ligand that stabilizes a metal-metal bond.

Apocynaceae species with antiproliferative and/or antiplasmodial properties: a review of ten genera

Apocynaceae is a large family of tropical trees, shrubs and vines with most species producing white latex. Major metabolites of species are triterpenoids, iridoids, alkaloids and cardenolides, which are known for a wide range of biological and pharmacological activities such as cardioprotective, hepatoprotective, neuroprotective, anti-inflammatory, anticancer and antimalarial properties. Prompted by their anticancer and antimalarial properties, the current knowledge on ten genera (Allamanda, Alstonia, Calotropis, Catharanthus, Cerbera, Dyera, Kopsia, Nerium, Plumeria and Vallaris) is updated. Major classes of metabolites are described using some species as examples. Species with antiproliferative (APF) and/or antiplasmodial (APM) properties have been identified. With the exception of the genus Dyera, nine genera of 22 species possess APF activity. Seven genera (Alstonia, Calotropis, Catharanthus, Dyera, Kopsia, Plumeria and Vallaris) of 13 species have APM properties. Among these species, Alstonia angustiloba, Alstonia macrophylla, Calotropis gigantea, Calotropis procera, Catharanthus roseus, Plumeria alba and Vallaris glabra displayed both APF and APM properties. The chemical constituents of these seven species are compiled for assessment and further research.

Collective Total Synthesis of (-)-Lundurines A-C

A collective asymmetric total synthesis of lundurines A-C using l-pyroglutamic acid derived from the chiral pool is described. The key steps include a tandem reductive amination/lactamization sequence to introduce the pyrrolidinone ring, a palladium-catalyzed intramolecular direct C-H vinylation of indole to construct the crucial polyhydroazocine ring, and a Lewis acid promoted formal [3 + 2] cycloaddition/N₂ extrusion process to install the polysubstituted cyclopropyl ring.

Unified Total Synthesis of Pyrroloazocine Indole Alkaloids Sheds Light on Their Biosynthetic Relationship

The total synthesis of seven members of the lapidilectine and grandilodine family of alkaloids has been accomplished in racemic and enantiopure form without protection/deprotection of functional groups. The two key steps, an 8- endo-dig hydroarylation and a 6- exo-trig photoredox cyclization, were catalyzed using gold. A rationale for the formation of the cyclopropane ring of the lundurines is also provided.

Biosynthetic Enantiodivergence in the Eburnane Alkaloids from Kopsia

Reexamination of the absolute configuration of recently isolated eburnane alkaloids from Malaysian Kopsia and Leuconotis species by X-ray diffraction analysis and ECD/TDDFT has revealed the existence of biosynthetic enantiodivergence. Three different scenarios are discerned with respect to the composition of the enantiomeric eburnane alkaloids in these plants: first, where the new eburnane congeners possess the same C-20, C-21 absolute configurations as the common eburnane alkaloids (eburnamonine, eburnamine, isoeburnamine, eburnamenine) occurring in the same plant; second, where the new eburnane congeners possess opposite or enantiomeric C-20, C-21 absolute configurations compared to the common eburnane alkaloids found in the same plant; and, third, where the four common eburnane alkaloids were isolated as racemic or scalemic mixtures, while the new eburnane congeners were isolated as pure enantiomers with a common C-20, C-21 configuration (20α, 21α). Additionally, the same Kopsia species (K. pauciflora) found in two different geographical locations (Peninsular Malaysia and Malaysian Borneo) showed different patterns in the composition of the enantiomeric eburnane alkaloids. Revision of the absolute configurations of a number of new eburnane congeners (previously assigned based on the assumption of a common biogenetic origin to that of the known eburnane alkaloids co-occurring in the same plant) is required based on the present results.

Enantioselective Catalysis Coupled with Stereodivergent Cyclization Strategies Enables Rapid Syntheses of (+)-Limaspermidine and (+)-Kopsihainanine A

Enantioselective Pd-catalyzed allylic alkylations of dihydropyrido[1,2-a]indolone (DHPI) substrates were used to construct the C20-quaternary stereocenters of multiple monoterpene indole alkaloids. Stereodivergent Pictet-Spengler and Bischler-Napieralski cyclization/reduction cascades furnish the cis- and trans-fused azadecalin subunits present in Aspidosperma and Kopsia alkaloids, respectively, en route to highly efficient syntheses of (+)-limaspermidine and (+)-kopsihainanine A.

Monoterpenoid Indole Alkaloids from Kopsia officinalis and the Immunosuppressive Activity of Rhazinilam

Six new monoterpenoid indole alkaloids, kopsinidines C-E (1-3), 11,12-methylenedioxychanofruticosinic acid (4), 12-methoxychanofruticosinic acid (5), and N(4)-methylkopsininate (7), as well as chanofruticosinic acid (6, as a natural product) and 23 known alkaloids, were obtained from the twigs and leaves of Kopsia officinalis. Their structures were characterized by physical data analysis. All isolated compounds were evaluated for their immunosuppressive activity on human T cell proliferation. Rhazinilam (29) significantly inhibited human T cell proliferation activated by anti-CD3/anti-CD28 antibodies (IC₅₀ = 1.0 μM) and alloantigen stimulation (IC₅₀ = 1.1 μM) without obvious cytotoxicity for naïve human T cells and peripheral blood mononuclear cells (0-320 μM). Although it did not affect T cell activation, it induced T cell cycle arrest in the G₂/M phase and inhibited proinflammatory cytokine production in activated T cells.

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

Two new monoterpene indole alkaloids, characterized by previously unencountered natural product skeletons, viz., arborisidine (1), incorporating indolizidine and cyclohexanone moieties fused to an indole unit, and arbornamine (2), incorporating an unprecedented 6/5/6/5/6 "arbornane" skeleton (distinct from the eburnan or tacaman skeleton), were isolated from a Malayan Kopsia arborea. The structures of the alkaloids were determined based on analysis of the NMR and MS data. Possible biogenetic pathways to these alkaloids from a common pericine precursor (3) are presented.

Aspidofractinine and Eburnane Alkaloids from a North Borneo Kopsia. Ring-Contracted, Additional Ring-Fused, and Paucidactine-Type Aspidofractinine Alkaloids from K. pauciflora

Eleven new indole alkaloids (1-11) comprising seven aspidofractinine and four eburnane alkaloids, were isolated from the stem-bark extract of Kopsia pauciflora occurring in Malaysian Borneo. The aspidofractinine alkaloids include a ring-contracted, an additional ring-fused, a paucidactine regioisomer, two paucidactine, and one kopsine alkaloid. The structures of several of these alkaloids were also confirmed by X-ray diffraction analyses. The bisindole alkaloids isolated, norpleiomutine and kopsoffinol, showed in vitro growth inhibitory activity against human PC-3, HCT-116, MCF-7, and A549 cells and moderate effects in reversing multidrug-resistance in vincristine-resistant human KB cells.

Monoterpene Indole Alkaloids from the Twigs of Kopsia arborea

The phytochemistry of Kopsia arborea Blume has received considerable attention, which has resulted in the isolation of a number of new unusual indole alkaloids with intriguing structures. In this study, a new eburnane-type alkaloid, phutdonginin (1), together with eight known alkaloids: 19-OH-(-)-eburnamonine (2), melodinine E (3), kopsinine (4), kopsilongine (5), kopsamine (6), (-)-methylenedioxy-11,12-kopsinaline (7), decarbomethoxykopsiline (8), and vincadifformine (9), were isolated from the twigs of K. arborea. Their structures were characterized extensively by 1D and 2D NMR spectroscopy and HR-ESI-MS. All compounds were submitted to TLC screening for acetylcholinesterase inhibitory activities. Only kopsamine and decarbomethoxykopsiline showed AChE inhibition with MIR values of 12.5 and 6.25 μg, respectively, compared with galanthamine (positive control, 0.004 μg). In addition, compounds 1 and 2 inhibited moderate antibacterial activity against E. coli TISTR 780 with the MIC value of 32 μg/mL.

Transformations of the 2,7-Seco Aspidosperma alkaloid leuconolam, structure revision of epi-leuconolam, and partial syntheses of leuconoxine and leuconodines A and F

Several transformations of the seco Aspidosperma alkaloid leuconolam were carried out. The based-induced reaction resulted in cyclization to yield two epimers, the major product corresponding to the optical antipode of a (+)-meloscine derivative. The structures and relative configuration of the products were confirmed by X-ray diffraction analysis. Reaction of leuconolam and epi-leuconolam with various acids, molecular bromine, and hydrogen gave results that indicated that the structure of the alkaloid, previously assigned as epi-leuconolam, was incorrect. This was confirmed by an X-ray diffraction analysis, which revealed that epi-leuconolam is in fact 6,7-dehydroleuconoxine. Short partial syntheses of the diazaspiro indole alkaloid leuconoxine and the new leuconoxine-type alkaloids leuconodines A and F were carried out.

Antiproliferative and phytochemical analyses of leaf extracts of ten Apocynaceae species

Background:

The anticancer properties of Apocynaceae species are well known in barks and roots but less so in leaves.

Materials and Methods:

In this study, leaf extracts of 10 Apocynaceae species were assessed for antiproliferative (APF) activities using the sulforhodamine B assay. Their extracts were also analyzed for total alkaloid content (TAC), total phenolic content (TPC), and radical scavenging activity (RSA) using the Dragendorff precipitation, Folin–Ciocalteu, and 1,1-diphenyl-2-picrylhydrazyl (DPPH) assays, respectively.

Results:

Leaf extracts of Alstonia angustiloba, Calotropis gigantea, Catharanthus roseus, Nerium oleander, Plumeria obtusa, and Vallaris glabra displayed positive APF activities. Extracts of Allamanda cathartica, Cerbera odollam, Dyera costulata, and Kopsia fruticosa did not show any APF activity. Dichloromethane (DCM) extract of C. gigantea, and DCM and DCM:MeOH extracts of V. glabra showed strong APF activities against all six human cancer cell lines. Against breast cancer cells of MCF-7 and MDA-MB-231, DCM extracts of C. gigantea and N. oleander were stronger than or comparable to standard drugs of xanthorrhizol, curcumin, and tamoxifen. All four extracts of N. oleander were effective against MCF-7 cells. Extracts of Kopsia fruticosa had the highest TAC while those of Dyera costulata had the highest TPC and RSA. Extracts of C. gigantea and V. glabra inhibited the growth of all six cancer cell lines while all extracts of N. oleander were effective against MCF-7 cells.

Conclusion:

Extracts of C. gigantea, V. glabra, and N. oleander therefore showed great promise as potential candidates for anticancer drugs. The wide-spectrum APF activities of these three species are reported for the first time and their bioactive compounds warrant further investigation.

Grandilodines A-C, biologically active indole alkaloids from Kopsia grandifolia

Three new indole alkaloids (1-3), named grandilodines A-C, and five known ones were obtained from the Malayan Kopsia grandifolia. The structures were established using NMR and MS analyses and, in the case of 1 and 2, were confirmed by X-ray diffraction analyses. Alkaloids 1, 3, and lapidilectine B (8) were found to reverse multidrug resistance in vincristine-resistant KB cells.

Strychnan and secoangustilobine A type alkaloids from Alstonia spatulata. Revision of the C-20 configuration of scholaricine

A total of 25 alkaloids were isolated from the leaf and stem-bark extracts of Alstonia spatulata, of which five are new alkaloids of the strychnan type (alstolucines A-E, 1-5) and the other, a new alkaloid of the secoangustilobine A type (alstolobine A, 6). The structures of these alkaloids were established using NMR and MS analysis and, in the case of alstolucine B (2), also confirmed by X-ray diffraction analysis. A reinvestigation of the stereochemical assignment of scholaricine (13) by NMR and X-ray analyses indicated that the configuration at C-20 required revision. Alkaloids 1, 2, 6, 7, 9, 10, and 13 reversed multidrug resistance in vincristine-resistant KB cells.

Leucoridines A-D, cytotoxic Strychnos-Strychnos bisindole alkaloids from Leuconotis

Four new bisindole alkaloids of the Strychnos-Strychnos type, leucoridines A-D (1-4), were isolated from the stem-bark extract of Leuconotis griffithii. Alkaloids 1-4 showed moderate cytotoxicity against drug-sensitive and vincristine-resistant human KB cells.

Leuconicines A-G and (-)-eburnamaline, biologically active strychnan and eburnan alkaloids from Leuconotis

Seven new indole alkaloids of the Strychnos type, leuconicines A-G (1-7), and a new eburnan alkaloid, (-)-eburnamaline (8), were isolated from the stem-bark extract of two Malayan Leuconotis species. The structures of these alkaloids were established using NMR and MS analysis and in the case of 8 also by partial synthesis. Alkaloids 1-5 reversed multidrug resistance in vincristine-resistant KB cells.

Leucophyllidine, a cytotoxic bisindole alkaloid constituted from the union of an eburnan and a new vinylquinoline alkaloid

A cytotoxic bisindole alkaloid possessing an unprecedented structure constituted from the union of an eburnan half and a novel vinylquinoline alkaloid has been isolated from Leuconotis griffithii. The structure was established by analysis of the spectroscopic data and confirmed by X-ray diffraction analysis. A possible biogenetic pathway to the novel quinolinic coupling partner is presented from an Aspidosperma precursor.