Alkaloids from Aspidosperma species from Bolivia

Synthesis of Hydrocarbazole Derivatives by Oxidative Dearomative Cyclization of Diarylamines Using a Hypervalent Iodine Reagent

Hydrocarbazole derivatives bearing a quaternary stereogenic center at C4a were synthesized by means of intramolecular oxidative dearomatization of diarylamines using hypervalent iodine in moderate to good yields. The hydrocarbazole bearing a cyclohexadienone moiety was further converted into the tetracyclic skeletons of Aspidosperma and akuammiline-type alkaloids via regioselective aza-Michael reaction at C4 and at C9a, respectively.

Aspidosperma pyrifolium, a medicinal plant from the Brazilian caatinga, displays a high antiplasmodial activity and low cytotoxicity

Background

Several species of Aspidosperma plants are referred to as remedies for the treatment of malaria, especially Aspidosperma nitidum. Aspidosperma pyrifolium, also a medicinal plant, is used as a natural anti-inflammatory. Its fractionated extracts were assayed in vitro for activity against malaria parasites and for cytotoxicity.

Methods

Aspidosperma pyrifolium activity was evaluated against Plasmodium falciparum using extracts in vitro. Toxicity towards human hepatoma cells, monkey kidney cells or human monocytes freshly isolated from peripheral blood was also assessed. Anti-malarial activity of selected extracts and fractions that presented in vitro activity were tested in mice with a Plasmodium berghei blood-induced infection.

Results

The crude stem bark extract and the alkaloid-rich and ethyl acetate fractions from stem extract showed in vitro activity. None of the crude extracts or fractions was cytotoxic to normal monkey kidney and to a human hepatoma cell lines, or human peripheral blood mononuclear cells; the MDL₅₀ values of all the crude bark extracts and fractions were similar or better when tested on normal cells, with the exception of organic and alkaloidic-rich fractions from stem extract. Two extracts and two fractions tested in vivo caused a significant reduction of P. berghei parasitaemia in experimentally infected mice.

Conclusion

Considering the high therapeutic index of the alkaloidic-rich fraction from stem extract of A. pyrifolium, it makes the species a candidate for further investigation aiming to produce a new anti-malarial, especially considering that the active extract has no toxicity, i.e., no mutagenic effects in the genototoxicity assays, and that it has an in vivo anti-malarial effect. In its UPLC-HRMS analysis this fraction was shown to have two major components compatible with the bisindole alkaloid Leucoridine B, and a novel compound, which is likely to be responsible for the activity against malaria parasites demonstrated in in vitro tests.

Concise Total Syntheses of (+)-Haplocidine and (+)-Haplocine via Late-Stage Oxidation of (+)-Fendleridine Derivatives

We report the first total syntheses of (+)-haplocidine and its N1-amide congener (+)-haplocine. Our concise synthesis of these alkaloids required the development of a late-stage and highly selective C-H oxidation of complex aspidosperma alkaloid derivatives. A versatile, amide-directed ortho-acetoxylation of indoline amides enabled our implementation of a unified strategy for late-stage diversification of hexacyclic C19-hemiaminal ether structures via oxidation of the corresponding pentacyclic C19-iminium ions. An electrophilic amide activation of a readily available C21-oxygenated lactam, followed by transannular cyclization and in situ trapping of a transiently formed C19-iminium ion, expediently provided access to hexacyclic C19-hemiaminal ether alkaloids (+)-fendleridine, (+)-acetylaspidoalbidine, and (+)-propionylaspidoalbidine. A highly effective enzymatic resolution of a non-β-branched primary alcohol (E = 22) allowed rapid preparation of both enantiomeric forms of a C21-oxygenated precursor for synthesis of these aspidosperma alkaloids. Our synthetic strategy provides succinct access to hexacyclic aspidosperma derivatives, including the antiproliferative alkaloid (+)-haplocidine.

Antimalarial plants used by indigenous people of the Upper Rio Negro in Amazonas, Brazil

ETHNOPHARMACOLOGICAL RELEVANCE

This is the first intercultural report of antimalarial plants in this region. The aim of this study was to document the medicinal plants used against malaria by indigenous people in the Upper Rio Negro region and to review the literature on antimalarial activity and traditional use of the cited species.

MATERIALS AND METHODS

Participant observation, semi-structured interviews, and ethnobotanical walks were conducted with 89 informants in five indigenous communities between April 2010 and November 2013 to obtain information on the use of medicinal plants against malaria. We reviewed academic databases for papers published in scientific journals up to January 2014 in order to find works on ethnopharmacology, ethnobotany, and antimalarial activity of the species cited.

RESULTS

Forty-six plant species belonging to 24 families are mentioned. Fabaceae (17.4%), Arecaceae (13.0%) and Euphorbiaceae (6.5%) account together for 36.9% of these species. Only seven plant species showed a relatively high consensus. Among the plant parts, barks (34.0%) and roots (28.0%) were the most widely used. Of the 46 species cited, 18 (39.1%) have already been studied for their antimalarial properties according to the literature, and 26 species (56.5%) have no laboratory essays on antimalarial activity.

CONCLUSIONS

Local traditional knowledge of the use of antimalarials is still widespread in indigenous communities of the Upper Rio Negro, where 46 plants species used against malaria were recorded. Our studies highlight promising new plants for future studies: Glycidendron amazonicum, Heteropsis tenuispadix, Monopteryx uaucu, Phenakospermum guianensis, Pouteria ucuqui, Sagotia brachysepala and notably Aspidosperma schultesii, Ampelozizyphus amazonicus, Euterpe catinga, E. precatoria, Physalis angulata, Cocos nucifera and Swartzia argentea with high-use consensus. Experimental validation of these remedies may help in developing new drugs for malaria.

Review of β-carboline Alkaloids from the Genus Aspidosperma

Plants belonging to the genus Aspidosperma, a member of the family Apocynaceae, provide a rich source of β-carboline alkaloids, which makes them potentially poisonous. However, some of these alkaloids possess antitumor and antimicrobial activity. The present review is a survey of the β-carboline alkaloids and shows that they comprise of a diverse array of structural modifications.

Antiparasitic Indole Alkaloids from Aspidosperma desmanthum and A. spruceanum from the Peruvian Amazonia

Twenty-three indole alkaloids were isolated from Aspidosperma desmanthum and A. spruceanum. Alkaloids 1-4 were isolated from the leaves, 5-8 from the stem bark and 9-15 from the root bark of A. desmanthum. Alkaloids 5, 11, 16, 17 and 19 were isolated from the stem bark, 18 and 20-22 from the root bark and 23 from the flowers of A. spruceanum. Compounds 4, 14, and 15 have not been previously reported as natural products while 16 and 20 have been isolated for the first time from the genus Aspidosperma. Their structures were determined by spectroscopic techniques including 1D and 2D NMR experiments (COSY, NOESY, HSQC, HMBC). The antiparasitic activity of these compounds was tested against Trypanosoma cruzi and Leishmania infantum and their non-specific cytotoxicity on mammalian cells. The most active compounds were 10, 12, 13, and 14 from A. desmanthum, and 19, 21 and 22 from A. spruceanum. Aspidolimine (10) aspidocarpine (12) and tubotaiwine (21) showed selective activity against L. infantum.

Synthesis of indole-based propellane derivatives via Weiss-Cook condensation, Fischer indole cyclization, and ring-closing metathesis as key steps

A variety of highly functionalized indole-based [n.3.3]propellane derivatives is described. The synthesis of the propellane derivatives involves a Weiss–Cook condensation, a Fischer indole cyclization, and a ring-closing metathesis as key steps.

¹H and ¹³C-NMR data of the simplest plumeran indole alkaloids isolated from Aspidosperma species

Indole alkaloids are the chemotaxonomic markers of the Aspidosperma genera. Those that have the simplest plumeran skeleton are classified as the precursors of biosynthetic routes and the intermediates for several synthetic reactions. This work aims to review the ¹H and ¹³C-NMR data, up to 2011, describing the skeleton of 35 different plumeran indole alkaloids, from a group of 46 of them, and highlight the main spectral differences amongst them.

Total synthesis of (+)-fendleridine (aspidoalbidine) and (+)-1-acetylaspidoalbidine

A total synthesis of the Aspidosperma alkaloids (+)-fendleridine and (+)-1-acetylaspidoalbidine is detailed, providing access to both enantiomers of the natural products and establishing their absolute configuration. Central to the synthetic approach is a powerful intramolecular [4+2]/[3+2] cycloaddition cascade of a 1,3,4-oxadiazole in which the pentacyclic skeleton and all the stereochemistry of the natural products are assembled in a reaction that forms three rings, four C-C bonds, and five stereogenic centers including three contiguous quaternary centers, and introduces the correct oxidation state at C19 in a single synthetic operation. The final tetrahydrofuran bridge is subsequently installed in one step, enlisting an intramolecular alcohol addition to an iminium ion generated by nitrogen-assisted opening of the cycloadduct oxido bridge, with a modification that permits release of useful functionality (a ketone) at the cleavage termini.

Antiplasmodial activity of aspidosperma indole alkaloids

The antiplasmodial activity of twelve alkaloids with an aspidospermane skeleton was estimated in vitro on chloroquine-resistant and sensitive strains of Plasmodium falciparum. Seven tetracyclic alkaloids possessing a free ethyl chain such aspidospermine, showed IC50 after incubation for 72 h between 3.2 and 15.4 microM. Moreover, four pentacyclic alkaloids with ethyl chain included in a tetrahydrofuran, such haplocine, showed a reduced activity, with IC50, after 72 h, between 22.6 and 52.6 microM. According to these results, a chloroquine-potentiating experiment was also performed with two of the most active compounds. Isobolograms were obtained and demonstrated a synergic effect of N-formyl-aspidospermidine and aspidospermine when associated with chloroquine. The cytotoxicity and the selectivity index of some alkaloids were also estimated.