Four tetracyclic oxindole alkaloids and a taberpsychine derivative from a Malayan Tabernaemontana

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.

Uncarialines A-E, new alkaloids from Uncaria rhynchophylla and their anticoagulant activity

Uncarialines A-E (1-5), five undescribed monoterpene indole alkaloids, together with five known analogues were obtained from the stems of Uncaria rhynchophylla. Alkaloids 1-3 were unique 3,4-seco-tricyclic alkaloids with a 6/5/10 ring system, while 4 and 5 possessed a rare rearranged scaffold originated from corynantheine-type alkaloids with C-2/C-7 oxidation. Their structures were characterized by a comprehensive analysis of MS, NMR, and ECD. Their effects on blood clotting times of human plasma were evaluated and alkaloid 5 had a slight prolongation effect on both thrombin time and activated partial thromboplastin time (p < 0.001).

Vobasine, vincamine, voaphylline, tacaman, and iboga alkaloids from Tabernaemontana corymbosa

Thirteen indole alkaloids comprising six vobasine/sarpagine, one vincamine, two voaphylline, two tacaman, one iboga, and one corynantheine alkaloid, were isolated from the leaf extract of Tabernaemontana corymbosa (sample from Taiping, Perak, Malaysia). The structures of these alkaloids were determined based on analysis of the spectroscopic data (NMR and MS), and in the case of vincarudine, the absolute configuration was established by ECD and X-ray diffraction analysis. Vobasidine E represents the first vobasine-type alkaloid characterized by a contracted ring C and loss of the ethylidene/ethyl side chain. A possible biogenetic pathway from a perivine precursor, which was also present in the leaf extract, is presented. Differences in the new alkaloid content between the present and previous sample of the same plant (occurring in a different location) are discussed.

Catalytic Asymmetric Alkynylation of 3,4-Dihydro-β-carbolinium Ions Enables Collective Total Syntheses of Indole Alkaloids

Chiral tetrahydro-β-carboline (THβC) is not only a prevailing structural feature of many natural alkaloids but also a versatile synthetic precursor for a vast array of monoterpenoid indole alkaloids. Asymmetric synthesis of C1-alkynyl THβCs remains rarely explored and challenging. Herein, we describe the development of two complementary approaches for the catalytic asymmetric alkynylation of 3,4-dihydro-β-carbolinium ions with up to 96 % yield and 99 % ee. The utility of chiral C1-alkynyl THβCs was demonstrated by the collective total syntheses of seven indole alkaloids: harmicine, eburnamonine, desethyleburnamonine, larutensine, geissoschizol, geissochizine, and akuammicine.

Studies on the Enantioselective Synthesis of E-Ethylidene-bearing Spiro[indolizidine-1,3'-oxindole] Alkaloids

A synthetic route for the enantioselective construction of the tetracyclic spiro[indolizidine-1,3′-oxindole] framework present in a large number of oxindole alkaloids, with a cis H-3/H-15 stereochemistry, a functionalized two-carbon substituent at C-15, and an E-ethylidene substituent at C-20, is reported. The key steps of the synthesis are the generation of the tetracyclic spirooxindole ring system by stereoselective spirocyclization from a tryptophanol-derived oxazolopiperidone lactam, the removal of the hydroxymethyl group, and the stereoselective introduction of the E-ethylidene substituent by acetylation at the α-position of the lactam carbonyl, followed by hydride reduction and elimination. Following this route, the 21-oxo derivative of the enantiomer of the alkaloid 7(S)-geissoschizol oxindole has been prepared.

Concise Total Synthesis of Tronocarpine

A concise total synthesis of tronocarpine, a chippiine-type indole alkaloid, was accomplished. The key feature of this total synthesis is a one-pot construction of the pentacyclic skeleton containing an azabicyclo[3.3.1]nonane core by tandem cyclization from an indole derivative with all carbon side chains and functional groups. This tandem cyclization consists of α,β-unsaturated aldehyde formation, intramolecular aldol reaction, six-membered lactamization, azide reduction, and seven-membered lactamization. The stereochemical outcome in this tandem cyclization is controlled by the stereocenter at the C14 position. This strategy can be utilized to synthesize other chippiine-type alkaloids with azabicyclo[3.3.1]nonane skeletons.

Alstonlarsines A-D, Four Rearranged Indole Alkaloids from Alstonia scholaris

Four indole alkaloids, alstonlarsines A-D (1-4), were isolated from Alstonia scholaris and structurally characterized. Compound 1 possesses a new carbon skeleton with a cage-shaped 9-azatricyclo[4.3.1.0^3,8]decane motif, and compounds 2-4 feature a rare carbon skeleton that was found in nature for the first time. Plausible biosynthetic routes for 1-4 are proposed. Compound 1 showed DRAK2 inhibitory activity with an IC₅₀ value of 11.65 ± 0.63 μΜ.

A review on ethnobotany, pharmacology and phytochemistry of Tabernaemontana corymbosa

OBJECTIVES

Tabernaemontana is a genus from the plant family, Apocynaceae with vast medicinal application and widespread distribution in the tropics and subtropics of Africa, Americas and Asia. The objective of this study is to critically evaluate the ethnobotany, medicinal uses, pharmacology and phytochemistry of the species, Tabernaemontana corymbosa (Roxb. ex Wall.) and provide information on the potential future application of alkaloids isolated from different parts of the plant.

KEY FINDINGS

T. corymbosa (Roxb. ex Wall.) parts are used as poultice, boiled juice, decoctions and infusions for treatment against ulceration, fracture, post-natal recovery, syphilis, fever, tumours and orchitis in Malaysia, China, Thailand and Bangladesh. Studies recorded alkaloids as the predominant phytochemicals in addition to phenols, saponins and sterols with vast bioactivities such as antimicrobial, analgesic, anthelmintic, vasorelaxation, antiviral and cytotoxicity.

SUMMARY

An evaluation of scientific data and traditional medicine revealed the medicinal uses of different parts of T. corymbosa (Roxb. ex Wall.) across Asia. Future studies exploring the structure-bioactivity relationship of alkaloids such as jerantinine and vincamajicine among others could potentially improve the future application towards reversing anticancer drug resistance.

Ibogan, tacaman, and cytotoxic bisindole alkaloids from tabernaemontana. Cononusine, an iboga alkaloid with unusual incorporation of a pyrrolidone moiety

Six new indole alkaloids, viz., cononusine (1, a rare example of an iboga-pyrrolidone conjugate), ervaluteine (2), vincamajicine (3), tacamonidine (4), 6-oxoibogaine (5), and N(4)-chloromethylnorfluorocurarine chloride (6), and two new vobasinyl-iboga bisindole alkaloids, ervatensines A (7) and B (8), in addition to other known alkaloids, were isolated from the stem-bark extract of the Malayan Tabernaemontana corymbosa. The structures of these alkaloids were established on the basis of NMR and MS analyses and, in one instance (7), confirmed by X-ray diffraction analysis. Vincamajicine (3) showed appreciable activity in reversing multidrug resistance in vincristine-resistant KB cells (IC50 2.62 μM), while ervatensines A (7) and B (8) and two other known bisindoles displayed pronounced in vitro growth inhibitory activity against human KB cells (IC50 < 2 μM). Compounds 7 and 8 also showed good growth inhibitory activity against A549, MCF-7, MDA-468, HCT-116, and HT-29 cells (IC50 0.70-4.19 μM). Cell cycle and annexin V-FITC apoptosis assays indicated that compounds 7 and 8 inhibited proliferation of HCT-116 and MDA-468 cells, evoking apoptotic and necrotic cell death.

Highly enantioselective aza-Henry reaction with isatin N-Boc ketimines

A highly enantioselective aza-Henry reaction with isatin N-Boc ketimines using a Cu(II)-BOX complex as a catalyst is described. The reaction, which does not require protection of the N1 atom, provides the corresponding nitroamines bearing a quaternary stereocentre with high yields and enantiomeric excesses (up to 99.9%).

Corynanthean, eburnan, secoleuconoxine, and pauciflorine alkaloids from Kopsia pauciflora

Eleven indole alkaloids, comprising four corynanthean, two eburnane, one aspidofractinine, one secoleuconoxine, one andranginine, and two pauciflorine type alkaloids were isolated from the stem-bark and leaf extracts of Kopsia pauciflora. Their structures were determined using NMR and MS analyses. The catharinensine type alkaloid kopsirensine B and the secoleuconoxine alkaloid arboloscine A showed moderate to weak activity in reversing MDR in vincristine-resistant KB cells. The alkaloid content was markedly different compared to that of a sample from Malaysian Borneo.

Cytotoxic vobasine, tacaman, and corynanthe-tryptamine bisindole alkaloids from Tabernaemontana and structure revision of tronoharine

Seven new indole alkaloids (1-7) comprising four vobasine, two tacaman, and one corynanthe-tryptamine bisindole alkaloid were isolated from the stem-bark extract of a Malayan Tabernaemontana. Two of the new vobasine alkaloids (1, 3), as well as 16-epivobasine (15) and 16-epivobasenal (17), showed appreciable cytotoxicity toward KB cells (IC50 ca. 5 μg/mL). The structure of the known Tabernaemontana alkaloid tronoharine (8) was revised based on newly acquired NMR data, as well as X-ray diffraction analysis.

Oxidized derivatives of macroline, sarpagine, and pleiocarpamine alkaloids from Alstonia angustifolia

A total of 20 new indole alkaloids comprising mainly oxidized derivatives of macroline- (including alstofonidine, a macroline indole incorporating a butyrolactone ring-F), pleiocarpamine-, and sarpagine-type alkaloids were isolated from the bark and leaf extracts of Alstonia angustifolia. The structures and relative configurations of these alkaloids were determined using NMR and MS analyses and in some instances confirmed by X-ray diffraction analyses. Alkaloids 3, 7, 35, and 41 showed moderate to weak activity, while 21 showed strong activity in reversing multidrug resistance in vincristine-resistant KB cells.

Monoterpene indole alkaloids from the stem bark of Mitragyna diversifolia and their acetylcholine esterase inhibitory effects

Five monoterpene indole alkaloids, mitradiversifoline, with a unique rearranged skeleton, specionoxeine-N(4)-oxide, 7-hydroxyisopaynantheine, 3-dehydropaynantheine, and 3-isopaynantheine-N(4)-oxide, and 10 known ones, were isolated from Mitragyna diversifolia. All the isolates were evaluated for their inhibition of acetylcholinesterase activities, and four showed moderate activities, with IC50 values of 4.1, 5.2, 10.2, and 10.3 μM, respectively.

Macrophyllionium and macrophyllines A and B, oxindole alkaloids from Uncaria macrophylla

An unusual oxindole alkaloid inner salt, macrophyllionium (1), and a pair of new tetracyclic oxindole alkaloids, macrophyllines A (2) and B (3), together with six known alkaloids, were isolated from the aerial parts of Uncaria macrophylla. Corynantheidine (8) exhibited moderate cytotoxicity against HL-60 and SW480 cells with IC(50) values of 13.96 and 23.28 μM, respectively. Dihydrocorynantheine (9) exhibited significant vasodilating activity against phenylephrine-induced contraction in rat thoracic aorta rings (IC(50) = 6.73 μg/mL). In addition, compounds 2, 6, and 9 showed weak inhibitory action on KCl-induced contraction.