Displaying publications 1 - 20 of 55 in total

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  1. Sim DS, Teoh WY, Sim KS, Lim SH, Thomas NF, Low YY, et al.
    J Nat Prod, 2016 Apr 22;79(4):1048-55.
    PMID: 26918761 DOI: 10.1021/acs.jnatprod.5b01117
    Six new bisindole alkaloids of the iboga-vobasine type, vobatensines A-F (1-6), in addition to four known bisindoles (8-11), were isolated from a stem bark extract of a Malayan Tabernaemontana corymbosa. The structures of these alkaloids were determined based on analysis of the spectroscopic data and in the case of vobatensines A (1), B (2), and 16'-decarbomethoxyvoacamine (8) also confirmed by partial syntheses. Nine of these alkaloids (1-5, 8-11) showed pronounced in vitro growth inhibitory activity against human KB, PC-3, LNCaP, HCT 116, HT-29, MCF7, MDA-MB-231, and A549 cancer cells.
  2. Nge CE, Gan CY, Low YY, Thomas NF, Kam TS
    Org. Lett., 2013 Sep 20;15(18):4774-7.
    PMID: 23991636 DOI: 10.1021/ol4021404
    Two new indole alkaloids, voatinggine (1) and tabertinggine (2), which are characterized by previously unencountered natural product skeletons, were isolated from a Malayan Tabernaemontana species. The structures and absolute configuration of these alkaloids were determined using NMR and MS analysis, and X-ray diffraction analysis. A possible biogenetic pathway to these novel alkaloids from an iboga precursor, and via a common cleavamine-type intermediate, is presented.
  3. Ezeoke MC, Krishnan P, Sim DS, Lim SH, Low YY, Chong KW, et al.
    Phytochemistry, 2018 Feb;146:75-81.
    PMID: 29247894 DOI: 10.1016/j.phytochem.2017.12.003
    From the leaves of Elaeocarpus tectorius (Lour.) Poir. four previously undescribed phenethylamine-containing alkaloids were isolated, namely, tectoricine, possessing an unprecedented isoquinuclidinone ring system incorporating a phenethylamine moiety, tectoraline, representing a rare alkamide incorporating two phenethylamine moieties, and tectoramidines A and B, representing the first naturally occurring trimeric and dimeric phenethylamine alkaloids incorporating an amidine function. The structures of these alkaloids were established by detailed spectroscopic analysis. The absolute configuration of tectoricine was determined by comparison of the experimental and calculated ECD spectra. Plausible biosynthetic pathways to the four alkaloids are proposed.
  4. Low YY, Hong FJ, Lim KH, Thomas NF, Kam TS
    J Nat Prod, 2014 Feb 28;77(2):327-38.
    PMID: 24428198 DOI: 10.1021/np400922x
    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.
  5. Hong FJ, Chong KW, Low YY, Thomas NF, Kam TS
    Chem Asian J, 2015 Oct;10(10):2207-20.
    PMID: 26097065 DOI: 10.1002/asia.201500488
    A systematic study on the FeCl3-induced oxidation of 1,2-diarylalkenes was carried out with the focus on the variation of product type as a function of aromatic substitution, as well as to compare the reactivity of stilbene cation radicals generated via Fe(III) oxidation with those generated by anodic oxidation. The aromatic substituents were found to fall into three main categories, namely those that give rise to tetralins and/or dehydrotetralins, those that give products possessing pallidol and ampelopsin F-type carbon skeletons, and last, those that give rise to trimeric products, indanes, and dehydrotetralins/tetralins. The latter are those stilbenes with a para-methoxy substituent in one ring and a para- or meta-EWG (CF3, NO2, Cl, F) in the other, and represent the most prominent departure when compared with the behavior of the same stilbenes under the conditions of anodic oxidation. Reaction pathways to rationalize the formation of the different products are presented.
  6. Tan CH, Yeap JS, Lim SH, Low YY, Sim KS, Kam TS
    J Nat Prod, 2021 05 28;84(5):1524-1533.
    PMID: 33872002 DOI: 10.1021/acs.jnatprod.1c00013
    A new linearly fused macroline-sarpagine bisindole, angustilongine M (1), was isolated from the methanolic extract of Alstonia penangiana. The structure of the alkaloid was elucidated based on analysis of the spectroscopic data, and its biological activity was evaluated together with another previously reported macroline-akuammiline bisindole from the same plant, angustilongine A (2). Compounds 1 and 2 showed pronounced in vitro growth inhibitory activity against a wide panel of human cancer cell lines. In particular, the two compounds showed potent and selective antiproliferative activity against HT-29 cells, as well as strong growth inhibitory effects against HT-29 spheroids. Cell death mechanistic studies revealed that the compounds induced mitochondrial apoptosis and G0/G1 cell cycle arrest in HT-29 cells in a time-dependent manner, while in vitro tubulin polymerization assays and molecular docking analysis showed that the compounds are microtubule-stabilizing agents, which are predicted to bind at the β-tubulin subunit at the Taxol-binding site.
  7. Tan SJ, Low YY, Choo YM, Abdullah Z, Etoh T, Hayashi M, et al.
    J Nat Prod, 2010 Nov 29;73(11):1891-7.
    PMID: 21043460 DOI: 10.1021/np100552b
    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.
  8. Krishnan P, Lee FK, Yap VA, Low YY, Kam TS, Yong KT, et al.
    J Nat Prod, 2020 01 24;83(1):152-158.
    PMID: 31935094 DOI: 10.1021/acs.jnatprod.9b01160
    Schwarzinicines A-G (1-7), representing the first examples of 1,4-diarylbutanoid-phenethylamine conjugates, were isolated from the leaves of Ficus schwarzii. The structures of these compounds were determined by detailed analysis of their MS, 1D and 2D NMR data. Compounds 1-4 exhibited pronounced vasorelaxant effects in the rat isolated aorta (Emax 106-120%; EC50 0.96-2.10 μM). However, compounds 1 and 2 showed no cytotoxic effects against A549, MCF-7, and HCT 116 human cancer cells (IC50 > 10 μM).
  9. Gan CY, Low YY, Thomas NF, Kam TS
    J Nat Prod, 2013 May 24;76(5):957-64.
    PMID: 23647487 DOI: 10.1021/np400214y
    Eight new indole alkaloids (1-8) belonging to the rhazinilam-leuconolam-leuconoxine group, in addition to 52 other alkaloids, were isolated from the stem-bark extract of Leuconotis griffithii, viz., nor-rhazinicine (1), 5,21-dihydrorhazinilam-N-oxide (2), 3,14-dehydroleuconolam (3), and leuconodines A-E (4-8). The structures of these alkaloids were determined using NMR and MS analyses and in some instances confirmed by X-ray diffraction analyses. Alkaloids 1, 5, and 7 showed only moderate to weak cytotoxicity toward KB cells (IC50 12-18 μg/mL), while 8 showed moderate activity in reversing MDR in vincristine-resistant KB cells.
  10. Chong KW, Thomas NF, Low YY, Kam TS
    J Org Chem, 2018 Dec 21;83(24):15087-15100.
    PMID: 30488699 DOI: 10.1021/acs.joc.8b02360
    The effect of ortho'-substituted side chains bearing nucleophilic groups such as CH2OH, CH2NHR, and CO2H on the reactivity of anodically generated 4-methoxy- and 3,4-dimethoxystilbene cation radicals was investigated, and results were compared with those of substrates where the nucleophilic groups such as OH and NHR are directly attached to the aromatic ring. It was found that when ortho'-substituted groups such as CH2OH or CH2NHR are present in the other ring, only direct intramolecular cation-nucleophile reactions occur to give bisbenzopyrans or bisisoquinolines. Crossover products (previously obtained when the ortho' substituents were OH and NH2) such as the fused benzoxepanes/fused benzoazepanes were not formed. When the ortho' substituent is COOH, direct intramolecular cation-nucleophile reaction occurs to give the corresponding bis-δ-lactones in high yield. The presence of an additional 3-methoxy substituent resulted in the formation of other fused polycyclic products due to competing aromatic substitution reactions. Reaction pathways leading to the different products and reasons for the difference in behavior shown by the present stilbenes are presented. The results have provided additional insight into the reactivity and behavior of anodically generated stilbene cation radicals.
  11. Chong KW, Thomas NF, Low YY, Kam TS
    J Org Chem, 2019 Jun 07;84(11):7279-7290.
    PMID: 31056921 DOI: 10.1021/acs.joc.9b00939
    The present investigation represents a continuation of studies on the effect of ortho'-substitution on the reactivity of anodically generated methoxystilbene cation radicals. Whereas previous studies have focused on the effect of ortho'-substituted nucleophilic groups such as OH, NH2, CH2OH, CH2NH2, and COOH, the present study extends the investigation to ortho'-substituted vinyl and formyl groups. The results show that when the ortho'-substituent is a vinyl group, the products include a bisdihydronaphthalene derivative and a doubly bridged, dibenzofused cyclononane from direct trapping of a bis carbocation intermediate. In the presence of an additional 3-methoxy substituent, the products are the tetracyclic chrysene derivatives. When the ortho'-substituent is a nonnucleophilic formyl group, the products include fused indanylnaphthalenes and indanylbenzopyran aldehydes. When an additional 3-methoxy group is present, an unusual fused benzofluorene-dibenzoannulene product is obtained. Mechanistic rationalization for the formation of the various products is presented. The results have contributed to a deeper understanding of how the reactivity of the methoxystilbene cation radicals is affected by the nature of the ortho'-substituents.
  12. Tan YS, Ng MP, Tan CH, Tang WK, Sim KS, Yong KT, et al.
    J Nat Prod, 2024 Feb 23;87(2):286-296.
    PMID: 38284153 DOI: 10.1021/acs.jnatprod.3c00960
    Nine new alkaloids, eugeniinalines A-H (1-8) and (+)-eburnamenine N-oxide (9), comprising one quinoline, six indole, and two isogranatanine alkaloids, were isolated from the stem-bark extract of the Malayan Leuconotis eugeniifolia. The structures and absolute configurations of these alkaloids were established based on the analysis of the spectroscopic data, GIAO NMR calculations, DP4+ probability analysis, TDDFT-ECD method, and X-ray diffraction analysis. Eugeniinaline A (1) represents a new pentacyclic quinoline alkaloid with a 6/6/5/6/7 ring system. Eugeniinaline G (7) and its seco-derivative, eugeniinaline H (8), were the first isogranatanine alkaloids isolated as natural products. The known alkaloids leucolusine (10) and melokhanine A (11) were found to be the same compound, based on comparison of the spectroscopic data of both compounds, with the absolute configuration of (7R, 20R, 21S). Eugeniinalines A and G (1 and 7) showed cytotoxic activity against the HT-29 cancer cell line with IC50 values of 7.1 and 7.2 μM, respectively.
  13. Lee FK, Chan NJ, Krishnan P, Datu Abdul Salam DS, Chee XW, Muhamad A, et al.
    J Nat Prod, 2024 Mar 05.
    PMID: 38442031 DOI: 10.1021/acs.jnatprod.3c00707
    Schwarzinicines A-D, a series of alkaloids recently discovered from Ficus schwarzii, exhibit pronounced vasorelaxant activity in rat isolated aorta. Building on this finding, a concise synthesis of schwarzinicines A and B has been reported, allowing further investigations into their biological properties. Herein, a preliminary exploration of the chemical space surrounding the structure of schwarzinicine A (1) was carried out aiming to identify structural features that are essential for vasorelaxant activity. A total of 57 analogs were synthesized and tested for vasorelaxant activity in rat isolated aorta. Both efficacy (Emax) and potency (EC50) of these analogs were compared. In addition to identifying structural features that are required for activity or associated with potency enhancement effect, four analogs showed significant potency improvements of up to 40.2-fold when compared to 1. Molecular dynamics simulation of a tetrameric 44-bound transient receptor potential canonical-6 (TRPC6) protein indicated that 44 could potentially form important interactions with the residues Glu509, Asp530, Lys748, Arg758, and Tyr521. These results may serve as a foundation for guiding further structural optimization of the schwarzinicine A scaffold, aiming to discover even more potent analogs.
  14. Tang SY, Tan CH, Sim KS, Yong KT, Lim KH, Low YY, et al.
    Phytochemistry, 2023 Apr;208:113587.
    PMID: 36646163 DOI: 10.1016/j.phytochem.2023.113587
    Eight undescribed iboga alkaloids, polyneurines A-H, were isolated from the bark of Tabernaemontana polyneura. The structures of these alkaloids were established by interpretation of the MS and NMR data, while the configurations were determined using GIAO NMR calculations and DP4+ probability analysis, TDDFT-ECD method, or X-ray diffraction analysis. Polyneurine A possesses a γ-lactone unit embedded within the iboga skeleton, while polyneurines D and E incorporate a formylmethyl moiety at C-3 of the iboga skeleton. Biosynthetic pathways towards the formation of polyneurines A, C, D, and E were proposed.
  15. Nett RS, Dho Y, Tsai C, Passow D, Martinez Grundman J, Low YY, et al.
    Nature, 2023 Dec;624(7990):182-191.
    PMID: 37938780 DOI: 10.1038/s41586-023-06716-y
    Plants synthesize numerous alkaloids that mimic animal neurotransmitters1. The diversity of alkaloid structures is achieved through the generation and tailoring of unique carbon scaffolds2,3, yet many neuroactive alkaloids belong to a scaffold class for which no biosynthetic route or enzyme catalyst is known. By studying highly coordinated, tissue-specific gene expression in plants that produce neuroactive Lycopodium alkaloids4, we identified an unexpected enzyme class for alkaloid biosynthesis: neofunctionalized α-carbonic anhydrases (CAHs). We show that three CAH-like (CAL) proteins are required in the biosynthetic route to a key precursor of the Lycopodium alkaloids by catalysing a stereospecific Mannich-like condensation and subsequent bicyclic scaffold generation. Also, we describe a series of scaffold tailoring steps that generate the optimized acetylcholinesterase inhibition activity of huperzine A5. Our findings suggest a broader involvement of CAH-like enzymes in specialized metabolism and demonstrate how successive scaffold tailoring can drive potency against a neurological protein target.
  16. Lim SH, Low YY, Tan SJ, Lim KH, Thomas NF, Kam TS
    J Nat Prod, 2012 May 25;75(5):942-50.
    PMID: 22559995 DOI: 10.1021/np300120p
    Three new bisindole alkaloids of the macroline-macroline type, perhentidines A-C (1-3), were isolated from the stem-bark extract of Alstonia macrophylla and Alstonia angustifolia. The structures of these alkaloids were established on the basis of NMR and MS analyses. The absolute configurations of perhentinine (4) and macralstonine (5) were established by X-ray diffraction analyses, which facilitated assignment of the configuration at C-20 in the regioisomeric bisindole alkaloids perhentidines A-C (1-3). A potentially useful method for the determination of the configuration at C-20 based on comparison of the NMR chemical shifts of the bisindoles and their acetate derivatives, in these and related bisindoles with similar constitution and branching of the monomeric units, is also presented.
  17. Cho ES, Krishnan P, Loh HS, Daly JM, Leong CO, Mai CW, et al.
    Phytochemistry, 2021 Oct 27;193:112988.
    PMID: 34717280 DOI: 10.1016/j.phytochem.2021.112988
    Four undescribed cucurbitacins, designated as petiolaticins A-D, and four known cucurbitacins were isolated from the bark and leaves of Elaeocarpus petiolatus (Jack) Wall. Their chemical structures were elucidated based on detailed analyses of the NMR and MS data. The absolute configuration of petiolaticin A was also determined by X-ray diffraction analysis. Petiolaticin A represents a cucurbitacin derivative incorporating a 3,4-epoxyfuranyl-bearing side chain, while petiolaticin B possesses a furopyranyl unit fused to the tetracyclic cucurbitane core structure. Petiolaticins A, B, and D were evaluated in vitro against a panel of human breast, pancreatic, and colorectal cancer cell lines. Petiolaticin A exhibited the greatest cytotoxicity against the MDA-MB-468, MDA-MB-231, MCF-7, and SW48 cell lines (IC50 7.4, 9.2, 9.3, and 4.6 μM, respectively). Additionally, petiolaticin D, 16α,23α-epoxy-3β,20β-dihydroxy-10αH,23βH-cucurbit-5,24-dien-11-one, and 16α,23α-epoxy-3β,20β-dihydroxy-10αH,23βH-cucurbit-5,24-dien-11-one 3-O-β-D-glucopyranoside were tested for their ability to inhibit cell entry of a pseudotyped virus bearing the hemagglutinin envelope protein of a highly pathogenic avian influenza virus. Petiolaticin D showed the highest inhibition (44.3%), followed by 16α,23α-epoxy-3β,20β-dihydroxy-10αH,23βH-cucurbit-5,24-dien-11-one (21.0%), and 16α,23α-epoxy-3β,20β-dihydroxy-10αH,23βH-cucurbit-5,24-dien-11-one 3-O-β-D-glucopyranoside showed limited inhibition (9.0%). These preliminary biological assays have demonstrated that petiolaticins A and D possess anticancer and antiviral properties, respectively, which warrant for further investigations.
  18. Tan SJ, Lim JL, Low YY, Sim KS, Lim SH, Kam TS
    J Nat Prod, 2014 Sep 26;77(9):2068-80.
    PMID: 25211145 DOI: 10.1021/np500439u
    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.
  19. Ng CT, Fong LY, Low YY, Ban J, Hakim MN, Ahmad Z
    Physiol Res, 2016 12 13;65(6):1053-1058.
    PMID: 27539106
    The endothelial barrier function is tightly controlled by a broad range of signaling cascades including nitric oxide-cyclic guanosine monophosphate (NO-cGMP) pathway. It has been proposed that disturbances in NO and cGMP production could interfere with proper endothelial barrier function. In this study, we assessed the effect of interferon-gamma (IFN-gamma), a pro-inflammatory cytokine, on NO and cGMP levels and examined the mechanisms by which NO and cGMP regulate the IFN-gamma-mediated HUVECs hyperpermeability. The flux of fluorescein isothiocyanate-labeled dextran across cell monolayers was used to study the permeability of endothelial cells. Here, we found that IFN-gamma significantly attenuated basal NO concentration and the increased NO levels supplied by a NO donor, sodium nitroprusside (SNP). Besides, application of IFN-gamma also significantly attenuated both the basal cGMP concentration and the increased cGMP production donated by a cell permeable cGMP analogue, 8-bromo-cyclic GMP (8-Br-cGMP). In addition, exposure of the cell monolayer to IFN-gamma significantly increased HUVECs basal permeability. However, L-NAME pretreatment did not suppress IFN-gamma-induced HUVECs hyperpermeability. L-NAME pretreatment followed by SNP or SNP pretreatment partially reduced IFN-gamma-induced HUVECs hyperpermeability. Pretreatment with a guanylate cyclase inhibitor, 6-anilino-5,8-quinolinedione (LY83583), led to a further increase in IFN-gamma-induced HUVECs hyperpermeability. The findings suggest that the mechanism underlying IFN-gamma-induced increased HUVECs permeability is partly related to the inhibition of NO production.
  20. Chan ZY, Krishnan P, Modaresi SM, Hii LW, Mai CW, Lim WM, et al.
    J Nat Prod, 2021 08 27;84(8):2272-2281.
    PMID: 34342431 DOI: 10.1021/acs.jnatprod.1c00374
    Seven new tropane alkaloids, including five monomeric (1-5), one dimeric (6), and one trimeric (7) 3α-nortropane ester, along with two known monomeric nortropane alkaloids (8 and 9), were isolated from the leaves and bark of Pellacalyx saccardianus. Their structures, including the absolute configuration of the enantiomeric pair of (±)-6, were elucidated by comprehensive spectroscopic analyses. Alkaloids 6 and 7 showed cytotoxicity toward human pancreatic cancer cell lines (AsPC-1, BxPC3, PANC-1, and SW1990). Alkaloids 1, 4, and 9 induced a smooth muscle relaxation effect comparable to that of atropine (Emax 106.1 ± 7.5%, 97.0 ± 5.2%, 100.9 ± 1.4%, 111.7 ± 1.7%, respectively) on isolated rat tracheal rings.
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