- Email: [email protected]
Ikirydinium A: a new indole alkaloid from the seeds of Hunteria umbellata (K. Schum)
Tetrahedron Letters 52 (2011) 7125–7127
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Ikirydinium A: a new indole alkaloid from the seeds of Hunteria umbellata (K. Schum) Olusegun S. Ajala a,b, Andrew M. Piggott a, Fabien Plisson a, Zeinab Khalil a, Xiao-cong Huang a, Sunday A. Adesegun b, Herbert A.B. Coker b, Robert J. Capon a,⇑ a b
Institute for Molecular Bioscience, The University of Queensland, St. Lucia, QLD 4072, Australia Department of Pharmaceutical Chemistry, Faculty of Pharmacy, The University of Lagos, CMUL/LUTH Campus, PMB12003 Idiaraba-Surulere, Lagos, Nigeria
a r t i c l e
i n f o
Article history: Received 20 September 2011 Revised 4 October 2011 Accepted 18 October 2011 Available online 25 October 2011 Keywords: Indole alkaloids Hunteria umbellata Nigerian medicinal plant Natural products chemistry
a b s t r a c t Chemical investigations into samples of Hunteria umbellata (K. Schum) collected in Osun State, Nigeria, led to the discovery of a new indole alkaloid, ikirydinium A, featuring an unprecedented 3-alkylpyridinium-indole-2-carboxylate scaffold. Ikirydinium A was found to exhibit antimicrobial activity (IC50 0.6 lM) against Bacillus subtilis ATCC 6051. The involvement of a common intermediate in the biosynthesis of ikirydinium A and vinblastine is hypothesized. Ó 2011 Elsevier Ltd. All rights reserved.
This Letter describes the results of our chemical investigations into Hunteria umbellata (K. Schum), leading to the discovery of the new indole alkaloid, ikirydinium A (1). Samples of the leaves, stem bark, and seeds of H. umbellata were collected from a cocoa plantation in Odofin Agbegi, Ikire, in the Irewole local government area of Osun State, Nigeria. Leaf biomass was air dried, pulverized, and subjected to soxhlet extraction, followed by partitioning between chloroform and water at pHs 6 and 10, sequential triturations with a range of solvents (n-hexane, acetonitrile, dichloromethane, methanol, n-butanol and water) and reversed-phase HPLC (see Supplementary data), to yield the known plant natural products ursolic acid (2),1 serpentine (3),2 and pseudoakuammigine (4).3 Comparable processing and fractionation of the stem bark yielded huntrabrine methochloride (5)4 and strictosidinic acid (6),5 while the seeds yielded two new compounds, N4-chloromethylakuammine (7) and ikirydinium A (1). Structures assigned on the basis of detailed spectroscopic analysis revealed that 2–6 were known plant alkaloids, while 7 was assessed to be a quaternary salt from the reaction with dichloromethane (handling artifact) of the known plant alkaloid akuammine (8).6 Ikirydinium A (1) was determined to possess a new chemical scaffold, the detailed structure proof for which is shown below. HRESI(+)MS analysis of 1 returned a highest mass ion (m/z 309.1607) consistent with the formula C19 H21 N2 O2 þ (Dmmu 0.9), while examination of the NMR (methanol-d4) data (Table 1) ⇑ Corresponding author. Tel.: +61 7 3346 2979; fax: +61 7 3346 2090. E-mail addresses: [email protected], [email protected] (R.J. Capon). 0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.10.106
revealed resonances and correlations consistent with five protonated structure fragments: (i) a 3-substituted pyridinium (dH 8.66, H-17; 8.47, H-13; 8.10, H-15, and 7.68, H-16), (ii) a 1,2-disubstituted benzene (dH 7.22, H-8; 6.91, H-7; 6.67, H-6, and 6.60, H-5), (iii) a 1,2-disubstituted ethane (dH 4.83, H2-11 and 3.36, H2-10), (iv) an aromatic ethyl (dH 2.46, H2-18 and 0.88, H3-19) and (v) an N-methyl moiety (dH 3.62, 1N-Me). Diagnostic 2D NMR correlations (Fig. 1) permitted the assembly of these structure fragments. Table 1 1 H (600 MHz) and Position 2 3 4 5 6 7 8 9 10 11 13 14 15 16 17 18 19 2-CO 2 1-NMe
13
C (150 MHz) NMR data (CD3OD) for ikirydinium A (1) dH, mult (J in Hz)
6.60, 6.67, 6.91, 7.22,
d (7.9) dd (7.9, 7.8) dd (8.0, 7.8) d (8.0)
3.36, t (5.9) 4.83, t (5.9) 8.47, s 8.10, 7.68, 8.66, 2.46, 0.88,
d (7.9) dd (7.9, 5.6) d (5.6) q (7.6) t (7.6)
3.62, s
dC 135.3 105.9 130.0 116.7 119.8 121.9 111.9 136.6 26.9 63.2 146.0 145.7 145.3 128.2 143.2 26.6 14.5 177.6 36.5
7126
O. S. Ajala et al. / Tetrahedron Letters 52 (2011) 7125–7127
Figure 1. Selected 2D NMR correlations for ikirydinium A (1).
Specifically, HMBC correlations from H-13 and H-15 to C-18 positioned the pyridinium ethyl substituent, while HMBC correlations from H-13 and H-17 to C-11, and reciprocal correlations from H2-11 to C-13 and C-17, positioned an ethane substituent on the pyridinium nitrogen. A series of HMBC correlations from H-5, H-6, and H-7 revealed quaternary sp2 carbons attributed to C-3, C-4, and C-9 of an indole residue. Likewise, HMBC correlations from the N-methyl positioned this moiety on 1N of the indole, with correlations to the flanking carbons C-9 and C-2. Placement of the 1N-methyl also established C-2 as quaternary and substituted by a carboxylated moiety. Finally, HMBC correlations from H2-10 to C-2 and C-3, supported by an H2-10/H-5 ROESY correlation, completed the assembly and defined the structure for ikirydinium A (1) as shown.
In our hands, 1 did not display cytotoxicity (IC50 >30 lM) when tested in a cell viability assay against the human colon cancer cell line SW620, and a derived multi-drug resistant (P-glycoprotein over expressing) cell line (SW620 Ad300). While 1 did not significantly inhibit growth of the fungus Candida albicans (ATCC 90028), the Gram-negative bacterium Escherichia coli (ATCC 11775), or the Gram-positive bacteria Staphylococcus aureus (ATCC 9144 and ATCC 25923) and Bacillus subtilis (ATCC 6633), it did display noteworthy inhibitory activity (IC50 0.6 lM) against B. subtilis (ATCC 6051) (Fig. 2). Ikirydinium A (1) represents a new natural chemical scaffold, with the only reported compound featuring a common indolyl-2carboxylate conjugated to a 3-alkylpyridinium being a synthetic intermediate prepared during a 1979 synthesis of the secodine class of plant alkaloids.7 Given its unusual structural characteristics it is interesting to speculate on the biosynthetic origins of ikirydinium A (1). Strictosidinic acid (6) has been noted as a
Figure 2. Antimicrobial activity of ikirydinium A (1) against Bacillus subtilis ATCC 6051.
precursor to a range of alkaloids including dehydrogeissoschizine, preakuammicine, and stemmadenine, which are proposed to in turn progress via a ‘hypothetical intermediate’ to catharanthine and vindoline, themselves key biosynthetic intermediates to the clinically important anticancer chemotherapeutic vinblastine (Fig. 3).8 This biosynthetic pathway is relevant to the current study in as much as the ‘hypothetical intermediate’ pivotal to vinblastine
biosynthesis is equally attractive as a biosynthetic precursor to 1. Prior investigations into H. umbellata alkaloids, and in particular those from seed extracts, extend back many decades. In a 1967 report,9 Bevan et al. described acetylcoryamine, coryamine, and isocoryamine from the seeds of a Western Nigerian collection of H. umbellata, while in a subsequent 1986 account,10 Adegoke et al. described four abereamine alkaloids. More recent accounts have noted the anti-obesity and antihyperlipidaemic activity of H. umbellata seed extracts, although the molecular basis behind this pharmacology has not been identified.11,12 Acknowledgments We thank S. Bates and R. Robey (NCI) for providing the SW620 and SW620 Ad300 cell lines. O.A. acknowledges financial support from the Nigerian Science and Technology Education Post – Basic
O. S. Ajala et al. / Tetrahedron Letters 52 (2011) 7125–7127
7127
Figure 3. Plausible biosynthesis of ikirydinium A (1).
(Step B) and the Education Trust Funds, Federal Ministry of Education, Nigeria. F.P., Z.K. and X.H. acknowledge financial support from The University of Queensland (UQ/IMB postgraduate award). This research was funded, in part, by the Institute for Molecular Bioscience, The University of Queensland and the Australian Research Council (LP0989954). Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tetlet.2011.10.106. References and notes 1. Seebacher, W.; Simic, N.; Weis, R.; Saf, R.; Kunert, O. Magn. Reson. Chem. 2003, 41, 636.
2. Wachsmuth, O.; Matusch, R. Phytochemistry 2002, 61, 705. 3. Joule, J. A.; Smith, G. F. J. Chem. Soc. 1962, 312. 4. Bartlett, M. F.; Korzun, B.; Sklar, R.; Smith, A. F.; Taylor, W. I. J. Org. Chem. 1963, 28, 1445. 5. Arbain, D.; Putra, D. P.; Sargent, M. V. Aust. J. Chem. 1993, 46, 977. 6. Lewin, G.; Le, M. P.; Rolland, Y.; Renouard, A.; Giesen-Crouse, E. J. Nat. Prod. 1992, 55, 380. 7. Kutney, J. P.; Badger, R. A.; Beck, J. F.; Bosshardt, H.; Matough, F. S.; RidauraSanz, V. E.; So, Y. H.; Sood, R. S.; Worth, B. R. Can. J. Chem. 1979, 57, 289. 8. Dewick, P. M. Medicinal Natural Products: A Biosynthetic Approach, 2nd ed.; John Wiley & Sons: Chichester, 2002. 9. Bevan, C. W. L.; Patel, M. B.; Rees, A. H.; Loudon, A. G. Tetrahedron 1967, 23, 3809. 10. Adegoke, E. A.; Alo, B. Phytochemistry 1986, 25, 1461. 11. Adeneye, A. A.; Adeyemi, O. O. J. Ethnopharmacol. 2009, 126, 238. 12. Adeneye, A. A.; Adeyemi, O. O.; Agbaje, E. O. J. Ethnopharmacol. 2010, 130, 307.
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Ikirydinium A: a new indole alkaloid from the seeds of Hunteria umbellata (K. Schum) Olusegun S. Ajala a,b, Andrew M. Piggott a, Fabien Plisson a, Zeinab Khalil a, Xiao-cong Huang a, Sunday A. Adesegun b, Herbert A.B. Coker b, Robert J. Capon a,⇑ a b
Institute for Molecular Bioscience, The University of Queensland, St. Lucia, QLD 4072, Australia Department of Pharmaceutical Chemistry, Faculty of Pharmacy, The University of Lagos, CMUL/LUTH Campus, PMB12003 Idiaraba-Surulere, Lagos, Nigeria
a r t i c l e
i n f o
Article history: Received 20 September 2011 Revised 4 October 2011 Accepted 18 October 2011 Available online 25 October 2011 Keywords: Indole alkaloids Hunteria umbellata Nigerian medicinal plant Natural products chemistry
a b s t r a c t Chemical investigations into samples of Hunteria umbellata (K. Schum) collected in Osun State, Nigeria, led to the discovery of a new indole alkaloid, ikirydinium A, featuring an unprecedented 3-alkylpyridinium-indole-2-carboxylate scaffold. Ikirydinium A was found to exhibit antimicrobial activity (IC50 0.6 lM) against Bacillus subtilis ATCC 6051. The involvement of a common intermediate in the biosynthesis of ikirydinium A and vinblastine is hypothesized. Ó 2011 Elsevier Ltd. All rights reserved.
This Letter describes the results of our chemical investigations into Hunteria umbellata (K. Schum), leading to the discovery of the new indole alkaloid, ikirydinium A (1). Samples of the leaves, stem bark, and seeds of H. umbellata were collected from a cocoa plantation in Odofin Agbegi, Ikire, in the Irewole local government area of Osun State, Nigeria. Leaf biomass was air dried, pulverized, and subjected to soxhlet extraction, followed by partitioning between chloroform and water at pHs 6 and 10, sequential triturations with a range of solvents (n-hexane, acetonitrile, dichloromethane, methanol, n-butanol and water) and reversed-phase HPLC (see Supplementary data), to yield the known plant natural products ursolic acid (2),1 serpentine (3),2 and pseudoakuammigine (4).3 Comparable processing and fractionation of the stem bark yielded huntrabrine methochloride (5)4 and strictosidinic acid (6),5 while the seeds yielded two new compounds, N4-chloromethylakuammine (7) and ikirydinium A (1). Structures assigned on the basis of detailed spectroscopic analysis revealed that 2–6 were known plant alkaloids, while 7 was assessed to be a quaternary salt from the reaction with dichloromethane (handling artifact) of the known plant alkaloid akuammine (8).6 Ikirydinium A (1) was determined to possess a new chemical scaffold, the detailed structure proof for which is shown below. HRESI(+)MS analysis of 1 returned a highest mass ion (m/z 309.1607) consistent with the formula C19 H21 N2 O2 þ (Dmmu 0.9), while examination of the NMR (methanol-d4) data (Table 1) ⇑ Corresponding author. Tel.: +61 7 3346 2979; fax: +61 7 3346 2090. E-mail addresses: [email protected], [email protected] (R.J. Capon). 0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.10.106
revealed resonances and correlations consistent with five protonated structure fragments: (i) a 3-substituted pyridinium (dH 8.66, H-17; 8.47, H-13; 8.10, H-15, and 7.68, H-16), (ii) a 1,2-disubstituted benzene (dH 7.22, H-8; 6.91, H-7; 6.67, H-6, and 6.60, H-5), (iii) a 1,2-disubstituted ethane (dH 4.83, H2-11 and 3.36, H2-10), (iv) an aromatic ethyl (dH 2.46, H2-18 and 0.88, H3-19) and (v) an N-methyl moiety (dH 3.62, 1N-Me). Diagnostic 2D NMR correlations (Fig. 1) permitted the assembly of these structure fragments. Table 1 1 H (600 MHz) and Position 2 3 4 5 6 7 8 9 10 11 13 14 15 16 17 18 19 2-CO 2 1-NMe
13
C (150 MHz) NMR data (CD3OD) for ikirydinium A (1) dH, mult (J in Hz)
6.60, 6.67, 6.91, 7.22,
d (7.9) dd (7.9, 7.8) dd (8.0, 7.8) d (8.0)
3.36, t (5.9) 4.83, t (5.9) 8.47, s 8.10, 7.68, 8.66, 2.46, 0.88,
d (7.9) dd (7.9, 5.6) d (5.6) q (7.6) t (7.6)
3.62, s
dC 135.3 105.9 130.0 116.7 119.8 121.9 111.9 136.6 26.9 63.2 146.0 145.7 145.3 128.2 143.2 26.6 14.5 177.6 36.5
7126
O. S. Ajala et al. / Tetrahedron Letters 52 (2011) 7125–7127
Figure 1. Selected 2D NMR correlations for ikirydinium A (1).
Specifically, HMBC correlations from H-13 and H-15 to C-18 positioned the pyridinium ethyl substituent, while HMBC correlations from H-13 and H-17 to C-11, and reciprocal correlations from H2-11 to C-13 and C-17, positioned an ethane substituent on the pyridinium nitrogen. A series of HMBC correlations from H-5, H-6, and H-7 revealed quaternary sp2 carbons attributed to C-3, C-4, and C-9 of an indole residue. Likewise, HMBC correlations from the N-methyl positioned this moiety on 1N of the indole, with correlations to the flanking carbons C-9 and C-2. Placement of the 1N-methyl also established C-2 as quaternary and substituted by a carboxylated moiety. Finally, HMBC correlations from H2-10 to C-2 and C-3, supported by an H2-10/H-5 ROESY correlation, completed the assembly and defined the structure for ikirydinium A (1) as shown.
In our hands, 1 did not display cytotoxicity (IC50 >30 lM) when tested in a cell viability assay against the human colon cancer cell line SW620, and a derived multi-drug resistant (P-glycoprotein over expressing) cell line (SW620 Ad300). While 1 did not significantly inhibit growth of the fungus Candida albicans (ATCC 90028), the Gram-negative bacterium Escherichia coli (ATCC 11775), or the Gram-positive bacteria Staphylococcus aureus (ATCC 9144 and ATCC 25923) and Bacillus subtilis (ATCC 6633), it did display noteworthy inhibitory activity (IC50 0.6 lM) against B. subtilis (ATCC 6051) (Fig. 2). Ikirydinium A (1) represents a new natural chemical scaffold, with the only reported compound featuring a common indolyl-2carboxylate conjugated to a 3-alkylpyridinium being a synthetic intermediate prepared during a 1979 synthesis of the secodine class of plant alkaloids.7 Given its unusual structural characteristics it is interesting to speculate on the biosynthetic origins of ikirydinium A (1). Strictosidinic acid (6) has been noted as a
Figure 2. Antimicrobial activity of ikirydinium A (1) against Bacillus subtilis ATCC 6051.
precursor to a range of alkaloids including dehydrogeissoschizine, preakuammicine, and stemmadenine, which are proposed to in turn progress via a ‘hypothetical intermediate’ to catharanthine and vindoline, themselves key biosynthetic intermediates to the clinically important anticancer chemotherapeutic vinblastine (Fig. 3).8 This biosynthetic pathway is relevant to the current study in as much as the ‘hypothetical intermediate’ pivotal to vinblastine
biosynthesis is equally attractive as a biosynthetic precursor to 1. Prior investigations into H. umbellata alkaloids, and in particular those from seed extracts, extend back many decades. In a 1967 report,9 Bevan et al. described acetylcoryamine, coryamine, and isocoryamine from the seeds of a Western Nigerian collection of H. umbellata, while in a subsequent 1986 account,10 Adegoke et al. described four abereamine alkaloids. More recent accounts have noted the anti-obesity and antihyperlipidaemic activity of H. umbellata seed extracts, although the molecular basis behind this pharmacology has not been identified.11,12 Acknowledgments We thank S. Bates and R. Robey (NCI) for providing the SW620 and SW620 Ad300 cell lines. O.A. acknowledges financial support from the Nigerian Science and Technology Education Post – Basic
O. S. Ajala et al. / Tetrahedron Letters 52 (2011) 7125–7127
7127
Figure 3. Plausible biosynthesis of ikirydinium A (1).
(Step B) and the Education Trust Funds, Federal Ministry of Education, Nigeria. F.P., Z.K. and X.H. acknowledge financial support from The University of Queensland (UQ/IMB postgraduate award). This research was funded, in part, by the Institute for Molecular Bioscience, The University of Queensland and the Australian Research Council (LP0989954). Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tetlet.2011.10.106. References and notes 1. Seebacher, W.; Simic, N.; Weis, R.; Saf, R.; Kunert, O. Magn. Reson. Chem. 2003, 41, 636.
2. Wachsmuth, O.; Matusch, R. Phytochemistry 2002, 61, 705. 3. Joule, J. A.; Smith, G. F. J. Chem. Soc. 1962, 312. 4. Bartlett, M. F.; Korzun, B.; Sklar, R.; Smith, A. F.; Taylor, W. I. J. Org. Chem. 1963, 28, 1445. 5. Arbain, D.; Putra, D. P.; Sargent, M. V. Aust. J. Chem. 1993, 46, 977. 6. Lewin, G.; Le, M. P.; Rolland, Y.; Renouard, A.; Giesen-Crouse, E. J. Nat. Prod. 1992, 55, 380. 7. Kutney, J. P.; Badger, R. A.; Beck, J. F.; Bosshardt, H.; Matough, F. S.; RidauraSanz, V. E.; So, Y. H.; Sood, R. S.; Worth, B. R. Can. J. Chem. 1979, 57, 289. 8. Dewick, P. M. Medicinal Natural Products: A Biosynthetic Approach, 2nd ed.; John Wiley & Sons: Chichester, 2002. 9. Bevan, C. W. L.; Patel, M. B.; Rees, A. H.; Loudon, A. G. Tetrahedron 1967, 23, 3809. 10. Adegoke, E. A.; Alo, B. Phytochemistry 1986, 25, 1461. 11. Adeneye, A. A.; Adeyemi, O. O. J. Ethnopharmacol. 2009, 126, 238. 12. Adeneye, A. A.; Adeyemi, O. O.; Agbaje, E. O. J. Ethnopharmacol. 2010, 130, 307.