Pyrrolizidine alkaloids of senecio sp from Peru

ARTIGO

Pyrrolizidine alkaloids of senecio sp from peru

Liliana Ruiz Vásquez and Matías Reina ArtilesI, ^*; Azucena González Coloma^II; Raimundo Cabrera Pérez^III; Lastenia Ruiz Mesia^IV

^IInstituto de Productos Naturales y Agrobiología, CSIC, Avda. Astrofísico Francisco Sánchez, 3, 38206, La Laguna, Tenerife, Spain

^IIInstituto de Ciencias Agrarias-(ICA), CSIC, Serrano 115-dpdo, 28006 Madrid, Spain

^IIIUnidad de Fitopatología, Facultad de Biología, Universidad de La Laguna (ULL), Avda. Astrofísico Francisco Sánchez, s/n, 38204, La Laguna, Tenerife, Spain

^IVLaboratorio de Investigación en Productos Naturales Antiparasitarios de la Amazonía, Universidad Nacional de la Amazonía Peruana (LIPNAA-UNAP), AA. HH. "Nuevo San Lorenzo" Pasaje los Paujiles S/N - San Juan, Iquitos, Peru

ABSTRACT

Six pyrrolizidine alkaloids (PAs) (two saturated macrocyclic, three unsaturated macrocyclic and one unsaturated seco-macrocyclic) were isolated from native Peruvian Senecio species. The structures of these alkaloids were established by a complete NMR spectroscopic analysis, chemical transformations and comparison of their NMR data with those published for similar alkaloids. Three PAs were then tested for antifungal activity against Fusarium moniliforme, F. (Sheldon), F. oxysporum fs. lycopersici (Scheldt) and F. solani (Mart), no significant activity being observed.

Keywords:Senecio; pyrrolizidine alkaloids; 19-acetoxysenkirkine.

INTRODUCTION

Pyrrolizidine alkaloids (PAs) are widely distributed in numerous plant families such as Asteraceae, Boraginaceae, Fabaceae, Convolvulaceae, Orchidaceae, Poaceae and Lamiaceae.¹ However, pyrrolizidine alkaloids are found predominantly in the genera Senecio (Asteraceae), Crotalaria (Fabaceae) and Heliotropium (Boraginaceae). In Peru, the genus Senecio comprises about 230 species, most of which are endemic and with restricted geographic distribution, the habitat of these species lying between 500 and 4700 m above sea level.² It should be noted that species of Senecio are used as popular remedies in folk medicine in different forms of preparation such as infusions, decoctions and poultices (to treat kidney disorders and ulcers), while there is great concern regarding public health since these species contain PAs with hepatotoxic activity.³ It is therefore necessary to identify the PA-containing plants to limit their use, or at least establish the maximum tolerated dose. Pyrrolizidine alkaloids (PAs) are of great interest as biopesticides.⁴ This has prompted us to carry out a phytochemical study of the genus Senecio, in addition to studying its biological activity.

Here, we wish to report on the isolation and characterization of six alkaloids of four species of Senecio from Peru: Senecio lasiocephalus (1, 2), S. usgorensis (3), S. chiquianensis (4) and S. laricifolius (5, 6). Alkaloid 3 was previously isolated from Senecio megaphyllus (Costa Rica) and Senecio usgorensis (Peru), and compound 6 from S. laricifolius (Peru).⁵

EXPERIMENTAL

General experimental procedures

1D and 2D ¹H, ¹³C NMR spectra were measured on a Bruker AMX 500 MHz spectrometer with pulsed-field gradient in CDCl₃ (500 MHz for ¹H and 125 MHz for ¹³C), using this solvent as internal standard. Optical rotations were measured at room temperature on a Perkin Elmer 343 Plus polarimeter. IR spectra were measured on a Perkin Elmer 1600 spectrophotometer. EIMS results were recorded on a VG-Micromass ZAB-2F instrument at 70 eV. Alumina (Merck Art. 1076-1077) was used for column chromatography, TLC and preparative TLC. Alkaloids were visualized with Dragendorff's reagent.

Plant material

All plants were collected in May 2008, at Markahuamachuco (Department of La Libertad), Peru, in the natural habitats between 3542-3597 m above sea level. Voucher specimens have been deposited at the Herbarium Amazonense of the Universidad Nacional de la Amazonía Peruana (Iquitos-Loreto): S. laricifolius H.B.K (voucher number 10456), S. usgorensis Cuart (voucher number 10453), S. chiquianensis Cabrera (voucher number 10455) and S. lasiocephalus (voucher number 10454).

Extraction and isolation of Pas

The dried samples were extensively extracted with ethanol at room temperature over 72 h and the extracts evaporated to dryness under reduced pressure. The crude EtOH extract was treated with a mixture of H₂SO₄ 0.5 M and CH₂Cl₂ (1:1). The aqueous phase was reduced with zinc dust for 4-6 h, filtered, basified with 30% NH₄OH (pH = 9) and the alkaloids were extracted with CH₂Cl₂. The purification was performed by column chromatography on neutral alumina using an EtOAc-MeOH gradient solvent system and subsequent purification with PTLC (20 x 20 cm, 0.25 mm) to obtain platyphylline and platyphylline N-oxide (1, 2, S. lasiocephalus), 13,19-epoxysenecionine (3, S. usgorensis), senecivernine (4, S. chiquianensis), senecionine and 19-acetoxysenkirkine (5, 6, S.laricifolius).

Platyphylline (1)

Mp 96.5-98.7 ºC (hexane-EtOAc): [α]_D ^25° -46.0° (c, 0.094, CHCl₃) [lit. [α]_D ^25° -46° in CHCl₃]⁶; EIMS m/z (int. rel., %): 337 (7) [M]⁺, 226 (5), 222 (4), 212 (6), 211 (50), 180 (4), 141 (10), 140 (100), 138 (57), 123 (53), 122 (93), 96 (22), 82 (50), 80 (11) and 55 (16). HREIMS m/z [M]⁺ 337.1887 (16), calculated for C₁₈H₂₇NO₅. ¹H and ¹³C NMR data identical with those reported.^7,8

Platyphylline N-oxide (2)

Amorphous solid: [α]_D^25° -38.2° (c, 0.136, CHCl₃). EIMS m/z (int. rel., %): 353 (2), 338 (3), 337 (11), 336 (6), 335 (7), 226 (11), 211 (51), 153 (20), 140 (97), 139 (14), 138 (84), 122 (66), 119 (74), 96 (35), 82 (100), 80 (22), and 55 (30). ESI-MS m/z [M+1]⁺ 354.1917 (2), calculated for C₁₈H₂₈NO₆.

13,19-epoxysenecionine (3)

EIMS m/z (int. rel., %): 349 (25) [M]⁺, 304 (6), 288 (7), 262 (4), 218 (8), 138 (29), 136 (100), 120 (91), 119 (63), 95 (27), 94 (46), 93 (59), 84 (23), 80 (27), 67 (10), 53 (19); HREIMS m/z [M]⁺ 349.1530, calculated for C₁₈H₂₃NO₆. ¹H and ¹³C NMR data identical with those reported.⁹

Senecivernine (4)

Amorphous solid: [α]_D^25° - 14.9° (c, 0.35, CHCl₃), [lit. [α]_D ^25° - 34.9° (EtOH)].¹⁰ IR (CHCl₃) Ν_máx 3400, 1718, 1710, 1654, 1560, 1508, 1458, 1255, 1160, 764 cm^-1. EIMS m/z [M]⁺ 335 (6), 291 (10), 248 (12), 218 (2), 153 (53), 136 (84), 120 (100), 93 (68), 81 (54), 55 (46); HREIMS, m/z [M]⁺ 335.0020, calculated for C₁₈H₂₅NO₆. ¹H and ¹³C NMR data identical with those reported.¹⁰

Senecionine (5)

Amorphous solid: [α]_D^25° - 67.2° (c, 0.054, CHCl₃) [lit. [α]_D ^25° -56.0° in CHCl₃]⁶. EIMS m/z [M]⁺ 335 (1), 333 (6), 291 (3), 248 (2), 220 (2), 153 (5), 136 (100), 120 (98), 119 (70), 93 (72), 80 (35), 53 (27); HREIMS, m/z [M]⁺ 335.1783, calculated for C₁₈H₂₅NO₅. ¹H and ¹³C NMR data identical with those reported.^7,8

19-acetoxysenkirkine (6)

Amorphous solid: IR (CHCl₃) Ν_máx 3400, 1720, 1650 and 1612 cm^-1, ESI (Electro Spray Ionization) m/z: [M+1]⁺ 424.1964 (74), calculated for C₂₁H₃₀NO₈. Data for ¹H and ¹³C NMR are given in Table 2.

Reduction of 2

Platyphylline N-oxide (85.6 mg) was treated with H₂SO₄ 0.5 N and zinc dust with stirring for 5 h at room temperature. The reaction product was filtered, basified with NH₄OH 30% to pH = 8-9 and extracted with CH₂Cl₂ several times. After removal of the solvent platyphylline (1, 57.0 mg, 70.0%) was obtained.

Antifungal assays of PAs

The antifungal activity of platyphylline, platyphylline N-oxide and 19-epoxysenecionine was tested against the following species of phytopathogenic fungi: Fusarium moniliforme (Sheldon), F. oxysporum fs. lycopersici (Scheldt) and F. solani (Mart) (Spanish Type Culture Collection (CECT) codes, CECT 2152, CECT 2715, and CECT 2199, respectively) and estimated as mycelial growth inhibition. The products were dissolved in ethanol and incorporated in PDA culture media (2% final concentration of solvent). The dose series used was: 0.01, 0.1 and 1.0 mg/mL. For each experiment 8 replicates, inoculated with the appropriate fungal sp., were incubated at 25 °C and the colony diameters measured after 48 h. Control experiments were ethanol. The inhibition values were determined with the SPS program.

RESULTS AND DISCUSSION

Alkaloid extract yield ranged between 0.03 and 0.1% with respect to plant dry weight. Senecio lasiocephalus and S. usgorensis showed the highest alkaloid content. Two saturated macrocyclic diester PAs, platyphylline and its N-oxide (1,2), three unsaturated macrocyclic diester PAs, 13,19-epoxysenecionine (3), senecivernine (4) senecionine (5), and one unsaturated macrocyclic diester seco-PAs, 19-acetoxysenkirkine (6), have been isolated from their respective alkaloid extracts (Table 1). All structures were established by spectroscopic data, chemical transformations and correlations of these data with those reported for similar alkaloids (Figure 1).

Alkaloids 1 and 2 were identified as platyphylline and its N-oxide (¹H and ¹³C NMR), previously isolated from different species of Senecio,^11,12 and were related via chemical by reduction of 2 with H₂SO₄/Zn dust. The most noteworthy differences observed in the ¹H and ¹³C NMR spectra of both compounds were the largest deshielding of H-8 at Δ_H 4.00 (1H, dd, J = 8.2 and 6.1 Hz) and resonance of carbons Δ_C 70.4 (t, C-3), 68.3 (t, C-5) and 86.0 (d, C-8) with respect to alkaloid 1, suggesting the presence of an extra oxygen molecule in the form of N-oxide.¹³

The structure of alkaloids 3-5 were elucidated by mono- and bidimensional NMR and by comparison with previously reported spectroscopic data.^8-10 Alkaloid 6, proved to be otonecine esterified with 5-ethylidene-2-hydroxy-2-methyl-3-methylacetyl-hexanodioic acid and was identified with 19-acetoxysenkirkine based on the following points: the IR spectrum showed absorption bands at Ν_max 3400, 1720, 1650 and 1612 cm^-1 which could be attributed to a carbonyl group, an ester, double bonds and a trans-annular carbonyl group O=C…N-Me,¹⁴ respectively. Its mass spectrum presents a molecular ion at m/z 424.1964 (74) [M+1]⁺ along with fragments of a macrocyclic ester, necic acid and otonecine base, characteristic of secopyrrolizidine alkaloids.^15,16 The ¹H NMR and ¹³C spectra of alkaloid 6 were very similar to those of 12-acetylsenkirkine, an alkaloid previously isolated from S. phillipicus and S. illinitus,¹⁷ except for the presence of a CH₂OCOCH₃ group at C-13 at Δ_H 4.11 (1H, dd, J = 12.2 y 2.9Hz, H-19a), 4.21 (1H, dd, J = 6.1, 11.8 Hz, H-19b), Δ_C 61.9 (t, C-19), two signals connected with carbon resonances of Δ_C 33.7 (t, C-14), 42.8 (d, C-13), 76.1 (s, C-12) and 170.9 (s, OCOCH₃) by an HMBC experiment. The complete NMR data of 19-acetoxysenkirkine (6) are presented in Table 2. The relative stereochemistry of alkaloids 3 and 6 was determined by a NOESY experiment, and observed NOE effects are shown in Figure 2.

Three APs, platyphylline, platyphylline N-oxide and 13,19-epoxysenecionine, were tested against the following fungal species Fusarium moniliforme (Sheldon), F. oxysporum fs. lycopersici (Scheldt) and F. solani (Mart) as previously described,¹⁸ but no significant antifungal activity was observed.

SUPPLEMENTARY MATERIAL

Available on http://quimicanova.sbq.org.br, in PFD file, with free access. ¹H NMR spectra of alkaloids 1-4 and 6.

ACKNOWLEDGEMENTS

This work was supported by a Collaborative Research Grant from CSIC-CONCYTEC, National University of the Peruvian Amazonia, Iquitos, Peru, Ministerio de Educación y Ciencia, Spain (CTQ2009-14629-C02-01) and a predoctoral MAEC-AECID fellowship to L. R. Vásquez.

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Recebido em 20/9/10; aceito em 15/12/10; publicado na web em 14/3/11

SUPPLEMENTARY MATERIAL

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Publication Dates

• Publication in this collection
  01 Aug 2011
• Date of issue
  2011

History

• Received
  20 Sept 2010
• Accepted
  15 Dec 2010