Abstracts

Article

Biological Activity of Quinoline Alkaloids from Raulinoa echinata and X-ray Structure of Flindersiamine

Maique W. Biavatti a^# * e-mail: paulo@dq.ufscar.br (chemistry); * e-mail: julio@dq.ufscar.br (X-ray crystallography); # Permanent address: Universidade do Vale do Itajaí, Curso de Farmácia, CP 360, 88303-202, Itajaí - SC, Brazil , Paulo C. Vieira^a* * e-mail: paulo@dq.ufscar.br (chemistry); * e-mail: julio@dq.ufscar.br (X-ray crystallography); # Permanent address: Universidade do Vale do Itajaí, Curso de Farmácia, CP 360, 88303-202, Itajaí - SC, Brazil , M. Fátima das G. F. da Silva^a, João B. Fernandes^a, Sandra R. Victor^b, Fernando C. Pagnocca^b, Sérgio Albuquerque^c, Ignez Caracelli^a and Julio Zukerman-Schpector^a,d* * e-mail: paulo@dq.ufscar.br (chemistry); * e-mail: julio@dq.ufscar.br (X-ray crystallography); # Permanent address: Universidade do Vale do Itajaí, Curso de Farmácia, CP 360, 88303-202, Itajaí - SC, Brazil

^aDepartamento de Química, Universidade Federal de São Carlos, CP 676, 13565-905, São Carlos - SP, Brazil

^bCentro de Estudos de Insetos Sociais, Universidade Estadual Paulista, Rio Claro - SP, Brazil

^cFaculdade de Ciências Farmacêuticas, Universidade de São Paulo, Av. do Café s/n, 14040-903, Ribeirão Preto - SP, Brazil

^dInstituto de Química, Universidade de São Paulo, CP 26077, 05513-970, São Paulo - SP, Brazil

Introduction

Raulinoa is a monospecific genus and the species Raulinoa echinata Cowan (Rutaceae) is endemic in the Itajaí Valley, SC, Brazil.¹ This perennial woody shrub is characterized by the presence of spines and has only been found in a short interval (1000 m) on the frequently inundated banks of Itajaí river in an approximate altitude of 100 m, showing a high degree of adaptation to the environment.

We have recently reported the isolation and biological activities of some compounds of R. echinata.² In continuation of our search for trypanocidal compounds and natural products to be used in the control of leaf-cutting ants, we have investigated the methanol extracts of stems and leaves of R. echinata leading to the isolation of typical rutaceous furoquinoline alkaloids: the widespread skimmianine (1), kokusaginine (2), maculine (3), flindersiamine (4) and also the quinolone derivatives: 2-n-nonyl-4-quinolone (5), 1-methyl-2-n-nonyl-4-quinolone (6) and 1-methyl-2-phenyl-quinolone(7).

Leaf-cutting ants are one of the most serious agricultural pests in South American countries and the biological control of these insects has been the aim of many studies. Their control can be achieved by inhibiting the growth of their symbiotic fungus.

Trypanocidal compounds are important for the treatment of Chagas´ disease which is responsible for a considerable number of deaths in Central and South American countries.

In this paper we describe the isolation, structural determination and evaluation of the activity of furoquinoline alkaloids as inhibitors of the growth of the symbiotic fungus Leucoagaricus gongylophorus of leaf-cutting ants. The alkaloids were also assayed in vitro against trypomastigote forms of Trypanosoma cruzi.

Experimental

General experimental procedures

NMR spectra: a Bruker DRX-400 spectrometer, operating at 400.13 MHz for ¹H and 100.62 MHz for ¹³C was used. All spectra were run in CDCl₃ using TMS as internal standard. For GC-MS analysis was used a Shimadzu GC-17A gas chromatograph fitted with a fused silica DB-1 (30 m x 0.25 mm i.d. x 0.25 mm film thickness) capillary column, with helium as the carrier gas at a flow rate of 2.6 mL.min^-1. The temperature was programmed initially at 70 °C for 1 min, then increased with a rate of 10 °C.min^-1 to 280 °C and kept for 15 min. The injection volume was 1 mL in a split mode and temperatures of the detector/injector were 300 °C/280 °C respectively. The chromatograph was coupled to a Shimadzu QP5000 mass selective detector at 70 eV with scans from 50 to 500 amu.

X-ray crystallographic data were recorded on an Enraf-Nonuis CAD-4 diffractometer using graphite monochromated MoKa radiation (l = 0.71073 Å) and w/2q scan mode at room temperature (291 K). During data collection, the intensity of three standard reflections were monitored every 30 min of X-ray exposure time showing no significant decay. Direct Methods solved the structure. H atoms were placed in calculated positions with fixed C-H distances (0.93 Å for Csp2 and 0.96 Å for Csp3), each riding on a carrier atom with isotropic displacement parameters amounting to 1.5 (for methyl H atoms) or 1.2 (for the other H atoms) times the value of the equivalent isotropic displacement parameter of the carrier atom. The CAD-4 Software³ was used for data collection; cell refinement was done on 25 centred reflections (10.23<q<18.11^o),³ data reduction: XCAD4,⁴ program used to solve the structure: SIR92;⁵ program used to refine structure: SHELXL97;⁶ software used to prepare material for publication and deposition: SHELXL97,⁶ PARST95⁷ and WinGX.⁸

Plant material

Stems and leaves of R. echinata were collected in Ressacada, Ibirama, SC, Brazil and identified by A. Reis (Universidade Federal de Santa Catarina) and J. R. Pirani (Universidade de São Paulo). Voucher specimens [A. Reis & M. Biavatti 2.570 (26/07/98)] were deposited in the Herbário Barbosa Rodrigues (HBR), Itajaí, Santa Catarina, Brazil.

Extraction and isolation of compounds

The dried and powdered stems (5 kg) and leaves (3 kg) were extracted with hexane and MeOH. Successively, half part (100.00 g) of the MeOH stems extract was fractionated over silica gel 60 (70-230 mesh) into three fractions: CH₂Cl₂, EtOAc and MeOH. The methanol fraction (89 g) was dissolved in H₂O-MeOH (3:7) and successively partitioned using solvents of increasing polarity (hexane, CH₂Cl₂ and EtOAc), affording the correspondent sub fractions. The CH₂Cl₂ sub fraction (2 g) was chromatographed over silica gel (230-400 mesh) using as eluent CH₂Cl₂-EtOAc (9:1 and gradient) to furnish the compounds (in eluting order): maculine (3) (36 mg), flindersiamine (4) (32 mg), kokusaginine (2) (42 mg), skimmianine (1) (29 mg), and the alkaloid 7 (10 mg).

The MeOH leaves extract (100 g) was also submitted to liquid partition with hexane, CH₂Cl₂, EtOAc and MeOH. Column chromatography (silica gel 230-400 mesh, hexane-acetone 7:3) of the leaves hexane fraction (10 g) yielded compounds 5 (600 mg) and 6 (300 mg). All the furoquinoline alkaloids described above (except flindersiamine) were also found in the leaves extract in considerable amounts.

Identification of the isolated compounds

The isolated alkaloids skimmianine⁹ (1), kokusaginine¹⁰ (2), maculine⁹ (3), and flindersiamine¹¹ (4) presented spectral data in agreement with the literature.

Flindersiamine (4) Yellow needles, mp 206-208^o. Crystallographic data: publication no. CCDC 151162.

2-n-Nonyl-4-quinolone¹² (5) Amorphous solid, mp 74-76^o, EIMS m/z (rel. int. %) [M⁺] 271 (10), 256 (3), 228 (5), 214 (4), 186 (11), 172 (60), 159 (100), 130 (30). ¹H and ¹³C NMR Table 2.

1-Methyl-2-n-nonyl-4-quinolone¹³ (6) Amorphous solid, mp 71-73^o, EIMS m/z (rel. int. %) [M⁺] 285 (10), 270 (3), 242 (5), 214 (4), 186 (60), 172 (100), 144 (50). ¹H and ¹³C NMR: Table 2.

1-Methyl-2-phenyl-4-quinolone¹⁴ (7) Amorphous solid, mp 118-120^o, EIMS m/z (rel. int. %) [M⁺] 235 (80), 207 (100), 191 (5), 178 (8), 165 (15), 130 (10), 102 (50), 89 (30), 77 (55), 51 (50).¹H and ¹³C NMR Table 2.

Fungicidal assay

The fungus Leucoagaricus gongylophorus Singer (Rozites gongylophora Möller) was isolated from a Atta sexdens Forel nest, kept in culture media and the fungicidal activity was performed according to established protocols.¹⁵

Trypanocidal activity

The in vitro trypomastigote forms of T. cruzi (Y strain) lethality assay was also performed according to established protocols.¹⁶

Results and Discussion

Compounds 1-4 were respectively identified as skimmianine, kokusaginine, maculine and flindersiamine, by comparison with the spectral data described in the literature.^9-11

Flindersiamine (4) has a methoxy group attached to C8, but spectroscopic data published could not furnish clear evidence of this moiety. In an attempt to confirm the extra methoxy group position on the furoquinoline nucleus, nOe experiments were performed. According to these experiments, by irradiation on the C4 methoxy group only the furan H3 signal was enhanced. Also, when the singlet corresponding to the quinoline hydrogen was irradiated, no enhancement was observed for the methoxy group as expected. These questionable irradiation results of 4 were verified by HMBC experiments. Unexpectedly, the C4 methoxy group showed correlation with the quinoline hydrogen. In order to solve these unclear findings, an X-ray analysis (Figure 1) was performed to confirm the C8 methoxy group position.

The molecule is planar except for C11, in fact the rms deviation for 16 non-H atoms fitted to the best least squares plane through them is of 0.026 and C11 being 1.069(3) Å out of that plane. A net of hydrogen bonds as shown in Table 3 joins the molecules in the crystal.

Special attention should be given to a C-H...p interaction and following Zukerman-Schpector et al.¹⁷ this interaction is characterized by three parameters, the H11C...Cgⁱ (Cg is the centroid of the O3-C1-C2-C3-C9 ring) distance of 3.39 Å, the C11-H11C...Cgⁱ angle of 128^o and the angle between the H...Cgⁱ vector and the plane of the ring of 92^o. (symmetry operation: i = 1-x, -0.5+y, 1.5-z).

Whether all the cited interactions are true hydrogen bonds is difficult to assert, because as pointed out by Cotton et al.,¹⁸ "..the field is getting muddier and muddier as the definition of a hydrogen bond is relaxed." In any case they are worth to be taken into account.

The crystallographic data obtained for the alkaloid 4 confirmed the previous HMBC spectroscopic attributions, but not the results obtained by nOe experiments.

The structures of compounds 5, 6 and 7 were elucidated on the basis of ¹H NMR and EIMS spectra and carbon attributions were made by HMBC and HSQC correlation spectra. Data are presented on Table 2.

Crude extracts of leaves and stems as well as fractions of R. echinata showed good inhibitory activity on L. gongylophorus growth. Among the isolated compounds of these fractions, each of the alkaloids 1-5 maintained or showed higher inhibition of the fungal growth at 100 mg mL^-1 (Table 1), compared with the original extracts. Contrary to the expectations, at a 50 mg mL^-1 dose the inhibitory effect was quite strongly reduced. In general, furoquinoline alkaloids showed higher activity than quinolones. Probably, the activity demonstrated by the methanol extract of leaves (fraction hexane) is due to the high concentration of total alkaloids, both furoquinoline and quinolone derivatives.

Only the quinolinone alkaloids 5 and 6 displayed some moderate activity on trypomastigote forms (Y strain) of T. cruzi with IC₅₀ of 134.9 and 100.9 mg mL^-1 respectively.

Fungicidal activities have been described for these alkaloids against plant pathogens Cladosporium cucumerinum¹⁹ and Phytophthora spp.²⁰ 4-Quinolones containing long alkyl chains at C-2 were first obtained from microorganisms (from Pseudomonas, also called pseudans²¹) and have been also isolated from rutaceous genera, mainly Evodia,²²Boronia,²³ Ruta¹³ and Esenbeckia.²⁴ Species from the former genus are used in Chinese herbal medicine and the isolated alkaloids have demonstrated inhibitory activity against the bacteria Helicobacter pilori.^25,26

Furoquinoline alkaloids have also demonstrated in vitro activity against Leishmania spp and Plasmodium falciparum.^27,28

Acknowledgements

We wish to thank Prof. A. Reis for collecting and identifying and Prof. J. R. Pirani for identifying the plant material, CAPES/PIDCT and UNIVALI for Ph.D. sponsorship of MWB and FAPESP, CNPq/PRONEX, MCT/FINEP for financial support. The X-ray facilities at Instituto de Química, USP was installed with a grant from FAPESP (94/2061-4).

Supplementary Information - Supplementary Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 151162. Copies of available material can be obtained, free of charge on application to the Director, CCDC, 12 Union Road, Cambridge CH2 1EZ, UK (fax: +44-1223-336-033 or e-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk)

References

3. CAD-4 Software; Version 5.0 Enraf-Nonius, Delft; The Netherlands, 1989.

Received: November 23, 2000

Published on the web: November 8, 2001

FAPESP helped in meeting the publication costs of this article.

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