16,17-Seco- and 2,3:16,17-di-Seco-pregnanes from Guarea guidonia

ARTICLE

16,17-Seco- and 2,3:16,17-di-Seco-pregnanes from Guarea guidonia

Walmir Silva Garcez^I,*; Fernanda Rodrigues Garcez^I; Luzinátia Ramos Soares^{I, II}

^IDepartamento de Química, Universidade Federal de Mato Grosso do Sul, 79070-900 Campo Grande-MS, Brazil

^IIUniversidade Estadual de Mato Grosso do Sul, Coxim, 79400-000 Campo Grande-MS, Brazil

ABSTRACT

Two new seco- and di-seco-pregnanes, 2α,3β-dihydroxy-16,17-seco-pregn-17-ene-16-oic acid methyl ester 2β,19-hemiketal (1) and 2,3:16,17-di-seco-pregn-17-ene-3-oic acid-16-oic acid methyl ester-19-hydroxy-2-carboxylic acid-2,19-lactone (2), have been obtained from the trunk bark of Guarea guidonia. Their structures have been established by a combination of 1D- and 2D-NMR spectroscopic techniques and MS data. The unique seco- and di-seco-pregnane carbocyclic skeletal types as found in compounds 1 and 2 are being reported in the Meliaceae for the first time as well as the occurrence of pregnanes in the genus Guarea.

Keywords:Guarea guidonia, Meliaceae, seco-pregnane, di-seco-pregnane

RESUMO

Dois novos seco- e di-seco-pregnanos, 2α,3β-diidrσxi-2β,19-hemicetal-16,17-seco-pregn-17-en-16-oato de metila (1) e ácido 2,3:16,17-di-seco-pregn-17-en-3-óico-16-oato de metila-19-hidróxi-2-carboxílico-2,19-lactona (2), foram isolados da casca do caule de Guarea guidonia. Suas estruturas foram determinadas com base em dados espectrais, particularmente RMN uni- e bidimensionais e massas. Os esqueletos pregnânicos incomuns seco- e di-seco destes compostos estão sendo relatados pela primeira vez em Meliaceae, assim como a ocorrência de pregnanos no gênero Guarea.

Introduction

Species of Guarea (Meliaceae) are found throughout tropical America and also in Africa.¹ This genus has been the subject of a number of investigations and is known as a rich source of secondary metabolites, including limonoids, sesqui-, di- and triterpenes and steroids.^2-5Guarea guidonia L. Sleumer is a tree widely distributed in Brazil and the wood bark of this plant is employed in traditional medicine as an abortive and febrifugal agent and the seeds are used to treat rheumatism while the leaves and fruits are reported to be toxic to cattle.^6,7 Moreover, seed extracts of G. guidonia were shown to have anti-inflammatory activity in rats while antiviral properties against pseudo-rabies virus were reported for crude extracts of leaves and fruits.^7,8 In previous works sesquiterpenes were isolated from the stem bark of a specimen of G. guidonia occurring in south-eastern Brazil³ while a specimen of G. guidonia collected in the central-western region of Brazil afforded a number of constituents, including di-, tri- and sesquiterpenes and steroids from the leaves⁴ and compounds of the sesquiterpene, limonoid and coumarin types from the trunk bark.⁵ In the course of the investigation of the trunk bark of the latter specimen,⁵ the CHCl₃-soluble fraction of the ethanol extract was shown to contain two minor steroidal derivatives which, nevertheless, were not further isolated. The present study of the trunk bark of this same specimen has now allowed the isolation of these constituents which were characterized as two novel seco- and di-seco-pregnane derivatives (1 and 2). Their structural elucidation was mainly based on 1D and 2D NMR spectroscopic techniques.

Results and Discussion

Compound 1 was obtained as an oil. Its molecular formula was established as C₂₂H₃₄O₅ by NMR spectroscopic data and HREIMS, which showed a molecular ion peak at m/z 378.2410, indicating six degrees of unsaturation. The IR absorptions at 3402 and 1734 cm^-1 were consistent with the presence of hydroxyl and ester carbonyl groups. The ¹H NMR spectrum of 1 (Table 1) showed signals attributable to a propenyl moiety [δ 5.36 (dq, J = 16.6, 6.0 Hz), 5.23 (brd, J = 16.6 Hz), 1.61 (brd, J = 6.0 Hz)], a methoxyl group [δ 3.60 (s)], a carbinolic hydrogen [δ 3.60 (m)] and two AB doublets at δ 3.92 (J = 8.1 Hz) and 3.67 (J = 8.1 Hz) ascribed to an oxymethylene group. The ¹³C NMR spectrum (Table 1) displayed signals for 22 carbon atoms, seven of which, with the aid of information afforded by the DEPT spectrum, confirmed the presence of propenyl (δ 142.0, 122.4, 18.2), carbomethoxy (δ 175.0, 51.6), oxymethylene (δ 67.4) and oxymethine (δ 73.6) groups. The remaining signals were attributed to a methyl, seven methylenes, four methines and three quaternary carbons, one of which could be assigned to a hemiketal carbon (δ 104.5). Comparison of ¹H and ¹³C NMR data of 1 and 2α,3β-dihydroxypregnan-16-one 2β,19-hemiketal (3), a pregnane previously isolated from Trichilia claussenii⁹ and Melia volkensii,¹⁰ revealed that they have identical rings A, B and C, with the same cyclic hemiketal funcionality with an oxy linkage between C-2 and C-19 and with a β-hydroxyl substituent at C-3, bearing a trans di-equatorial relationship with the hydroxyl group at C-2. The presence of the aforementioned propenyl and carbomethoxy groups in 1, in addition to the long-range connectivities observed in the HMBC spectrum between the methylene hydrogens at δ 2.27 and 1.88 assigned to H₂-15 and the carbonyl ester carbon and between H₃-18 (δ 0.79) and the olefinic carbon at δ 142.0 of the propenyl moiety were in accordance with a 16,17-seco-pregnane skeleton. The location of the 1-propenyl and the carbomethoxy groups at C-13 and C-14, respectively, was then established. Accordingly, two-bond proton-carbon correlations were observed between H₂-15 and C-14 (δ 46.5) and the olefinic hydrogen H-17 (δ 5.23) and C-13 (δ 39.5). The cis-orientation of C-18 methyl and the C-14 CH₂CO₂Me substituents was revealed by the correlations observed in the NOESY spectrum between H-18 and H-15. Hence, considering that the carbon backbones of 1 and 3 are likely to be formed by the same biosynthetic route accepted for all pregnane structural types, the orientation of both C-18 methyl group and H-14 of these compounds are assumed to be the same. The structure of this 16,17-seco-pregnane was then characterized as 1.

Compound 2 was isolated as an oil. The HRESITOFMS ([MH⁺], m/z 393.2328) and NMR spectroscopic data of 2 suggested a molecular formula of C₂₂H₃₂O₆. Its IR spectrum showed absorption bands at 3451 (broad), 1774 and 1732 (broad) cm^-1 indicating the presence of lactone and ester carbonyl and carboxyl groups. The ¹H and ¹³C NMR spectra of 2 (Table 1) were similar in many respects to those of 1. The proton and carbon shifts of the rings B and C corresponded to each other as well as the signals relative to the propenyl and methylene carbomethoxy groups. The ¹³C NMR spectrum of 2, however, lacked the hemiketal and carbinolic carbon resonances at δ 104.5 and 73.6 assigned to C-2 and C-3, respectively, in 1 and instead displayed signals ascribed to a carboxyl group (δ 177.2) and a lactone moiety (δ 175.3). Likewise, the multiplet at δ 3.60 attributed to the carbinolic hydrogen at C-3 in 1 was absent in the ¹H NMR spectrum of 2. When 2 was methylated with diazomethane, a methyl ester derivative (2a) was obtained. Its ¹H NMR spectrum showed, in addition to a singlet at δ 3.69 assignable to a second carbomethoxy group and besides the one located at C-15, an AB pattern of two doublets at δ 4.03 and 4.20 (J = 10.7 Hz), suggestive of methylene hydrogens linked to an oxygen ester group. This pair of doublets showed a connectivity in the HMQC spectrum with the carbon signal at δ 69.1, and three-bond correlations in the HMBC spectrum with the carbon signals attributable to C-5 (δ 41.2) and C-9 (δ 50.3). These information, along with the long-range correlations discernible between the two-proton singlet at δ 2.48 (H-1) and the carbon signals at δ 69.1 (C-19), 176.7 (lactone carbonyl) and 45.3 (C-10), and between H₂-4 and the carbonyl group at δ 172.6 attributable to C-3, supported the presence of a 2,3-seco-A ring in 2a, and also, in compound 2. Accordingly, the location of a carboxyl moiety at C-3 as well as the formation of a γ-lactone ring between C-19 and C-2, with the latter bearing the carbonyl group, were established. As in the case of 1, the β-configurations suggested for C-19 and C-4 are based on biogenetic grounds. The above-mentioned data could be satisfactorily assembled to give structure 2 for this 2,3:16,17-di-seco-pregnane steroid, whose unusual skeleton could have been originated from 1 through an oxidative cleavage of the C-2/C-3 bond. In order to allow a better comparison between the spectral data of 1 and 2, ¹H and ¹³C NMR spectra of 1 were also obtained in pyridine-d₅(Table 2) whose results further supported the proposed structures for these compounds.

The finding of pregnane steroids in the Guarea genus is noteworthy, since this class of compounds has so far been described in only seven among the almost fifty genera comprising the Meliaceae, namely Trichilia,⁹ Melia,¹⁰Aglaia,¹¹ Dysoxylum,¹²Munronia,¹³Toona¹⁴ and Turraea.¹⁵ In addition, seco-pregnane steroid derivatives are not of widespread occurrence in plant taxa. The great majority of these compounds has been described in members of the Asclepiadaceae, namely 14,15-, 15,16- and 8,14-seco- and 13,14:14,15-di-seco-pregnanes and their glycosides isolated from Cynanchum species¹⁶ and from Adelostemma gracillimum,¹⁷Biondia insignis,¹⁸Sarcostemma viminale,¹⁹ Solenostemma argel²⁰ and Tylophora tanakae.²¹ However, no records are available for the occurrence in the family Meliaceae of the unique 16,17-seco- and 2,3:16,17-di-seco-pregnane carbocyclic skeletal types displayed by compounds 1 and 2.

Experimental

General experimental procedures

Optical rotations were determined on a Perkin-Elmer 341 polarimeter (Na filter, λ=589 nm). IR spectra were recorded as films on a Bomem-Hartmann & Braun FT IR spectrometer. 1D- and 2D-NMR experiments were run on a Bruker DPX-300 spectrometer at 300 MHz (¹H) and 75 MHz (¹³C), using TMS as an internal standard. HREIMS data was obtained at 70 eV on a VG Autospec spectrometer (Instituto de Química, UNICAMP, SP, Brazil), and HRQTOFESIMS spectrum (positive mode) was performed with an UltrOTOF mass spectrometer (Fiocruz/Farmanguinhos, RJ, Brazil). Silica gel 70-230 mesh and Sephadex LH-20 were used for column chromatography. Preparative TLC was performed on silica gel 60 PF₂₅₄ and compounds were visualized by exposure of the edges of the plate to iodine vapors.

Plant material

The trunk bark of Guarea guidonia L. Sleumer was collected in Campo Grande, MS, Brazil, in June, 2002. The plant material was identified by Dr. Humberto Barreiros, Jardim Botânico do Rio de Janeiro, RJ, Brazil, where a voucher specimen (No. 1870) is deposited.

Extraction and isolation of chemical constituents

Air-dried and powdered trunk bark of G. guidonia (2.2 kg) was extracted successively at room temperature with hexane and then with EtOH. After evaporation of the solvent under reduced pressure, the residue obtained from the EtOH extract (145.3 g) was subsequently partitioned between MeOH-H₂O 1:1 and hexane, CHCl₃ and EtOAc. The dried CHCl₃ partition (29.5 g) was applied to a silica gel column and eluted with hexane-CHCl₃, CHCl₃-EtOAc and EtOAc-MeOH gradient systems to give 9 fractions. Gel filtration chromatography over Sephadex LH-20 of fraction 5 (CHCl₃-EtOAc 1:1, 315.3 mg) using CHCl₃-MeOH 7:3 as eluent gave 80 fractions. Fractions 47-52 from this column (18.0 mg) afforded 1 (7.8 mg) and 2 (4.9 mg) after prep. TLC on silica gel (CH₂Cl₂-EtOAc-MeOH-AcOH 10:4:0.15:0.1), R_f values 0.4 (1) and 0.5 (2).

2α,3β-dihydroxy-16,17-seco-pregn-17-ene-16-oic acid methyl ester 2β,19-hemiketal (1)

Yellowish oil; - 4.5º (c 0.88, CHCl₃); IR (film) ν_max/cm^-1: 3402, 2930, 2856, 1734; ¹H and ¹³C NMR data: see Table 1; HREIMS: m/z 378.2410 [M]⁺ (calcd for C₂₂H₃₄O₅, 378.2407).

2,3:16,17-di-seco-pregn-17-ene-3-oic acid-16-oic acid methyl ester-19-hydroxy-2-carboxylic acid-2,19-lactone (2)

Yellowish oil; - 7.8º (c 0.25, CHCl₃); IR (film) ν_max/cm^-1: 3451, 2962, 2927, 1774, 1732; ¹H and ¹³C NMR data: see Table 1; HRESIMS: m/z 393.2328 [M+H]⁺ (calcd for C₂₂H₃₃O₆, 393.2278). Compound 2 was treated with an ethereal solution of diazomethane to yield the corresponding dimethyl ester 2a. ¹H NMR spectral data (CDCl₃): δ 0.88 (3H, s, H-18), 1.26 (1H, m, H-9), 1.56 (2H, m, H-11), 1.58 (2H, m, H-7), 1.66 (3H, dd, J 6.8, 0.9 Hz, H-21), 1.70 (1H, m, H-14), 1.96 (1H, dd, J 16.8, 8.4 Hz, H-15b), 1.98 (1H, m, H-5), 2.11 (1H, dd, J 16.0, 10.1 Hz, H-4b), 2.37 (1H, dd, J 16.8, 2.4 Hz, H-15a), 2.48 (2H, s, H-1), 2.48 (1H, dd, J 16.0, 2.8 Hz, H-4a), 3.64 (3H, s, OCH₃-16), 3.69 (3H, s, OCH₃-3), 4.03 (1H, d, J 10.7 Hz, H-19b), 4.20 (1H, d, J 10.7 Hz, H-19a), 5.27 (1H, dq, J 16.8, 6.8 Hz, H-20), 5.42 (1H, dd, J 16.8, 0.9 Hz, H-17). ¹³C NMR spectral data (CDCl₃): δ 16.5 (q, C-18), 18.2 (q, C-21), 21.6 (t, C-11), 29.2 (t, C-6), 29.9 (t, C-7), 35.1 (t, C-15), 36.2 (t, C-4), 37.1 (t, C-1), 38.8 (d, C-8), 39.1 (s, C-13), 39.1 (t, C-12), 41.2 (d, C-5), 45.3 (s, C-10), 46.1 (d, C-14), 50.3 (d, C-9), 51.7 (q, OCH₃-3), 52.0 (q, OCH₃-16), 69.1 (t, C-19), 122.8 (d, C-20), 141.6 (d, C-17), 172.6 (s, C-3), 174.8 (s, C-16), 176.7 (s, C-2).

Acknowledgments

The authors are grateful to FUNDECT-MS and CPq-PROPP/UFMS for their financial support and to CAPES for the scholarship awarded. Dr. H. Barreiros (Jardim Botânico do Rio de Janeiro-RJ, Brazil ) is acknowledged for his assistance in the identification of the plant material and Dr. A. Magalhães (Fiocruz/Farmanguinhos, Rio de Janeiro, RJ, Brazil) for HRESITOFMS measurements.

Supplementary Information

¹H NMR, ¹³C NMR, ¹H-¹1H COSY, HMQC and HMBC NMR spectra of compounds 1, 2 and 2a are available free of charge at http://jbcs.org.br, as PDF file.

References

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3. Núñez, C. V.; Roque, N. F.; J. Essent. Oil Res. 1999, 11, 439.

4. Brochini, C. B.; Roque, N. F.; J. Braz. Chem. Soc. 2000, 11, 361; Lago, J. H. G.; Brochini, C. B.; Roque, N. F.; Phytochemistry 2002, 60, 333.

5. Garcez, F. R.; Núñez, C. V.; Garcez, W. S.; Almeida, R. M.; Roque, N. F.; Planta Med. 1998, 64, 79.

6. Correa, M. P.; Dicionário das Plantas Úteis do Brasil e das Exóticas Cultivadas, Imprensa Nacional: Rio de Janeiro, 1978; Camargo, W. V. A.; O Biológico 1966, 32, 23.

7. Oga, S.; Sertie, J. A.; Brasilie, A. C.; Hanada, S.; Planta Med. 1981, 42, 310.

8. Simoni, I. C.; Munford, V.; Felício, J. D.; Lins, A. P.; Braz. J. Med. Biol. Res. 1996, 29, 647.

9. Pupo, M. T.; Vieira, P. C.; Fernandes, J. B.; da Silva, M. F. G. F.; Rodrigues Filho, E.; Phytochemistry 1997, 45, 1495.

10. Rogers, L. L.; Zeng, L.; McLaughlin, J. L.; J. Org. Chem. 1998, 63, 3781.

11. Inada, A.; Murata, H.; Inatomi, Y.; Nakanishi, T.; Darnaedi, D.; Phytochemistry 1997, 45, 1225.

12. Mohamad, K.; Martin, M. T.; Litaudon, M.; Gaspard, C.; Sévenet, T.; Païs, M.; Phytochemistry 1999, 52, 1461.

13. Cai, X. H.; Luo, X. D.; Zhou, J.; Hão, X. J.; J. Integr. Plant Biol. 2006, 48, 1126.

14. da Silva, M. F. G. F.; Agostinho, S. M. M.; Paula, J. R.; Oiano Neto, J.; Castro-Gamboa, I.; Rodrigues Filho, E.; Fernandes, J. B.; Vieira, P. C.; Pure Appl. Chem. 1999, 71, 1083.

15. Wang, X. N.; Fan, C. Q.; Yue, J. M.; Steroids 2006, 71, 720.

16. Bai, H.; Li, W.; Koike, K.; Steroids 2008, 73, 96; Liu, Y.; Qu, J.; Yu, S. S.; Hu, Y. C.; Huang, X. Z.; Steroids 2007, 72, 313; Yanmei, W. L.; Li, S.; Chen, X.; Shen, Y.; Zhang, Z.; He, H.; Xu, W.; Shu, Y.; Liang, G.; Fang, R.; Hao, X.; Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 8083; Warashina, T.; Noro, T.; Chem. Pharm. Bull. 2006, 54, 1551; Kanchanapoom, T.; Kasai, R.; Ohtani, K.; Andriantsiferana, M.; Yamasaki, K.; Chem. Pharm. Bull. 2002, 50, 1031.

17. Mu, Q. Z.; Shen, Y. M.; Zhou, Q. L.; Wang, S. Q.; Wu, B.; Zheng, Q. T.; Planta Med. 1992, 58, 200.

18. Zhang, Y. H.; Shen, Y. M.; Wen, Y. Y.; Kuang, T. Y.; Chin. Chem Lett. 2000, 11, 1065.

19. Vleggaar, R.; van Heerden, F. R.; Anderson, L. A. P.; Erasmus, G. L.; J. Chem. Soc. Perkin Trans. I 1993, 483.

20. Perrone, A.; Plaza, A.; Ercolino, S. F.; Hamed, A. I.; Parente, L.; Pizza, C.; Piacente, S.; J. Nat. Prod. 2006, 69, 50; Plaza, A.; Perrone, A.; Balestrieri, M. L.; Bifulco, G.; Carbone, V.; Hamed, A. I.; Pizza, C.; Piacente, S.; Tetrahedron 2005, 61, 7470.

21. Abe, F.; Hirokawa, M.; Yamauchi, T.; Honda, K.; Hayashi, N.; Nishida, R.; Chem. Pharm. Bull. 1999, 47, 1384.

Received: November 13, 2007

Web Release Date: July 8, 2008

Supplementary Information

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• ¹
  http://mobot.mobot.org/W3T/Search/vast.html, accessed in November 2007.
  » link
• 2. Garcez, F. R.; Garcez, W. S.; da Silva, A. F. G.; Bazzo, R. C.; Resende, U. M.; J. Braz. Chem. Soc. 2004, 15, 767;
• Lago, J. H. G.; Roque, N. F. Phytochemistry 2002, 60, 329;
• Harding, W. W.; Jacobs, H.; McLean, S.; Reynolds, W. F. Magn. Reson. Chem. 2001, 39, 719;
• Camacho, M. R.; Phillipson, J. D.; Croft, S. L.; Kirby, G. C.; Warhurst, D. C.; Solis, P. N. Phytochemistry 2001, 56, 203;
• Furlan, M.; Lopes, M. N.; Fernandes, J. B.; Pirani, J. R. Phytochemistry 1996, 41, 1159;
• Mootoo, B. S.; Játiva, C.; Tinto, W. F.; Reynolds, W. F.; McLean, S. Can. J. Chem. 1992, 70,1260;
• Akinniyi, J. A.; Connolly J. D.; Rycroft, D. S.; Sondengam, B. L.; Ifeadike, N. P.; Can. J. Chem. 1980, 58, 1865.
• 3. Núñez, C. V.; Roque, N. F.; J. Essent. Oil Res. 1999, 11, 439.
• 4. Brochini, C. B.; Roque, N. F.; J. Braz. Chem. Soc. 2000, 11, 361;
• Lago, J. H. G.; Brochini, C. B.; Roque, N. F.; Phytochemistry 2002, 60, 333.
• 5. Garcez, F. R.; Núñez, C. V.; Garcez, W. S.; Almeida, R. M.; Roque, N. F.; Planta Med. 1998, 64, 79.
• 6. Correa, M. P.; Dicionário das Plantas Úteis do Brasil e das Exóticas Cultivadas, Imprensa Nacional: Rio de Janeiro, 1978;
• Camargo, W. V. A.; O Biológico 1966, 32, 23.
• 7. Oga, S.; Sertie, J. A.; Brasilie, A. C.; Hanada, S.; Planta Med. 1981, 42, 310.
• 8. Simoni, I. C.; Munford, V.; Felício, J. D.; Lins, A. P.; Braz. J. Med. Biol. Res. 1996, 29, 647.
• 9. Pupo, M. T.; Vieira, P. C.; Fernandes, J. B.; da Silva, M. F. G. F.; Rodrigues Filho, E.; Phytochemistry 1997, 45, 1495.
• 10. Rogers, L. L.; Zeng, L.; McLaughlin, J. L.; J. Org. Chem. 1998, 63, 3781.
• 11. Inada, A.; Murata, H.; Inatomi, Y.; Nakanishi, T.; Darnaedi, D.; Phytochemistry 1997, 45, 1225.
• 12. Mohamad, K.; Martin, M. T.; Litaudon, M.; Gaspard, C.; Sévenet, T.; Païs, M.; Phytochemistry 1999, 52, 1461.
• 13. Cai, X. H.; Luo, X. D.; Zhou, J.; Hão, X. J.; J. Integr. Plant Biol. 2006, 48, 1126.
• 14. da Silva, M. F. G. F.; Agostinho, S. M. M.; Paula, J. R.; Oiano Neto, J.; Castro-Gamboa, I.; Rodrigues Filho, E.; Fernandes, J. B.; Vieira, P. C.; Pure Appl. Chem. 1999, 71, 1083.
• 15. Wang, X. N.; Fan, C. Q.; Yue, J. M.; Steroids 2006, 71, 720.
• 16. Bai, H.; Li, W.; Koike, K.; Steroids 2008, 73, 96;
• Liu, Y.; Qu, J.; Yu, S. S.; Hu, Y. C.; Huang, X. Z.; Steroids 2007, 72, 313;
• Yanmei, W. L.; Li, S.; Chen, X.; Shen, Y.; Zhang, Z.; He, H.; Xu, W.; Shu, Y.; Liang, G.; Fang, R.; Hao, X.; Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 8083;
• Warashina, T.; Noro, T.; Chem. Pharm. Bull. 2006, 54, 1551;
• Kanchanapoom, T.; Kasai, R.; Ohtani, K.; Andriantsiferana, M.; Yamasaki, K.; Chem. Pharm. Bull. 2002, 50, 1031.
• 17. Mu, Q. Z.; Shen, Y. M.; Zhou, Q. L.; Wang, S. Q.; Wu, B.; Zheng, Q. T.; Planta Med. 1992, 58, 200.
• 18. Zhang, Y. H.; Shen, Y. M.; Wen, Y. Y.; Kuang, T. Y.; Chin. Chem Lett. 2000, 11, 1065.
• 19. Vleggaar, R.; van Heerden, F. R.; Anderson, L. A. P.; Erasmus, G. L.; J. Chem. Soc. Perkin Trans. I 1993, 483.
• 20. Perrone, A.; Plaza, A.; Ercolino, S. F.; Hamed, A. I.; Parente, L.; Pizza, C.; Piacente, S.; J. Nat. Prod. 2006, 69, 50;
• Plaza, A.; Perrone, A.; Balestrieri, M. L.; Bifulco, G.; Carbone, V.; Hamed, A. I.; Pizza, C.; Piacente, S.; Tetrahedron 2005, 61, 7470.
• 21. Abe, F.; Hirokawa, M.; Yamauchi, T.; Honda, K.; Hayashi, N.; Nishida, R.; Chem. Pharm. Bull. 1999, 47, 1384.

Publication Dates

• Publication in this collection
  12 Sept 2008
• Date of issue
  2008

History

• Accepted
  08 July 2008
• Received
  13 Nov 2007