Abstracts

Article

Dibenzylbutyrolactone lignans and coumarins from Ipomoea cairica^* * Based in part on the M.Sc. thesis submitted by O.O. de A.L. to the Universidade Federal Rural do Rio de Janeiro, Seropédica, Rio de Janeiro - RJ, Brazil (1989).

Olga O. de A. Lima and Raimundo Braz-Filho ^#

Curso de Pós-graduação em Química Orgânica, Universidade Federal Rural do Rio de Janeiro, 23851-970 Rio de Janeiro - RJ, Brazil;

^# Prof. Emérito-UFRRJ. Prof. Honoris causa - UFC

Received: March 14, 1996; September 18, 1996

Introduction

Ipomoea cairica, Convolvulaceae family, is a climbing plant growing in Bahia, Rio de Janeiro, Paraná, and probably in other Brazilian states¹. Hexane and methanol extracts of the dried aerial parts of a specimen of this plant afforded three lignoids (1-3) and two coumarins (4 and 5), along with palmitic and stearic acids and b-sitosterol. Lignoids 1-3 and the two coumarins have been previously reported. Earlier chemical investigations of the Convolvulaceae family did not report the occurrence of lignoids², except for pinoresinol dimethyl ether isolated from Humbertia madagascariensis³. The nomenclature and numbering of compounds 1-3 follow the rules outlined in a recent review⁴.

Results and Discussion

The aqueous methanol extract of a specimen of Ipomoea cairica was extracted successively with chloroform and n-butanol. The separation of lignoids 1-3 and coumarins 4 and 5 was achieved by chromatography on an alumina column and silica gel preparative thin layer chromatography of the chloroform extract. Compounds 1-5 have been previously reported. Lignoid 1 has been isolated from Arctium lappa (Compositae)⁵, Forsythia xoreana, and F. viridissima (Oleaceae)², 2 from Stellera chamaejasme (Thymelaeaceae), Tsuga chinensis (Pinaceae), F. xorena, and F. viridissima (Oleaceae)², and 3 from Trachelospermum asiaticum (Apocynaceae) ⁶. Coumarins umbelliferone (4) and scopoletin (5) have been recorded in several species of the Convolvulaceae family. Scopoletin appears in Ipomoea batatas as a phytoalexin^7,8.

During the course of our analysis of the ¹³C-NMR spectra of compound 1 and the mixture of 2 and 3 it was noted that some chemical shift assignments recently published for these and other lignoids^9,10 required revision (Table 1).

As anticipated, the comparison of the chemical shifts of the carbon atoms of lignoid 7 with those of the 9’-oxo compound (e.g. 6) clearly showed a g-effect at C-7’ due to the introduction of the carbonyl oxygen at C-9’ (Table 1). Thus, some assignments have been revised for the non-aromatic carbon atoms of compounds 6⁹ and 8¹⁰ (Table 1). The chemical shifts for the aromatic carbon C-5 (methine ortho to the methoxyl group) and C-5’ (methine ortho to the hydroxyl group) in lignoids 1 and 3 were distinguished on the basis of the g-effect of the methyl of the 3-OMe group (e.g. 9-11)¹¹. Model compounds 9-11 may also be used to demonstrate the known shielding effects induced by OH and OMe groups in the para and ortho positions, and the deshielding effect on the ipso-carbon by methylation. The oxygen atom of the hydroxyl group at C-8’ is responsible for an upfield shift of the C-1’ and C-7 (g-effect).

Conclusions

Convolvulaceae is a large family comprising about 59 genera and more than 1800 species, which are found in tropical and subtropical regions as herbs, climbers, or shrubs12. The natural products reported in the species of this family may be classified as polyketides, terpenoids (mono-, sesqui-, di-, and triterpenoids), steroids, shikimides (coumarins and benzoic and cinnamic acid derivatives), flavonoids (flavonols, anthocyanins, flavones, flavanones), one xanthone (mangiferin), alkaloids (pyrrolidines, tropanes, indolizidines, and ergolines), and other compounds. To the best of our knowledge, lignoids such as 1-3 have been hitherto unreported as natural products of the Convolvulaceae family, with the exception of pinoresinol dimethyl ether (12) which has been isolated from Humbertia madagascariensis³. The fact that umbelliferone (4) was isolated points to the possibility that the specimen used in our investigation had been infested by a fungus, since this substance was recently found to be a phytoalexin in I. batatas^7,8. Moreover, it can be assumed that lignoids were also biosynthesized as phytoalexins. At the ecological level, there is evidence that lignans play a role in plant-plant, plant-fungus, and plant-insect interactions¹³.

Matairesinol (2), formed via the coupling of two phenylpropane units having the ferulic substitution pattern, would be a logical precursor of the other lignoids 1 and 3 through methylation of the hydroxyl group localized at C-4’¹⁴.

Phytochemical studies of other specimens of Ipomoea cairica have also revealed the presence of the lignanolides arctigenin 1 and trachelogenin 3 in an aqueous methanol extract¹⁵. That investigation was probably developed during the same period as our study of the same species.

Experimental

General

Mps are uncorrected. NMR (¹H: 100 and 200 MHz; ¹³C: 25.2 and 50.3 MHz) spectra were recorded in a CDCl₃ solution with TMS as the internal standard. EIMS were measured by direct inlet at 70 eV ionization. GC/MS spectra were measured on a GCHR/MS-C 5987A (HP) spectrometer, using a capillary glass column (OV-1701; 20 m).

Plant material

The plant material of Ipomoea cairica (L.) Sweet, Convolvulaceae family, was collected on the campus of the Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil. The specimen was identified by Rosa Fuks, Jardim Botânico do Rio de Janeiro (voucher specimen nº 257337), Rio de Janeiro, Brazil.

Isolation of the constituents

Dried and milled whole plant (1.35 kg) was extracted at room temperature with hexane and then with MeOH. A portion (1 g) of the residue obtained from the hexane extract (98 g) was dissolved in ether and treated with aqueous 10% NaHCO₃. After neutralization and extraction with ether, the aqueous solution gave a mixture of fatty acids (60 mg). The remaining residue (97 g) was dissolved in hexane and filtered on a CaCO₃ column to eliminate chlorophyll. The hexane solution was concd. in vacuo and the residue was submitted to silica gel column chromatography. Elution with hexane and hexane-EtOAc (9:1) gave aliphatic material and b-sitosterol, respectively.

A portion (50 g) of the MeOH extract (250 g) was treated with tartaric acid 1% aqueous solution and successively extracted with CHCl₃ and n-BuOH. The CHCl₃ solution was concd in vacuo and a portion of the residue (300 mg) was chromatographed on a basic alumina (V) column. Elution with CHCl₃, CHCl₃-MeOH (mixts of the indicated composition), and MeOH furnished frs 1-6 (CHCl₃), 7 (99:1), 8 (95:5), 9 (1:1), and 10 (MeOH). Preparative TLC of fr-2 [silica gel GF-254, CHCl₃-MeOH (99:1)] and crystallization from MeOH-ether yielded arctigenin (1, 20 mg), mp 98-100 °C. The frs 3-6 were submitted to preparative TLC (silica gel H) with CHCl₃-MeOH (1:1), affording matairesinol (2) + trachelogenin (3). This mixture was identified by ¹³C-NMR spectra (full decoupled and SFORD), and the purification of the individual compounds was achieved by preparative TLC (silica gel H) with MeOH. The less polar spot gave matairesinol (2,5 mg), mp 117-119 °C, after crystallization from MeOH; the more polar band was crystallized from MeOH to furnish trachelogenin (3,10 mg), mp 139-141 °C. Preparative TLC of fr-7 [silica gel H, CHCl₃-MeOH (99:1)] gave umbelliferone (4) and scopoletin (5).

Identification

The known compounds 1⁵, 2, 3², 4¹⁵, and 5¹⁶ were identified by comparison with authentic samples (2-5) and by spectral data, including derivatives obtained by acetylation (1a) with Ac₂O/Py and methylation (2a) with CH₂N₂/ether. The fatty acids mixture was methylated with CH₂N₂/ether, and the methyl esters of palmitic and stearic acids were identified by GC/MS.

Acknowledgments

This study was supported by a research fellowship and by grants from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), the Financiadora de Estudos e Projetos (FINEP), and the Coordenação de Aperfeiçoamento de Pessoal de Ensino Superior (CAPES). The authors are grateful to Professors Antônio J.R. da Silva (NPPN, Universidade Federal do Rio de Janeiro) for the NMR spectra (¹H: 100 MHz; ¹³C: 25.2 MHz) and MS spectra, Jary Nóbrega (Instituto de Química, Universidade Federal do Rio de Janeiro) for providing the GC/MS, Sansei Nishibe (Department of Pharmacognosy, School of Pharmaceutical Sciences, Japan) for samples of compounds 2 and 3, Victor Rumjanek (Departamento de Química, Universidade Federal Rural do Rio de Janeiro) for NMR (¹H: 200 MHz) spectra, and Ceres M.R. Gomes (Departamento de Química, Universidade Federal Rural do Rio de Janeiro) for the interest and useful suggestions in the conclusion of the M. Sc. thesis.

References

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