New volatile constituents from leaves of Stemodia trifoliata (Link.) Reichb. (Schrophulariaceae)

ARTICLE

New volatile constituents from leaves of Stemodia trifoliata (Link.) Reichb. (Schrophulariaceae)

Wildson Max B. da Silva; João Carlos da C. Assunção; Renata M. Araújo; Edilberto R. Silveira; Otília D. L. Pessoa^*

Departamento de Química Orgânica e Inorgânica, Centro de Ciências, Universidade Federal do Ceará, CP 12.200, 60021-970 Fortaleza-CE, Brazil

ABSTRACT

The leaf essential oils of Stemodia trifoliata (Scrophulariaceae), collected at the same month of two different years (August 2005 and 2006), were analized by GC-MS and GC-FID. A total of 22 volatile components represented by sesquiterpenes and diterpenes was identified. β-Caryophyllene (9.4-15.4%) and caryophyllene oxide (6.2-9.0%) were the major compounds identified in the sesquiterpene fraction, while the diterpenoids 6α-acetoxymanoyl oxide (13.9-23.2%) and 6α-hydroxymanoyl oxide (25.1-29.7%) were the main constituents of the diterpene fraction. The two novel manoyl oxide derivatives had their structures established by means of spectroscopic methods, particularly 1D and 2D NMR. This work describes for the first time the chemical investigation on the volatile composition of S. trifoliata.

Keywords:Stemodia trifoliata, essential oil, manoyl oxide diterpenes, sesquiterpenes

RESUMO

Os óleos essenciais das folhas de Stemodia trifoliata (Scrophulariaceae), coletadas nos meses de agosto de 2005 e 2006, foram analisados por CG-EM e CG-DIC. Um total de 22 componentes voláteis, representados por sesquiterpenos e diterpenos, foi identificado. β-Cariofileno (9,4-15,4%) e óxido de cariofileno (6,2-9,0%) foram os principais componentes identificados na fração sesquiterpênica, enquanto os diterpenóides 6α-acetóxi-óxido de manoíla (13,9-23,2%) e 6α-hidróxi-óxido de manoíla (25,1-29,7%) foram os constituintes majoritários da fração diterpênica. Estes diterpenos, derivados do óxido de manoíla, são novos e suas estruturas foram determinadas por métodos espectroscópicos, particularmente RMN 1D e 2D. A investigação química dos componentes voláteis de S.trifoliata é descrita pela primeira vez neste trabalho.

Introduction

The genus Stemodia (Scrophulariaceae) comprises approximately 40 species represented by annual herbs and shrubs distributed in Asia, Africa, Australia and the Americas.¹ In accordance with a literature survey only five Stemodia species (S. maritima, S. chilensis, S. kingii, S. viscosa and S. foliosa) have been previously studied.^1-5 Secondary metabolites such as cucurbitacins, stearic acid derivatives and flavonoids have been isolated, but diterpenes containing an unusual tetracyclic skeleton, known as stemodane, are the most characteristic compounds elaborated by plants of this genus. These compounds bear a close resemblance to aphidicolin, a potent cytotoxic and antiviral diterpenoid isolated from Cephalosporium aphidicola and, by this reason have been the subject of innumerous biotransformations.^6-9S. trifoliata (Link) Reichb. is an annual plant, herbaceous and pubescent. Due to its preference by fertile and humid soils it is commonly found in flower beds, orchards and back yard gardens.¹⁰

In the present work, the chemical composition of the essential oil from the leaves of a wild population of S. trifoliata, as well as the isolation of two novel labdane diterpenes, are described. This is the first report on the phytochemical investigation of S. trifoliata and on the volatile chemical composition of a Stemodia species.

Results and Discussion

The essential oils of S. trifoliata, collected in August of two different years (2005 and 2006), but from the same site, were analyzed by GC-MS and GC-FID. The chemical composition of the oils, including the retention index and the percentage of each constituent is presented at Table 1. A total of 22 components was identified, representing 87.0 and 92.6% of the whole oils samples. Except by the presence of (E)-3-hexenol (2.6%), detected just in one of the analyzed oils, they were composed mainly by sesquiterpenes (33.2-48.0%) and diterpenes (44.6-51.2%), but the prevalent components were the same in both oils. In spite of this, marked qualitative differences were observed for the two oils, since a higher number of sesquiterpenes were identified for the sample B. Among the identified sesquiterpenes β-caryophyllene (9.4-15.4%) and caryophyllene oxide (6.2-9.0%) were the major ones. The diterpene fraction was dominated by manoyl oxide derivatives, which were identified as 6α-hydroxymanoyl oxide (25.1-29.7%) and 6α-acetoxymanoyl oxide (13.9-23.2%). In the GC-MS preliminary analysis of the oils, the peaks corresponding to 6α-hydroxymanoyl oxide (KI 1878) and 6α-acetoxymanoyl oxide (KI 1895), were not identified from their Kovats Indices and mass spectra. For this reason, an aliquot of the combined essential oils (samples A and B), was subjected to flash chromatography yielding both compounds, in the pure forms. Their structures were unequivocally elucidated by spectroscopy, especially NMR, including 1D and 2D experiments (Table 2).

Compound 1 was isolated as a colorless oil and its molecular formula C₂₂H₃₆O₃ was deduced by a combination of ¹³C NMR spectral data and EIMS (m/z 333, [M]⁺ - CH₃). Even though the molecular ion of 1 has not been detected it is plausible to speculate the loss of Me-16 to afford peak m/z 333 (5%) after the stability conferred by the conjugation of the positive charge with the vinyl group increased by the donation of the electron pair from the oxygen. From this ion the deacetylation would generate a more abundant ion at m/z 273 (38%), see supplementary data. The molecular formula accounted for five degrees of unsaturation, one of them represented by a acetoxyl group at d 170.3 in the ¹³C NMR spectrum, corroborated by the 1733 cm^-1 absorption in the FTIR spectrum. A second unsaturation was related to a vinyl group inferred from the typical ¹³C NMR signals at 147.8 (CH-14) and 110.7 (CH₂-15), and ¹H NMR signals at d 5.87 (1H, dd, J = 17.3 and 10.7 Hz, H-14), 5.16 (1H, dd, J = 17.3 and 1.4 Hz, H-15a) and 4.94 (1H, dd, J = 10.7, 1.4 Hz, H-15b). The ¹³C NMR (CPD) spectrum showed the presence of 22 carbon signals assigned to five methyls, six methylenes, three methines, a vinyl moiety, one acetyl group, as well as to four non-hydrogenated carbon atoms after comparison with both DEPT 135 and HMQC spectra. The above data are consistent with a manoyl oxide diterpene with an acetoxy substituent. The presence of the acetylated hydroxyl group at the C-6 position was supported by the observed ³J_C-H long-range correlations displayed for H-6 (d 5.11, td, J = 4.0 and 11.4 Hz) and the carbon signals at d 170.3 (C-21) and 74.5 (C-8). In the HMBC spectrum the correlation of H-14 (d 5.87, dd, J = 10.7 and 17.3 Hz) with the carbon signal at d 29.4 was sufficient to assure the correct assignment of CH₃-16.¹¹ The 4-axial orientation inferred to this methyl group was in agreement with the chemical shift at d 29.4. The proposed relative stereochemistry was further supported by the NOESY experiment, which clearly revealed dipolar interaction between CH₃-16/CH₃-17 and CH₃-16/H-12eq. Similarly, the α-equatorial orientation suggested to the acetyl group was defined based on nOe correlations of H-6 with the methyl groups 17, 19 and 20. Additional nOe effects observed for H-5/H-7ax/H-9ax; H-1ax/H-9ax and, H-9ax/H-12ax (Figure 2) supported the stereochemistry established for 1. Complete assignments of the NMR data of 1 were achieved by ¹H, ¹H-COSY, HMQC, HMBC and NOESY experiments. Thus, the structure of the new diterpene was unambiguously established as the 6α-acetoxymanoyl oxide.

Compound 2 was also isolated as colorless oil and its molecular formula C₂₀H₃₄O₂ was deduced by a combination of NMR ¹³C spectral data and EIMS (m/z 291, [M]⁺ - CH₃). The same kind of explanation used for 1 should be used here to justify the peak at m/z 273 (12%) after dehydration from m/z 291 (24%). This molecular formula along with the ¹H and ¹³C NMR spectra similarities for both compound suggested that compound 2 could be the corresponding alcohol of 6α-acetoxymanoyl oxide, 1. This was reinforced by the absence of the typical signals of the acetyl group. Moreover, the oxymethine proton (H-6) was shifted highfield (d 3.88) in 2 compared with that at d 5.11 in 1. The expected deshielding effect on the chemical shifts of carbons C-5 (d 61.9) and C-7 (d 54.8) when compared with those of 1 (d 59.1, C-5; d 50.1, C-7) was in agreement with the presence of a hydroxyl group in C-6 after the disappearance of the γ-effect of the acetyl group. This was also supported by the HMBC data (Figure 1). The NOESY experiment (Figure 2) showed that the stereochemistry of diterpene 2 was consistent with that established for 1. It is worthy of notice that the isomer 6α-hydroxy-13-epimanoyl oxide has been previously isolated from southern pine (Pinus spp.) tall oil.¹²

Experimental

General experimental procedures

The qualitative analysis was carried out on a Shimadzu GC-17A/QP5050 using a non-polar OV-5 fused silica capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness); carrier gas helium, flow rate 1 mL min^-1 and with split mode (ratio 1:48). The injector temperature and detector temperature were 250 and 280 ºC, respectively. The column temperature was programmed from 40 to 180 ºC at 4 ºC min^-1 and then 180 to 250 ºC at 20 ºC min^-1 and held isothermal for 7 min. Mass spectra were recorded from 30-450 m/z. NMR spectra were recorded on a Bruker Avance DRX-500 (500 MHz for ¹H and 125 MHz for ¹³C) spectrometer equipped with 5 mm inverse detection z-gradient probe. The chemical shifts, given on the d scale, were referenced to the residual undeuterated CHCl₃ portion (d_H 7.27) and, the center peak of the deuterated CDCl₃ (d_C 77.23).

Column chromatography was run using silica gel 60, particle size 0.040-0.063 (230-400 mesh, EMD/Merck), while TLC was performed on precoated plates (silica gel 60 F₂₅₄, Merck, Germany) and compounds detected by spraying with vanillin/perchloric acid/EtOH solution followed by heating at 120 ºC.

Plant material

The leaves of S. trifoliata were collected during the flowering stage, in August 2005 and 2006, from a population growing on the road margins at Pico Alto, Guaramiranga ridge (Ceará State, Brazil), at an elevation of approximately 1000 m. A voucher specimen (No. 39550) has been deposited at the Herbário Prisco Bezerra (EAC) of the Departamento de Biologia, Universidade Federal do Ceará.

Essential oil distillation

The fresh leaves of S. trifoliata were hydrodistilled in a Clevenger-type apparatus for a period of 2 h to afford yellowish oils, which were dried over Na₂SO₄, stored in sealed glass vials and preserved under refrigeration before analysis. The yield of the oils (0.02%, m/m), were calculated on fresh weight of the plant materials.

Isolation of compounds 1 and 2

An aliquot of the leaf essential oil (86 mg) of S. trifoliata, was subjected to silica gel column flash chromatography using a gradient of n-hexane:CH₂Cl₂ 4:6 to give 72 fractions of approximately 8 mL each. After TLC analysis, fractions 65 to 67 were combined (48 mg) and further submitted to silica gel flash chromatography using CH₂Cl₂ as eluent to yield the pure compounds 1 (17 mg) and 2 (10 mg).

Compound identification

The individual components of the oil were identified on the basis of their GC retention indices (RI) with reference to a homologous series of C₈-C₂₆n-alkanes, and by matching their 70 eV mass spectra with those of the spectrometer data base using the Wiley Class-5000 library and, by comparison of the fragmentation patterns of the mass spectra with those reported in the literature.¹³ The structures of the new volatile diterpenes (1 and 2) were elucidated based on spectroscopic analysis and comparison with published data for known manoyl oxide diterpenes.¹¹

6α-Acetoxymanoyl oxide (1)

Colorless oil; +25º (c 0.3, CHCl₃); IR (KBr) ν_max/cm^-1 2928, 2869, 1733, 1459, 1374, 1244, 1026; EM (70 eV) 333, 273, 255, 190, 175, 135, 119, 81; ¹H (500 MHz) and ¹³C NMR (125 MHz) data, see Table 1.

6α-Hydroxy-13-epimanoyl oxide (2)

Colorless oil; +20º (c 0.2, CHCl₃); IR (KBr) ν_max/cm^-1 3431, 3005, 2924, 1465, 1378, 1070; EM (70 eV) 291, 273, 255, 2001, 190, 135, 121, 109, 81, 69; ¹H (500 MHz) and ¹³C NMR (125 MHz) data, see Table 1.

Acknowledgments

The authors are grateful to CNPq, CAPES, FUNCAP and PRONEX for financial support and research fellowships.

Supplementary Information

Supplementary information for compounds 1 and 2 is available free of charge as PDF file at http://jbcs.sbq.org.br.

Received: April 22, 2008

Web Release Date: October 21, 2008

Supplementary Information

Figure S1 - click to extend

Figure S2 - click to extend

Figure S11 - click to extend

Figure S12 - click to extend

• 1. Chamy, M. C.; Piovano, M.; Garbarino, J. A.; Gambaro, V.; Phytochemistry 1991, 30, 1719.
• 2. Hufford, C. D.; Oguntimein, B. O.; Muhammad, I.; J. Nat. Prod 1992, 55, 48.
• 3. Allen, J. G.; Colegate, S. M.; Mitchell, A. A.; Mulder, R. J.; Raisbeck, M. F.; Phytochem. Anal 2006, 17, 226.
• 4. Ramesh, P.; Ramavhandran, N. A. G.; Sankara, S. S.; Pondicherry 1979, 48, 67.
• 5. Silva, L. L. D.; Nascimento, M.; Silva, D. H. S.; Furlan, M.; Bolzani, V. S.; Planta Med 2002, 68, 1137.
• 6. Chen, A. R. M.; Reese, P. B.; Phytochemistry 2002, 59, 57.
• 7. Martin, G. D. A.; Reynolds, W. F.; Reese, P. B.; Phytochemistry 2005, 66, 901.
• 8. Chen, A. R. M.; Ruddock, P. L. D.; Lamm, A. S.; Reynolds, W. F.; Reese, P. B.; Phytochemistry 2005, 66, 1898.
• 9. Lamm, A. S.; Reynolds, W. F.; Reese, P. B.; Phytochemistry 2006, 67, 1088.
• 10. Lorenzi H.; Plantas Daninhas do Brasil. Terrestres, Aquáticas, Parasitas e Tóxicas Instituto Plantarum de Estudos da Flora Ltda. 3Ş. ed. Nova Odessa: São Paulo, 2000. p. 546.
• 11. Chang, C. W. J.; Flament, I.; Matson, J. A.; Nishida, T.; Ohloff, G.; Wehrli, F. W.; Weinheimer, A. J.; Progress in the Chemistry of Organic Natural Products Springer Verlag Wien: New York, vol. 36, 1979, p. 56.
• 12. Conner, A. H.; Rowe, J. W.; Phytochemistry 1977, 16, 1777.
• 13. Adams, R. P.; Identification of Essential Oil Components by Gas Chromatography/Quadrupole Mass Spectroscopy. Allured Publishing Corporation, Carol Stream, Illinois: USA, 2001.

Publication Dates

• Publication in this collection
  14 Jan 2009
• Date of issue
  2009

History

• Received
  22 Apr 2008
• Accepted
  21 Oct 2008