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A sesquiterpene acid from Eremophila interstans
0031-9422/90 S3.00+0.00 Q 1990 Pergamon Press plc
Vol. 29, No. 8, pp. 2700 2701, 1990. Printed in Great Britain.
Phytochemistry,
A SESQUITERPENE
ACID FROM EREMOPHILA
ZNTERSTANS
EMILIO L. GHISALBERTI, PHILLIP R. JEFFERIES and HIEU THI NGOC Vu Department of Organic Chemistry, The University
of Western
Australia,
Nedlands,
6009, Western
Australia
(Received2 January 1990) Key Word Index--Eremophila
interstuns;
Abstract-A new bicyclic sesquiterpene related sesquiterpene acids previously species.
Myoporaceae;
sesquiterpene
acid.
acid has been isolated from Eremophila interstans. The occurrence of two other isolated from E. uirgata establishes a chemotaxonomic link between the two
The genus Eremophila (family Myoporaceae) consists mainly of desert adapted shrubs or small trees, with over one hundred species listed as indigenous to Western Australia alone [l]. Eremophiln species are of limited economic significance, with a few species finding some use in horticulture. The majority of the species exhibit bright and colourful flowers during the spring months and about half of the species secrete resin coatings onto the leaves and terminal branches. Typically these resins consist of oxygenated diterpenes and flavones [Z], although recently we have uncovered two examples [3,4] in which sesquiterpenes replaced the diterpene component. We now report on a third case where sesquiterpenes are the dominant resin components. E. interstans (S. Moore) Morrison accumulates three sesquiterpene acids which have been isolated as their methyl esters (l-3). Of these, 1 and 2 have previously been isolated from E. uirgata [3] and 3 is the methyl ester of a new sesquiterpene hydroxy acid. Evidence for the structure of 3 is the subject of this report. The extract (13% dry wt) obtained by soaking the leaves and terminal branches of a sample of E. interstans in ether was partitioned into neutral and acidic fractions. TLC of the methylated sodium bicarbonate soluble portion showed the presence of two main components which could be separated with difficulty by rapid silica gel filtration. The ‘H NMR spectrum of the less polar component showed it to be a mixture of 1 and 2 previously isolated from E. uirgata [3]. The more polar component (C,,H,,OJ was shown to be the new hydroxy ester (3) on the following evidence. The molecular ion at m/z 264 in the mass spectrum of 3 showed it to be isomeric with 1 and 2. Comparison of the NMR spectral data of l-3 indicated that in 3 a vinylic methyl (S, 1.70, dd, J- 1.3, 0.8 Hz; 6, 19.3, 4) had replaced the allylic primary alcohol, and a carbinol methyl (& 1.17, s; 6c72.0, s; 20.6, 4) had replaced the secondary methyl at C-l in 1 and 2. Importantly, the signal for the /&proton of the a$unsaturated ester functionality at 6, 6.78 appeared as a relatively sharp signal W,,, 4 Hz, a feature characteristic for the trans-fused series represented by 1 and its derivatives. The cis-octahydronaphthalene system of 2 and derivatives shows a more complicated muitiplet, W,,, 10 Hz, for the same proton [3]. Further evidence for the structure assigned to 3 came from a comparison (Table 1)
3
4
R
5
R=H
6
R=OH
7
of the 13CNMR spectra of 3 and 4, a compound previously prepared [3] in the structural elucidation of 1. The deshielding effect observed for C-2, C-3 and C-10 (A69,3.7,6.9 ppm respectively) and the shielding effect.for C9 (65.2 ppm) correspond closely with those observed for the corresponding carbons in the model trans-bicyclo[4.4.O]decanes (5, 6) [S]. The chemical shifts of C-l (6, 72.0) is in agreement with that of the carbinol carbon of 6 (6, 72.7) but not with that (6, 70.9) obtained for the epimer of 6 [S]. Furthermore the axial disposition of the
2700
Short Reports Table 1. “C NMR spectral data of 3 and 4* C
3
4
1 2 3 4 5 6 7 8
72.0 41.9 28.9 40.4’ 50.2” 141.0 130.3 25.3
32.4 32.9 25.0 36.8’ 50.ga 144.9 129.4 27.3
C 9 10 11 12 13 14 15 OMe
3
4
21.8 48.5 20.6 112.9 146.5 19.3 169.1 51.6
27.2 41.6 12.5 112.7 147.1 19.4 173.3 -
*CDC13, 75.5 MHz. Multiplicities were determined by SFORD and GASPE techniques and are consistent with assignments. “Values may be interchanged.
C-l methyl group is evident from its chemical shift (Sc 20.6), which is in better agreement with the value of the similar carbon in 6 (a, 21.3) than that (6, 28.4) of the epimer of 6 in which the methyl group is equatorial, and matches the value (S, 19.8) calculated from empirical rules [6]. Thus the structure for this new sesquiterpene is as shown in 3, with the absolute stereochemistry assumed to be the same as that established [3] for 1. A number of points are worth highlighting. Once the structure of 1 was postulated, its separation was facilitated by treating the mixture of 1-3 in dichloromethane with pyridinium dichromate. The less polar aldehydes derived from 1 and 2 could easily be removed by radial plate chromatography (Experimental). Secondly, a chemotaxomic relationship between E. interstans and E. uirgata has been established. These become the first cases among Eremophila species in which
sesquiterpene acids have been shown to be present as metabolites. Comparative GLC studies show that the methylated acidic fraction of E. oirgata contains 52% of 1 and 31% of 2, the remainder consisting of about equal amounts of derivatives of 1 [S]. In E. interstans the relative proportions of 1, 2 and 3 are 21, 46 and 18% respectively. Finally the occurrence of the acid corresponding to 3 is interesting when considered with the fact that (+)-oplopanone (7) is a known metabolite of E. miniata [7]. Both share the same absolute stereochemistry and would appear to be derived from a common bicyclic precursor.
2701 EXPERIMENTAL
General experimental details have been reported previously
PI.
Isolation of metabolites from Eremophila interstans. Leaves and branches (162 g) of a sample of the flowering plant, collected 84 km south of Coolgardie in Western Australia, were soaked in ether (1.5 1).The ether extract was partitioned into neutral (4 g), 8% aq. NaHCO, soluble (11.7 g) and 5% aq. NaOH soluble (5.6 g) fractions. The NaHCO, soluble fraction was treated with ethereal CH,N,. TLC (silica gel; EtOAc-petrol; 7:3) analysis indicated the presence of two major metabolites whereas GLC [HP Ultra-l, methylsilicone gum phase, 50 m x 0.2 mm, T, 100 (0.5 min) at 7.5” min- ’ to T, 250” (5 min)] showed that the extract was composed of three compounds: R, 12.73 min (18%), 14.30 (21) and 14.43 (46). The last two were shown to be identical to 1 and 2. Rapid silica gel chromatography of the mixture led to the isolation of a small amount of 3. More conveniently, the mixture (150mg) was dissolved in CH,Cl, and treated with pyridinium dichromate (334 mg) for 16 hr. The product recovered showed two spots on TLC (silica gel; EtOAc-petro13: 7) at R, 0.2 and 0.35. Separation by radial plate chromatography (EtOAc-petrol 1: 3) afforded fractions (45 mg) of the less polar material, as a mixture of aldehydes derived from 1 and 2, and fractions (28 mg) of the hydroxy ester (3), [a]o -22”, [alsTs - 23”, [a],,,-27”, [a],,,-53”, [a&-39” (CHCl,; c 19), oil, bp (bath) 158%/0.1 mm (Found: C, 72.41; H 9.15. C,,H,,OJ requires: C, 72.68; H, 9.08%). ‘H NMR (300 MHz, CDCI,) 61.17 (3H, s, H,-11), 1.70 (3H, dd, J= 1.3, 0.8 Hz, H,-14), 3.70 (3H, s, CO,CH,), 4.79 (lH, dq, J= 1.7, 0.8 Hz, H-12a), 4.90 (lH, dq, J =1.7, 1.3 Hz, H-12b), 6.78 (IH, s, W,,, 4H2, H-6); “CNMR: Table 1. EIMSm/z(rel.int.). 264[M]+ (13%),246(19),232(100), 187 (58), 174 (35), 161 (26), 147 (49), 145 (41), 131 (44), 107 (65). REFERENCES
1. Grieve, B. J. and Blackall, E. W. (1975) in How to Know Western Australian Wildjowers, Part IV. The University of
Western Australian Press. Forster, P. G., Ghisalberti, E. L. and Jefferies, P. R. (1987) Tetrahedron 43, 2999.
Ghisalberti, E. L., Jefferies, P. R., Skelton, B. W., White, A. H. and Williams, R. S. F. (1989) Tetrahedron 45, 6297. Ghisalberti, E. L., Jefferies, P. R. and Vu, H. T. N. (1990) Phytochemistry 29, 316.
Whitesell, J. K. and Minton, M. A. (1987) in Stereochemical Analysis of Aiicylic Compounds by C-13 Nh4R Spectroscopy,
Chap. 17, p. 211. Chapman & Hall, New York. Crews, P. and Kho-Wiseman, E. (1978) Tetrahedron Letters 2483.
Dastlik, K., Forster, P. G., Ghisalberti, E. L. and Jefferies, P. R. (1989) Phytochemistry 28, 1425. Ghisalberti, E. L. Jefferies, P. R., Mori, T. A., Skelton, B. W. and White, A. H. (1986) Tetrahedron 41, 2517.
Vol. 29, No. 8, pp. 2700 2701, 1990. Printed in Great Britain.
Phytochemistry,
A SESQUITERPENE
ACID FROM EREMOPHILA
ZNTERSTANS
EMILIO L. GHISALBERTI, PHILLIP R. JEFFERIES and HIEU THI NGOC Vu Department of Organic Chemistry, The University
of Western
Australia,
Nedlands,
6009, Western
Australia
(Received2 January 1990) Key Word Index--Eremophila
interstuns;
Abstract-A new bicyclic sesquiterpene related sesquiterpene acids previously species.
Myoporaceae;
sesquiterpene
acid.
acid has been isolated from Eremophila interstans. The occurrence of two other isolated from E. uirgata establishes a chemotaxonomic link between the two
The genus Eremophila (family Myoporaceae) consists mainly of desert adapted shrubs or small trees, with over one hundred species listed as indigenous to Western Australia alone [l]. Eremophiln species are of limited economic significance, with a few species finding some use in horticulture. The majority of the species exhibit bright and colourful flowers during the spring months and about half of the species secrete resin coatings onto the leaves and terminal branches. Typically these resins consist of oxygenated diterpenes and flavones [Z], although recently we have uncovered two examples [3,4] in which sesquiterpenes replaced the diterpene component. We now report on a third case where sesquiterpenes are the dominant resin components. E. interstans (S. Moore) Morrison accumulates three sesquiterpene acids which have been isolated as their methyl esters (l-3). Of these, 1 and 2 have previously been isolated from E. uirgata [3] and 3 is the methyl ester of a new sesquiterpene hydroxy acid. Evidence for the structure of 3 is the subject of this report. The extract (13% dry wt) obtained by soaking the leaves and terminal branches of a sample of E. interstans in ether was partitioned into neutral and acidic fractions. TLC of the methylated sodium bicarbonate soluble portion showed the presence of two main components which could be separated with difficulty by rapid silica gel filtration. The ‘H NMR spectrum of the less polar component showed it to be a mixture of 1 and 2 previously isolated from E. uirgata [3]. The more polar component (C,,H,,OJ was shown to be the new hydroxy ester (3) on the following evidence. The molecular ion at m/z 264 in the mass spectrum of 3 showed it to be isomeric with 1 and 2. Comparison of the NMR spectral data of l-3 indicated that in 3 a vinylic methyl (S, 1.70, dd, J- 1.3, 0.8 Hz; 6, 19.3, 4) had replaced the allylic primary alcohol, and a carbinol methyl (& 1.17, s; 6c72.0, s; 20.6, 4) had replaced the secondary methyl at C-l in 1 and 2. Importantly, the signal for the /&proton of the a$unsaturated ester functionality at 6, 6.78 appeared as a relatively sharp signal W,,, 4 Hz, a feature characteristic for the trans-fused series represented by 1 and its derivatives. The cis-octahydronaphthalene system of 2 and derivatives shows a more complicated muitiplet, W,,, 10 Hz, for the same proton [3]. Further evidence for the structure assigned to 3 came from a comparison (Table 1)
3
4
R
5
R=H
6
R=OH
7
of the 13CNMR spectra of 3 and 4, a compound previously prepared [3] in the structural elucidation of 1. The deshielding effect observed for C-2, C-3 and C-10 (A69,3.7,6.9 ppm respectively) and the shielding effect.for C9 (65.2 ppm) correspond closely with those observed for the corresponding carbons in the model trans-bicyclo[4.4.O]decanes (5, 6) [S]. The chemical shifts of C-l (6, 72.0) is in agreement with that of the carbinol carbon of 6 (6, 72.7) but not with that (6, 70.9) obtained for the epimer of 6 [S]. Furthermore the axial disposition of the
2700
Short Reports Table 1. “C NMR spectral data of 3 and 4* C
3
4
1 2 3 4 5 6 7 8
72.0 41.9 28.9 40.4’ 50.2” 141.0 130.3 25.3
32.4 32.9 25.0 36.8’ 50.ga 144.9 129.4 27.3
C 9 10 11 12 13 14 15 OMe
3
4
21.8 48.5 20.6 112.9 146.5 19.3 169.1 51.6
27.2 41.6 12.5 112.7 147.1 19.4 173.3 -
*CDC13, 75.5 MHz. Multiplicities were determined by SFORD and GASPE techniques and are consistent with assignments. “Values may be interchanged.
C-l methyl group is evident from its chemical shift (Sc 20.6), which is in better agreement with the value of the similar carbon in 6 (a, 21.3) than that (6, 28.4) of the epimer of 6 in which the methyl group is equatorial, and matches the value (S, 19.8) calculated from empirical rules [6]. Thus the structure for this new sesquiterpene is as shown in 3, with the absolute stereochemistry assumed to be the same as that established [3] for 1. A number of points are worth highlighting. Once the structure of 1 was postulated, its separation was facilitated by treating the mixture of 1-3 in dichloromethane with pyridinium dichromate. The less polar aldehydes derived from 1 and 2 could easily be removed by radial plate chromatography (Experimental). Secondly, a chemotaxomic relationship between E. interstans and E. uirgata has been established. These become the first cases among Eremophila species in which
sesquiterpene acids have been shown to be present as metabolites. Comparative GLC studies show that the methylated acidic fraction of E. oirgata contains 52% of 1 and 31% of 2, the remainder consisting of about equal amounts of derivatives of 1 [S]. In E. interstans the relative proportions of 1, 2 and 3 are 21, 46 and 18% respectively. Finally the occurrence of the acid corresponding to 3 is interesting when considered with the fact that (+)-oplopanone (7) is a known metabolite of E. miniata [7]. Both share the same absolute stereochemistry and would appear to be derived from a common bicyclic precursor.
2701 EXPERIMENTAL
General experimental details have been reported previously
PI.
Isolation of metabolites from Eremophila interstans. Leaves and branches (162 g) of a sample of the flowering plant, collected 84 km south of Coolgardie in Western Australia, were soaked in ether (1.5 1).The ether extract was partitioned into neutral (4 g), 8% aq. NaHCO, soluble (11.7 g) and 5% aq. NaOH soluble (5.6 g) fractions. The NaHCO, soluble fraction was treated with ethereal CH,N,. TLC (silica gel; EtOAc-petrol; 7:3) analysis indicated the presence of two major metabolites whereas GLC [HP Ultra-l, methylsilicone gum phase, 50 m x 0.2 mm, T, 100 (0.5 min) at 7.5” min- ’ to T, 250” (5 min)] showed that the extract was composed of three compounds: R, 12.73 min (18%), 14.30 (21) and 14.43 (46). The last two were shown to be identical to 1 and 2. Rapid silica gel chromatography of the mixture led to the isolation of a small amount of 3. More conveniently, the mixture (150mg) was dissolved in CH,Cl, and treated with pyridinium dichromate (334 mg) for 16 hr. The product recovered showed two spots on TLC (silica gel; EtOAc-petro13: 7) at R, 0.2 and 0.35. Separation by radial plate chromatography (EtOAc-petrol 1: 3) afforded fractions (45 mg) of the less polar material, as a mixture of aldehydes derived from 1 and 2, and fractions (28 mg) of the hydroxy ester (3), [a]o -22”, [alsTs - 23”, [a],,,-27”, [a],,,-53”, [a&-39” (CHCl,; c 19), oil, bp (bath) 158%/0.1 mm (Found: C, 72.41; H 9.15. C,,H,,OJ requires: C, 72.68; H, 9.08%). ‘H NMR (300 MHz, CDCI,) 61.17 (3H, s, H,-11), 1.70 (3H, dd, J= 1.3, 0.8 Hz, H,-14), 3.70 (3H, s, CO,CH,), 4.79 (lH, dq, J= 1.7, 0.8 Hz, H-12a), 4.90 (lH, dq, J =1.7, 1.3 Hz, H-12b), 6.78 (IH, s, W,,, 4H2, H-6); “CNMR: Table 1. EIMSm/z(rel.int.). 264[M]+ (13%),246(19),232(100), 187 (58), 174 (35), 161 (26), 147 (49), 145 (41), 131 (44), 107 (65). REFERENCES
1. Grieve, B. J. and Blackall, E. W. (1975) in How to Know Western Australian Wildjowers, Part IV. The University of
Western Australian Press. Forster, P. G., Ghisalberti, E. L. and Jefferies, P. R. (1987) Tetrahedron 43, 2999.
Ghisalberti, E. L., Jefferies, P. R., Skelton, B. W., White, A. H. and Williams, R. S. F. (1989) Tetrahedron 45, 6297. Ghisalberti, E. L., Jefferies, P. R. and Vu, H. T. N. (1990) Phytochemistry 29, 316.
Whitesell, J. K. and Minton, M. A. (1987) in Stereochemical Analysis of Aiicylic Compounds by C-13 Nh4R Spectroscopy,
Chap. 17, p. 211. Chapman & Hall, New York. Crews, P. and Kho-Wiseman, E. (1978) Tetrahedron Letters 2483.
Dastlik, K., Forster, P. G., Ghisalberti, E. L. and Jefferies, P. R. (1989) Phytochemistry 28, 1425. Ghisalberti, E. L. Jefferies, P. R., Mori, T. A., Skelton, B. W. and White, A. H. (1986) Tetrahedron 41, 2517.