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Phenanthrene derivatives from the orchid Coelogyne cristata
Phytochemistry 58 (2001) 581–586 www.elsevier.com/locate/phytochem
Phenanthrene derivatives from the orchid Coelogyne cristata P.L. Majumder*, S. Sen, S. Majumder Department of Chemistry, University College of Science, 92, Acharya Prafulla Chandra Road, Calcutta 700 009, India Received 14 February 2001; received in revised form 15 May 2001
Abstract Coeloginanthridin, a 9,10-dihydrophenanthrene derivative, and coeloginanthrin, the corresponding phenanthrene analogue, were isolated from the orchid Coelogyne cristata which earlier afforded coelogin (1a) and coeloginin (1b). The structures of coeloginanthridin and coeloginanthrin were established as 3,5,7-trihydroxy-1,2-dimethoxy-9,10-dihydrophenanthrene (2a) and 3,5,7-trihydroxy-1,2-dimethoxyphenanthrene (2c), respectively, from spectral and chemical evidence including the conversion of coeloginanthridin triacetate (2b) to coeloginanthrin triacetate (2d) by dehydrogenation with DDQ. In the light of earlier reports on structurally similar compounds, 2a and 2c may have biological activities of phytoalexins and endogenous plant growth regulators. # 2001 Published by Elsevier Science Ltd. Keywords: Coelogyne cristata; Orchidaceae; Coeloginanthridin; Coeloginanthrin; Phenanthrene derivatives
1. Introduction We reported earlier the isolation of a fairly large number of compounds from a series of Indian orchids. These compounds encompass a wide variety of stilbenoids, viz. stilbene (Majumder et al., 1998b), bibenzyls (Majumder et al., 1999a), phenanthrenes (Majumder and Sen, 1991) and 9,10-dihydrophenanthrenes (Majumder et al., 1996) and their dimers (Majumder and Banerjee, 1988; Majumder et al., 1998a, 1999b), phenanthropyrans and pyrones (Majumder and Sabzabadi, 1988; Majumder and Maiti, 1991) and their 9,10dihydro derivatives (Majumder et al., 1999c), fluorenonone (Majumder and Chakraborty, 1989) and a few other polyphenolics (Majumder et al., 1994, 1995), several triterpenoids (Majumder and Ghosal, 1991), steroids of biogenetic importance (Majumder and Pal, 1990) and some simple aromatic compounds (Majumder and Lahiri, 1989). As part of this general programme of research we chemically investigated the orchid Coelogyne cristata which earlier afforded the first pair of naturally occurring 9,10-dihydrophenanthropyran and pyrone derivatives 1a and 1b, respectively (Majumder et al., 1982). Further investigation of this orchid has now resulted in the isolation of two new * Corresponding author. Tel.: +91-33-351-9754. E-mail address: [email protected] (P.L. Majumder).
phenanthrene derivatives, designated coeloginanthridin and coeloginanthrin. The structures of coeloginanthridin and coeloginanthrin were established as 2a and 2c, respectively, from 1H and 13C NMR spectral and chemical evidence.
2. Results and discussion Coeloginanthridin (2a), C16H16O5 ([M+] at m/z 288) showed UV absorptions [lEtOH max 220.5, 275.5 and 298.5 nm (log " 4.37, 4.02 and 3.90)] which are strikingly similar to those of 9,10-dihydrophenanthrene derivatives (Majumder et al., 1982). The UV spectrum of coeloginanthrin (2c), C16H14O5 ([M+] at m/z 286) [lEtOH max 223.5, 260.5, 347 and 364 nm (log " 4.53, 4.57, 3.63 and 3.64)], on the other hand, exhibited close resemblance to those of fully aromatic phenanthrene derivatives (Majumder and Kar, 1987). The phenolic nature of both 2a and 2c was indicated by their characteristic colour reactions with FeCl3 (violet) and phosphomolybdic acid reagent (deep blue), alkali-induced bathochromic shifts of their UV maxima and by their IR absorption bands [2a: max 3240–3480 cm 1; 2c: max 3220–3490 cm 1]. The presence of three phenolic hydroxyl groups in both 2a and 2c was confirmed by the formation of their respective triacetyl derivatives [coeloginanthridin triacetate (2b), C22H22O8
0031-9422/01/$ - see front matter # 2001 Published by Elsevier Science Ltd. PII: S0031-9422(01)00287-4
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(M+) at m/z 414; coeloginanthrin triacetate (2d), C22H20O8 ([M+] at m/z 412)] with Ac2O and pyridine. The presence of three phenolic hydroxyl groups in each of 2a and 2c was also corroborated by the 1H NMR spectra of the compounds [2a: 5.76 (3H, br. s, disappeared on deuterium exchange); 2c: 4.99, 5.65 and 5.70 (each 1H, br.s; also disappeared on dueterium exchange)]. The presence of two aromatic methoxyl groups in each compound was evident from the three-proton singlets at 3.83 and 3.95 (2a) and 4.00 and 4.10 (2c). The 1H NMR spectrum of 2a showed two two-proton multiplets at 2.57 and 2.62, which are typical of the protons of the two benzylic methylene groups of a 9,10dihydrophenanthrene derivative (Majumder et al., 1982). The signals at 2.57 and 2.62 of 2a were replaced
by a pair of one-proton doublets at 7.35 and 7.85 (each having J=8.7 Hz), which are characteristic of H-9 and H-10 of a phenanthrene derivative (Majumder and Kar, 1987). This would suggest that 2a is a 9,10-dihydrophenanthrene derivative and 2c is the corresponding phenanthrene analogue. Furthermore, the 1H NMR spectrum of 2a exhibited a one-proton singlet at 7.52 corresponding to H-4 or H-5 of 9,10-dihydrohenanthrene derivative (Majumder and Lahiri, 1990), while the spectrum of 2c was characterized by a one-proton singlet at 8.85 similar to that of the corresponding proton of a phenanthrene (Majumder and Lahiri, 1990). The relatively upfield shifts of these proton signals in the 1H NMR spectra of both 2a and 2c by about 0.5 ppm (H-4 and H-5 of 9,10-dihydrophenanthrene normally appear at ca 8.0 and the corresponding protons
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of phenanthrene usually resonate at ca 9.5) were attributed to the presence of electron-donating groups (OH or OMe) at ortho and para to these protons in both 2a and 2c. Now, if these downfield aromatic proton signals of 2a and 2c are assigned to H-4, then C-5 of both the compounds must be substituted by a hydroxyl or methoxyl group. Again, since the above downfield proton signals of both 2a and 2c appeared as sharp singlets, C-3 and C-2 of both the compounds must also be substituted by OH or OMe. The remaining two aromatic protons of 2a and 2c appeared as a pair of doublets corresponding to the meta-coupled aromatic protons [2a: 6.29 and 6.33 (each 1H, d, J=2.0 Hz); 2c: 6.55 and 6.75 (each illresolved meta-coupled doublets)]. The signals at 6.29 and 6.33 in the spectrum of 2a resemble those of H-6 and H-8 of calaphenanthrenol (2e) (Yoshikawa et al., 1998) having two hydroxyl groups at C-5 and C-7. The corresponding signals at 6.55 and 6.75 of 2c may be similarly assigned to H-6 and H-8 of the compound possessing a phenanthrene skeleton having two hydroxyl functions at C-5 and C-7. This would imply that while two hydroxyl groups in both 2a and 2c may be located at C-5 and C-7, the remaining hydroxyl function and the two methoxyl groups in each compound must be distributed between C-3, C-2 and C-1. The relative positions of the hydroxyl and methoxyl groups in both 2a and 2c were ascertained by the observed changes in the chemical shifts of H-4, H-6 and H-8 of the compounds in the 1H NMR spectra of their respective triacetyl derivatives 2b and 2d. It has been shown that H-5 of a 4-hydroxyphenanthrene and its 9,10-dihydro derivative is shifted upfield by ca 0.5 and 0.2 ppm, respectively on acetylation (Letcher and Nhamo, 1971; Letcher and Wong, 1978; Majumder and Lahiri, 1990). A similar upfield shift of H-4 was observed in 2b and 2d by 0.28 and 0.09 ppm, respectively, affirming the placement of one of the three hydroxyl groups at C-5 in both 2a and 2c. The relatively smaller upfield shifts of H-4 of both 2a and 2c than those usually observed on acetylation of their hydroxyl function at C-5 may be attributed to the presence of a hydroxyl group at C-3 in both the compounds, which on acetylation produces a deshielding effect on H-4. The remaining hydroxyl group in both 2a and 2c was placed at C-7 from the observed downfield shifts of H-6 (ortho to two hydroxyl groups at C-5 and C-7) and H-8 (ortho to one hydroxyl group at C-7 and para to another hydroxyl function at C-5) by 0.52 and 0.61 ppm, and 0.65 and 0.82 ppm, respectively, in the 1H NMR spectra of their triacetyl derivatives 2b and 2d. The greater deshielding of H-6 and H-8 of 2c in the spectrum of its triacetyl derivative 2d may be partly attributed to the acetylation of its hydroxyl group at C-3, causing a deshielding of its H-9 by 0.23 ppm in the spectrum of 2d. The two methoxyl groups in both 2a and 2c must then be placed at C-1
and C-2. Coeloginanthridin and coeloginanthrin may, therefore, be represented as 3,5,7-trihydroxy-1,2-dimethoxy-9,10-dihydrophenanthrene (2a) and 3,5,7-trihydroxy-1,2-dimethoxyphenanthrene (2c), respectively. The structure of coeloginanthridin (2a) was also affirmed by the 13C NMR spectral data of the compound and its triacetyl derivative 2b (Table 1). The degree of protonation of the carbon atoms in 2a and 2b was determined by DEPT experiments and the carbon chemical shifts were assigned by comparison with the C values of the structurally similar compound calaphenanthrenol (2e) (Yoshikawa et al., 1998). Thus, except that of C-8a, the C values of C-4b, C-5, C-6, C-7 and C-8 of 2a are essentially similar to those of the corresponding carbon atoms of 2e supporting the placement of two hydroxyl groups at C-5 and C-7 in 2a. The 13C NMR chemical shift assignments of 2a were confirmed by HMQC and HMBC experiments and on the basis of these experiments C-8a of 2a was assigned the signal at C 142.0. It appears from the following 2D NMR spectral analysis of coeloginanthridin (2a) that the signal at C 131.4 earlier assigned to C-8a of calaphenanthrenol (2e) (Yoshikawa et al., 1998) should be interchanged with that at C 142.0 assigned to its C-3. The HMQC spectrum of 2a (with JCH parameter set to 160 Hz) showed correlations of the hydrogens at H Table 1 13 C NMR spectral data of compounds 2a, 2b and 2e Chemical shift (, ppm)a
C
1 2 3 4 4a 4b 5 6 7 8 8a 9 10 10a OMe OAc
a
2a
2b
2eb
149.4 137.8 147.1 108.6 128.7 114.4 153.7 102.1 155.0 107.7 142.0 30.1 21.3 123.6 60.7 (OMe at C-1) 60.9 (OMe at C-2) – – –
149.3 144.1 142.5 117.2 128.4 126.4 147.8 115.2 151.3 118.6 141.7 29.6 21.4 130.8 60.7 (2xOMe)
108.0 145.7 142.0 142.1 119.2 113.0 154.7 104.0 156.0 108.0 131.4 31.6 30.5 137.2 61.8 (OMe at C-4) 56.4 (OMe at C-2) –
168.6 168.7 168.8 20.6 20.9 21.0
–
Spectra were run in CDCl3 and the chemical shifts were measured with (TMS)=(CDCl3)+76.9 ppm. b The carbon resonances of 2e and their assignments shown here are those reported by (Yoshikawa et al., 1998).
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2.57 (H2-9), 2.62 (H2-10), 7.52 (H-4), 6.29 (H-6), 6.33 (H-8), 3.83 (OMe) and 3.95 (OMe) with the carbon atoms at C 30.1 (C-9), 21.3 (C-10), 108.6 (C-4), 102.1 (C-6), 107.7 (C-8), 60.7 and 60.9, respectively. In the HMBC spectrum of 2a (with JCH parameter set to 7 Hz), the signal at H 2.57 (H2-9) exhibited fairly strong 3-bond correlations with the carbon signals at C 114.4, 107.7 and 123.6, which were thus assigned to C-4b, C-8 and C-10a, respectively. The signal at H 2.57 (H2-9) also showed a 2-bond correlation with the carbon at C 142.0 assigned to C-8a. In aromatic compounds 3-bond 1 H-13C correlations were found to be a common feature, while 2-bond and other long range (4-bond or more remote) correlations are usually not observed. However, in polyoxygenated phenanthrene derivatives such 2-bond and other long range correlations have sometimes been observed and were assumed to occur due to the presence of oxygenated functions like OH and OMe attached to the aromatic ring, which alter the electron distribution in favour of such couplings (Majumder et al., 1997). In light of the observation that presence of a substituent like methoxyl group at C-1 in 9,10-dihydrophenenthrene derivatives causes an upfield shift of C10 by ca 7 ppm as in coelogin (1a), coeloginin diacetate and coeloginin dimethyl ether (Majumder et al., 1982), the upfield benzylic methylene carbon at C 21.3 of 2a having a methoxyl group at C-1 must be assigned to C10 which showed 1-bond correlation to the protons at H 2.62. The signal at H 2.62 of 2a must then be assigned to its H2-10 which showed 3-bond correlations with the carbons resonating at C 142.0, 149.4 and 128.7, which may, therefore, be assigned to C-8a, C-1 and C-4a, respectively, of 2a. The signal at H 2.62 (H210) also exhibited a 2-bond correlation with the carbon at C 123.6 which has already been assigned to C-10a. Since both 2a and 2e have identical substituents in their ring C (OH at C-5 and C-7), the confirmation of the assignment of the carbon resonance at C 142.0 of 2a to its C-8a implies that the signal at C 142.0 of 2e (earlier attributed to its C-3) may, by analogy, be also assigned to its C-8a. The signal at C 131.4 of 2e (earlier assigned to its C-8a) would correspond to its C-3. The proton at H 7.52 corresponding to H-4 showed 3-bond correlations with the carbons at C 114.4 (C-4b), 123.6 (C-10a) and 137.8 which must correspond to C-2. H-4 (H 7.52) also exhibited a 2-bond correlation with the carbon at C 147.1 (C-3) and a weak 4-bond correlation with the carbon at C 149.4 (C-1). The proton at H 6.29 assigned to H-6 was found to exhibit 3-bond correlations with the carbons at C 107.7 and 114.4 which were already assigned to C-8 and C-4b, respectively. H-6 (H 6.29) also showed 2-bond correlations with the nonprotonated carbons at C 153.7 and 155.0, which corresponded to C-5 and C-7, respectively. Similarly, the proton at H 6.33 assigned to H-8 showed 3-bond cor-
relations with the carbons at C 102.1 (C-6), 114.4 (C4b) and 30.1 (C-9) and a weak 2-bond correlation with the carbon at C 155.0 (C-7). The methoxyl protons at H 3.83 and 3.95, which were associated with methoxyl carbons at C 60.7 and 60.9, respectively, as evident from HMQC spectrum of 2a, showed 3-bond correlations with the carbons at C 149.4 (C-1) and 137.8 (C2), respectively. All the above 1H–13C correlations observed in the HMBC spectra of 2a are depicted in its structural diagram. The above spectral data thus unequivocally established the structure of coeloginanthridin (2a). Although similar spectral analysis could not be made with coeloginanthrin (2c) due to paucity of material, the structure of the compound was confirmed by the conversion of coeloginanthridin triacetate (2b) to coeloginanthrin triacetate (2d) by dehydrogenation of the former to the latter with 2,3-dichloro-5,6-dicyano-1,4benzoquinone (DDQ) in dry benzene. Acid-hydrolysis of the dehydrogenation product finally afforded coeloginanthrin (2c). Coeloginanthridin (2a) and coeloginanthrin (2c) are thus two new additions to the growing lists of the naturally occurring 9,10-dihydrophenanthrenes and phenanthrenes respectively. In the light of the earlier reports that a variety of phenanthrene derivatives were found to be potent phytoalexins (Gaumann and Kern, 1959a,b; Gorham, 1980; Purkayastha, 1985, 1995), while several others were shown to act as endogenous plant growth regulators (Gorham, 1980), it would be interesting to study 2a and 2c for similar biological activities.
3. Experimental Melting points: uncorr. CC: Silica gel (100–200 mesh). MPLC: silica gel (230–400 mesh). TLC: silica gel G. UV: 95% aldehyde-free EtOH. IR: KBr discs. 1H and 13 C NMR: 300 and 75 MHz, respectively (for 2b, 2c and 2d); 500 MHz (for 1H, HMBC and HMQC of 2a), 125 MHz (for 13C NMR of 2a). NMR spectra were recorded in CDCl3 using TMS as internal standard and chemical shifts are expressed in (ppm). MS: direct inlet system, 70 eV. All analytical samples were routinely dried over P2O5 for 24 h in vacuo and were tested for purity by TLC and MS. The petrol used had bp 60– 80 C. 3.1. Plant materials Coelogyne cristata was collected from Kalimpong (Darjeeling), India in October, 1998. A voucher specimen (Majumder s.n.) was deposited in the Herbarium of the Department of Botany, University of Calcutta (CUH).
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3.2. Isolation of coeloginanthridin (2a) and coeloginanthrin (2c) Air-dried finely powdered whole plant of Coelogyne cristata (5 kg) was kept soaked in MeOH (10 l) for 3 weeks. The MeOH extract was concentrated to ca. 100 ml, diluted with H2O (750 ml) and exhaustively extracted with Et2O. The Et2O extract was fractionated into acidic and nonacidic fractions with 2 M NaOH. The aqueous alkaline solution was acidified in the cold with conc. HCl and the liberated solids were extracted with Et2O, washed with H2O, dried and the solvent removed. The residue was subjected to CC. The petrol–EtOAc (7:1) eluate gave a mixture of 1a and 1b. The above mixture of 1a and 1b was then subjected to CC and the early fractions of the petrol–EtOAc (7:1) eluate also gave a mixture of 1a and 1b containing predominantly 1b. The later fractions of the same eluate also afforded a mixture of 1a and 1b containing mostly 1a. The fractions containing mostly 1a and those containing mostly 1b were separately mixed together and evaporated. The two residues enriched with respect to 1a and 1b were then separately subjected to MPLC using petrol–EtOAc (5:1) as the solvent system. The early fractions of the residue containing mostly 1b gave pure 1b, while the later fractions of the same eluate gave pure 1a. Similarly, MPLC of the residue containing mostly 1a gave in the early fractions pure 1b, while the later fractions afforded pure 1a. The fractions containing pure 1b and 1a were separately mixed together and the solvent removed to give pure 1b (0.3 g), crystallized from petrol–EtOAc in yellowish needles, mp 198 C and pure 1a (0.25 g), also crystallized from the same solvent mixture in fine colourless needles, mp 151 C. Washing the main column with petrol–EtOAc (3:1) gave a gummy residue comprising mainly coeloginanthridin (2a) and coeloginanthrin (2c). The above mixture was subjected to MPLC using petrol–EtOAc (2:1) as the solvent system. The early fractions gave a residue which contained coeloginanthridin (2a) as the major constituent, while the later fractions gave a mixture containing mostly coeloginanthrin (2c). The above two mixtures were again separately subjected to MPLC using the same solvent system to give finally pure coeloginanthridin (2a) (0.10 g) and coeloginanthrin (2c) (0.02 g), both as semisolid mass. Coeloginanthridin (2a) (found: C, 66.61; H, 5.48; C16H16O5 requires: C, 66.67; H, 5.60%). UV 0:1 M NaOH lEtOH nm: 212.5, 286.0 and 311.5 (log " 4.51, max 3.98 and 4.00); IR max cm 1: 3240–3480 (OH), 1610, 1550, 1480, 880 and 840 (aromatic nucleus); MS m/z (rel. int.): 288 [M+] (100), 273 (25), 258 (15) and 230 (10); 1H NMR: 7.52 (1H, s; H-4), 6.33 and 6.29 (each 1H, d, J=2.0 Hz, H-8 and H-6), 5.76 (3H, br. s, 3xArOH), 3.95 and 3.83 (each 3H, s, 2xArOMe), 2.62 and 2.57 (each 2H, m, H2-10 and H2-9). Acetylation of
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coeloginanthridin (2a) with Ac2O and pyridine in the usual manner gave coeloginanthridin triacetate (2b), also as a semisolid mass. (found: C, 63.71; H, 5.25; C22H22O8 requires: C, 63.77; H, 5.35%). IR: max cm 1: 1215, 1220, 1225, 1745, 1755 and 1770 (OAc), 1615, 1600, 1580, 1475, 820, 775, 735, 715, 690 and 595 (aromatic nucleus); 1H NMR: 7.43 (1H, s, H-4), 6.94 and 6.81 (each 1H, d, J=2.34 Hz, H-8 and H-6), 3.89 and 3.84 (each 3H, s, 2xArOMe), 2.80 and 2.77 (each 2H, m, H2-10 and H2-9), 2.35, 2.29 and 2.27 (each 3H, s, 3xOAc); MS m/z (rel. int.): 414 [M+] (25), 372 (30), 330 (35), 288 (100), 273 (20), 258 (10) and 230 (10). Coeloginanthrin (2c): (found: C, 67.02, H, 4.83. C16H14O5 requires: C, 67.13; H, 4.93%). UV 0:1 M NaOH lEtOH nm: 212, 228.5, 277, 307 and 377 (log " max 4.47, 4.45, 4.44, 4.15 and 3.69); IR max cm 1: 3220– 3490 (OH), 1625, 1615, 1585, 1480, 885, 840 and 650 (aromatic nucleus); MS m/z (rel. int.) 286 [M+] (100), 271 (30), 256 (20) and 228 (10); 1H NMR: 8.85 (1H, s, H-4), 7.85 and 7.35 (each 1H, d, J=8.7 Hz, H-10 and H-9), 6.75 and 6.55 (each 1H, ill-resolved meta-coupled d, H-8 and H-6), 5.70, 5.65 and 4.99 (each 1H, br.s, 3xArOH), 4.10 and 4.00 (each 3H, s, 2xArOMe). Acetylation of coeloginanthrin (2c) with Ac2O and pyridine in the usual manner gave coeloginanthrin triacetate (2d), also as a semisolid mass. (found: C, 63.97; H, 4.78. C22H20O8 requires: C, 64.07; H, 4.89%). IR nmax cm 1: 1205, 1220, 1230, 1775, 1780 and 1785 (OAc), 1615, 1585, 1555, 1485, 845, 820, 810, 765 and 740 (aromatic nucleus); 1H NMR: 8.57 (1H, s, H-4), 8.08 and 7.67 (each 1H, d, J=9.2 Hz, H-10 and H-9), 7.57 and 7.20 (each 1H, ill-resolved meta-coupled d, H-8 and H-6), 4.07 and 4.03 (each 3H, s, 2xArOMe), 2.49, 2.42 and 2.36 (each 3H, s, 3xOAc); MS m/z (rel. int.) 412 [M+] (35), 370 (30), 328 (20), 286 (100), 271 (25), 256 (10) and 228 (8). 3.3. Conversion of coeloginanthridin triacetate (2b) to coeloginanthrin triacetate (2d) by dehydrogenation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and acid-hydrolysis of coeloginanthrin triacetate (2d) to coeloginanthrin (2c) To a soln. of coeloginanthridin triacetate (2b) (0.03 g) in dry C6H6 (25 ml) was added 0.07 g of 2,3-dichloro5,6-dicyano-1,4-benzoquinone. The resultant mixture was heated under reflux for 16 h. The solvent was removed under red. pres. to give a solid which was extracted with Et2O, washed three times with 2 M aq. NaOH and then with H2O, dried and the solvent removed. The residue was chromatographed. The petrol-EtOAc (10:1) eluate gave coeloginanthrin triacetate (2d) (0.018 g) as a semi-solid mass. To a soln. of 0.01 g of coeloginanthrin triacetate (2d) was added 5 ml of 2 M aq. methanolic HCl and the mixture was heated under reflux for 5 h. MeOH was removed under red. pres. and
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the residue was extracted with Et2O, washed with H2O, dried and the solvent removed. The residue was chromatographed. The petrol–EtOAc (3:1) eluate gave coeloginanthrin (2c) (0.008 g) as a semi-solid mass.
Acknowledgements The work was supported by C.S.I.R., New Delhi, India.
References Gaumann, E., Kern, H., 1959a. U¨ber die Isolierung and den Chemischen Nachweis des Orchinobs. Phytopathologische Zeitschrift 35, 347–356. Gaumann, E., Kern, H., 1959b. U¨ber chemische Abwehrreaktionen bei Orchideen. Phytopathologische Zeitschrift 36, 1–26. Gorham, J., 1980. The Stilbenoids. Progress in Phytochemistry 6, 203– 252. Letcher, R.M., Nhamo, L.R.M., 1971. Chemical constituents of the combretaceae Part 1. Substituted phenanthrenes and 9,10-dihydrophenanthrenes from the heartwood of Combretum apiculatum. Journal of the Chemical Society (C) 3070–3076. Letcher, R.M., Wong, K.M., 1978. Structure and synthesis of the phenanthrenes TaI and TaV from Tamus communis. Journal of the Chemical Society Perkin Trans. I 739–742. Majumder, P.L., Bandyopadhyay, D., Joardar, S., 1982. Coelogin and coeloginin: two novel 9,10-dihydrophenanthrene derivatives from the orchid Coelogyne cristata. Journal of the Chemical Society Perkin Trans I 1131–1136. Majumder, P.L., Banerjee, S., 1988. Structure of flavanthrin, the first dimeric 9,10-dihydrophenanthrene derivative from the orchid Eria flava. Tetrahedron 44, 7303–7308. Majumder, P.L., Banerjee, S., Lahiri, S., Mukhoti, N., Sen, S., 1998a. Dimeric phenanthrenes from two Agrostophyllum species. Phytochemistry 47, 855–860. Majumder, P.L., Banerjee, S., Sen, S., 1996. Three stilbenoids from the orchid Agrostophyllum callosum. Phytochemistry 42, 847–852. Majumder, P.L., Chakraborty, J., 1989. Chemical constituents of the orchid Dendrobium farmerii: Further evidence for the revised structure of dengibsin. Journal of the Indian Chemical Society 66, 834– 837. Majumder, P.L., Ghosal, S., 1991. Arundinol, a new triterpene from the orchid Arundina bambusifolia. Journal of the Indian Chemical Society 68, 88–91. Majumder, P.L., Guha, S., Sen, S., 1999a. Bibenzyl derivatives from the orchid Dendrobium amoenum. Phytochemistry 52, 1365–1369.
Majumder, P.L., Kar, A., 1987. Confusarin and confusaridin, two phenanthrene derivatives of the orchid Eria confusa. Phytochemistry 26, 1127–1129. Majumder, P.L., Kar, A., Banerjee, S., Pal, A., Joardar, M., 1997. Application of 2D NMR spectroscopy in structural elucidation of some phenanthrene derivatives from Indian Orchidaceae plants. Journal of the Indian Chemical Society 74, 908–916. Majumder, P.L., Lahiri, S., 1989. Chemical constituents of the orchid Lusia indivisa. Indian Journal of Chemistry 28B, 771–774. Majumder, P.L., Lahiri, S., 1990. Lusianthrin and lusianthridin, two stilbenoids from the orchid Lusia indivisa. Phytochemistry 29, 621– 624. Majumder, P.L., Lahiri, S., Mukhoti, N., 1995. Chalcone and dihydrochalcone derivatives from the orchid Lusia volucris. Phytochemistry 40, 271–274. Majumder, P.L., Lahiri, S., Pal, S., 1994. Occurrence of lignans in the Orchidaceae plants Lusia volucris and Bulbophyllum triste. Journal of the Indian Chemical Society 71, 645–647. Majumder, P.L., Maiti, D.C., 1991. Isoflaccidinin and isooxoflaccidinin, stilbenoids from Coelogyne flaccida. Phytochemistry 30, 971–974. Majumder, P.L., Pal, S., 1990. A steroidal ester from Coelogyne uniflora. Phytochemistry 29, 2717–2720. Majumder, P.L., Pal, S., Majumder, S., 1999b. Dimeric phenanthrenes from the orchid Bulbophyllum reptans. Phytochemistry 50, 891– 897. Majumder, P.L., Roychowdhury, M., Chakraborty, S., 1998b. Thunalbene, a stilbene derivative from the orchid Thunia alba. Phytochemistry 49, 2375–2378. Majumder, P.L., Sabzabadi, E., 1988. Agrostophyllin, a naturally occurring phenanthropyran derivative from Agrostophyllum khasiyanum. Phytochemistry 27, 1899–1901. Majumder, P.L., Sen, R.C., 1991. Pendulin, A polyoxygenated phenanthrene derivative from the orchid Cymbidium pendulum. Phytochemistry 30, 2432–2434. Majumder, P.L., Sen, S., Banerjee, S., 1999c. Agrostophyllol and isoagrostophyllol, two novel diastereomeric 9,10-dihydrophenanthropyran derivatives from the orchid Agrostophyllum callosum. Tetrahedron 55, 6691–6702. Purkayastha, R.P., 1985. Phytoalexins. In: Mukerji, K.G., Pathak, N.C., Singh, V.P. (Eds.), Frontiers in Applied Microbiology. Vol. 1. Print House (India), Lucknow, pp. 363–407. Purkayastha, R.P., 1995. Progress in Phytoalexin Research during the Past 50 Years. In: Daniel, M., Purkayastha, R.P. (Eds.), Handbook of Phytoalexin Metabolism and Action. Marcel Dekker, New York, pp. 1–39. Yoshikawa, M., Murakami, T., Kishi, A., Sakurama, T., Matsuda, H., Nomura, M., Matsuda, H., Kubo, M., 1998. Novel indole S,Obisdesmoside, calanthoside, the precursor glycoside of tryptanthrin, indirubin, and isatin, with increasing skin blood flow promoting effects, from two Calanthe species (Orchidaceae). Chemical and Pharmaceutical Bulletin 46, 886–888.
Phenanthrene derivatives from the orchid Coelogyne cristata P.L. Majumder*, S. Sen, S. Majumder Department of Chemistry, University College of Science, 92, Acharya Prafulla Chandra Road, Calcutta 700 009, India Received 14 February 2001; received in revised form 15 May 2001
Abstract Coeloginanthridin, a 9,10-dihydrophenanthrene derivative, and coeloginanthrin, the corresponding phenanthrene analogue, were isolated from the orchid Coelogyne cristata which earlier afforded coelogin (1a) and coeloginin (1b). The structures of coeloginanthridin and coeloginanthrin were established as 3,5,7-trihydroxy-1,2-dimethoxy-9,10-dihydrophenanthrene (2a) and 3,5,7-trihydroxy-1,2-dimethoxyphenanthrene (2c), respectively, from spectral and chemical evidence including the conversion of coeloginanthridin triacetate (2b) to coeloginanthrin triacetate (2d) by dehydrogenation with DDQ. In the light of earlier reports on structurally similar compounds, 2a and 2c may have biological activities of phytoalexins and endogenous plant growth regulators. # 2001 Published by Elsevier Science Ltd. Keywords: Coelogyne cristata; Orchidaceae; Coeloginanthridin; Coeloginanthrin; Phenanthrene derivatives
1. Introduction We reported earlier the isolation of a fairly large number of compounds from a series of Indian orchids. These compounds encompass a wide variety of stilbenoids, viz. stilbene (Majumder et al., 1998b), bibenzyls (Majumder et al., 1999a), phenanthrenes (Majumder and Sen, 1991) and 9,10-dihydrophenanthrenes (Majumder et al., 1996) and their dimers (Majumder and Banerjee, 1988; Majumder et al., 1998a, 1999b), phenanthropyrans and pyrones (Majumder and Sabzabadi, 1988; Majumder and Maiti, 1991) and their 9,10dihydro derivatives (Majumder et al., 1999c), fluorenonone (Majumder and Chakraborty, 1989) and a few other polyphenolics (Majumder et al., 1994, 1995), several triterpenoids (Majumder and Ghosal, 1991), steroids of biogenetic importance (Majumder and Pal, 1990) and some simple aromatic compounds (Majumder and Lahiri, 1989). As part of this general programme of research we chemically investigated the orchid Coelogyne cristata which earlier afforded the first pair of naturally occurring 9,10-dihydrophenanthropyran and pyrone derivatives 1a and 1b, respectively (Majumder et al., 1982). Further investigation of this orchid has now resulted in the isolation of two new * Corresponding author. Tel.: +91-33-351-9754. E-mail address: [email protected] (P.L. Majumder).
phenanthrene derivatives, designated coeloginanthridin and coeloginanthrin. The structures of coeloginanthridin and coeloginanthrin were established as 2a and 2c, respectively, from 1H and 13C NMR spectral and chemical evidence.
2. Results and discussion Coeloginanthridin (2a), C16H16O5 ([M+] at m/z 288) showed UV absorptions [lEtOH max 220.5, 275.5 and 298.5 nm (log " 4.37, 4.02 and 3.90)] which are strikingly similar to those of 9,10-dihydrophenanthrene derivatives (Majumder et al., 1982). The UV spectrum of coeloginanthrin (2c), C16H14O5 ([M+] at m/z 286) [lEtOH max 223.5, 260.5, 347 and 364 nm (log " 4.53, 4.57, 3.63 and 3.64)], on the other hand, exhibited close resemblance to those of fully aromatic phenanthrene derivatives (Majumder and Kar, 1987). The phenolic nature of both 2a and 2c was indicated by their characteristic colour reactions with FeCl3 (violet) and phosphomolybdic acid reagent (deep blue), alkali-induced bathochromic shifts of their UV maxima and by their IR absorption bands [2a: max 3240–3480 cm 1; 2c: max 3220–3490 cm 1]. The presence of three phenolic hydroxyl groups in both 2a and 2c was confirmed by the formation of their respective triacetyl derivatives [coeloginanthridin triacetate (2b), C22H22O8
0031-9422/01/$ - see front matter # 2001 Published by Elsevier Science Ltd. PII: S0031-9422(01)00287-4
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(M+) at m/z 414; coeloginanthrin triacetate (2d), C22H20O8 ([M+] at m/z 412)] with Ac2O and pyridine. The presence of three phenolic hydroxyl groups in each of 2a and 2c was also corroborated by the 1H NMR spectra of the compounds [2a: 5.76 (3H, br. s, disappeared on deuterium exchange); 2c: 4.99, 5.65 and 5.70 (each 1H, br.s; also disappeared on dueterium exchange)]. The presence of two aromatic methoxyl groups in each compound was evident from the three-proton singlets at 3.83 and 3.95 (2a) and 4.00 and 4.10 (2c). The 1H NMR spectrum of 2a showed two two-proton multiplets at 2.57 and 2.62, which are typical of the protons of the two benzylic methylene groups of a 9,10dihydrophenanthrene derivative (Majumder et al., 1982). The signals at 2.57 and 2.62 of 2a were replaced
by a pair of one-proton doublets at 7.35 and 7.85 (each having J=8.7 Hz), which are characteristic of H-9 and H-10 of a phenanthrene derivative (Majumder and Kar, 1987). This would suggest that 2a is a 9,10-dihydrophenanthrene derivative and 2c is the corresponding phenanthrene analogue. Furthermore, the 1H NMR spectrum of 2a exhibited a one-proton singlet at 7.52 corresponding to H-4 or H-5 of 9,10-dihydrohenanthrene derivative (Majumder and Lahiri, 1990), while the spectrum of 2c was characterized by a one-proton singlet at 8.85 similar to that of the corresponding proton of a phenanthrene (Majumder and Lahiri, 1990). The relatively upfield shifts of these proton signals in the 1H NMR spectra of both 2a and 2c by about 0.5 ppm (H-4 and H-5 of 9,10-dihydrophenanthrene normally appear at ca 8.0 and the corresponding protons
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of phenanthrene usually resonate at ca 9.5) were attributed to the presence of electron-donating groups (OH or OMe) at ortho and para to these protons in both 2a and 2c. Now, if these downfield aromatic proton signals of 2a and 2c are assigned to H-4, then C-5 of both the compounds must be substituted by a hydroxyl or methoxyl group. Again, since the above downfield proton signals of both 2a and 2c appeared as sharp singlets, C-3 and C-2 of both the compounds must also be substituted by OH or OMe. The remaining two aromatic protons of 2a and 2c appeared as a pair of doublets corresponding to the meta-coupled aromatic protons [2a: 6.29 and 6.33 (each 1H, d, J=2.0 Hz); 2c: 6.55 and 6.75 (each illresolved meta-coupled doublets)]. The signals at 6.29 and 6.33 in the spectrum of 2a resemble those of H-6 and H-8 of calaphenanthrenol (2e) (Yoshikawa et al., 1998) having two hydroxyl groups at C-5 and C-7. The corresponding signals at 6.55 and 6.75 of 2c may be similarly assigned to H-6 and H-8 of the compound possessing a phenanthrene skeleton having two hydroxyl functions at C-5 and C-7. This would imply that while two hydroxyl groups in both 2a and 2c may be located at C-5 and C-7, the remaining hydroxyl function and the two methoxyl groups in each compound must be distributed between C-3, C-2 and C-1. The relative positions of the hydroxyl and methoxyl groups in both 2a and 2c were ascertained by the observed changes in the chemical shifts of H-4, H-6 and H-8 of the compounds in the 1H NMR spectra of their respective triacetyl derivatives 2b and 2d. It has been shown that H-5 of a 4-hydroxyphenanthrene and its 9,10-dihydro derivative is shifted upfield by ca 0.5 and 0.2 ppm, respectively on acetylation (Letcher and Nhamo, 1971; Letcher and Wong, 1978; Majumder and Lahiri, 1990). A similar upfield shift of H-4 was observed in 2b and 2d by 0.28 and 0.09 ppm, respectively, affirming the placement of one of the three hydroxyl groups at C-5 in both 2a and 2c. The relatively smaller upfield shifts of H-4 of both 2a and 2c than those usually observed on acetylation of their hydroxyl function at C-5 may be attributed to the presence of a hydroxyl group at C-3 in both the compounds, which on acetylation produces a deshielding effect on H-4. The remaining hydroxyl group in both 2a and 2c was placed at C-7 from the observed downfield shifts of H-6 (ortho to two hydroxyl groups at C-5 and C-7) and H-8 (ortho to one hydroxyl group at C-7 and para to another hydroxyl function at C-5) by 0.52 and 0.61 ppm, and 0.65 and 0.82 ppm, respectively, in the 1H NMR spectra of their triacetyl derivatives 2b and 2d. The greater deshielding of H-6 and H-8 of 2c in the spectrum of its triacetyl derivative 2d may be partly attributed to the acetylation of its hydroxyl group at C-3, causing a deshielding of its H-9 by 0.23 ppm in the spectrum of 2d. The two methoxyl groups in both 2a and 2c must then be placed at C-1
and C-2. Coeloginanthridin and coeloginanthrin may, therefore, be represented as 3,5,7-trihydroxy-1,2-dimethoxy-9,10-dihydrophenanthrene (2a) and 3,5,7-trihydroxy-1,2-dimethoxyphenanthrene (2c), respectively. The structure of coeloginanthridin (2a) was also affirmed by the 13C NMR spectral data of the compound and its triacetyl derivative 2b (Table 1). The degree of protonation of the carbon atoms in 2a and 2b was determined by DEPT experiments and the carbon chemical shifts were assigned by comparison with the C values of the structurally similar compound calaphenanthrenol (2e) (Yoshikawa et al., 1998). Thus, except that of C-8a, the C values of C-4b, C-5, C-6, C-7 and C-8 of 2a are essentially similar to those of the corresponding carbon atoms of 2e supporting the placement of two hydroxyl groups at C-5 and C-7 in 2a. The 13C NMR chemical shift assignments of 2a were confirmed by HMQC and HMBC experiments and on the basis of these experiments C-8a of 2a was assigned the signal at C 142.0. It appears from the following 2D NMR spectral analysis of coeloginanthridin (2a) that the signal at C 131.4 earlier assigned to C-8a of calaphenanthrenol (2e) (Yoshikawa et al., 1998) should be interchanged with that at C 142.0 assigned to its C-3. The HMQC spectrum of 2a (with JCH parameter set to 160 Hz) showed correlations of the hydrogens at H Table 1 13 C NMR spectral data of compounds 2a, 2b and 2e Chemical shift (, ppm)a
C
1 2 3 4 4a 4b 5 6 7 8 8a 9 10 10a OMe OAc
a
2a
2b
2eb
149.4 137.8 147.1 108.6 128.7 114.4 153.7 102.1 155.0 107.7 142.0 30.1 21.3 123.6 60.7 (OMe at C-1) 60.9 (OMe at C-2) – – –
149.3 144.1 142.5 117.2 128.4 126.4 147.8 115.2 151.3 118.6 141.7 29.6 21.4 130.8 60.7 (2xOMe)
108.0 145.7 142.0 142.1 119.2 113.0 154.7 104.0 156.0 108.0 131.4 31.6 30.5 137.2 61.8 (OMe at C-4) 56.4 (OMe at C-2) –
168.6 168.7 168.8 20.6 20.9 21.0
–
Spectra were run in CDCl3 and the chemical shifts were measured with (TMS)=(CDCl3)+76.9 ppm. b The carbon resonances of 2e and their assignments shown here are those reported by (Yoshikawa et al., 1998).
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2.57 (H2-9), 2.62 (H2-10), 7.52 (H-4), 6.29 (H-6), 6.33 (H-8), 3.83 (OMe) and 3.95 (OMe) with the carbon atoms at C 30.1 (C-9), 21.3 (C-10), 108.6 (C-4), 102.1 (C-6), 107.7 (C-8), 60.7 and 60.9, respectively. In the HMBC spectrum of 2a (with JCH parameter set to 7 Hz), the signal at H 2.57 (H2-9) exhibited fairly strong 3-bond correlations with the carbon signals at C 114.4, 107.7 and 123.6, which were thus assigned to C-4b, C-8 and C-10a, respectively. The signal at H 2.57 (H2-9) also showed a 2-bond correlation with the carbon at C 142.0 assigned to C-8a. In aromatic compounds 3-bond 1 H-13C correlations were found to be a common feature, while 2-bond and other long range (4-bond or more remote) correlations are usually not observed. However, in polyoxygenated phenanthrene derivatives such 2-bond and other long range correlations have sometimes been observed and were assumed to occur due to the presence of oxygenated functions like OH and OMe attached to the aromatic ring, which alter the electron distribution in favour of such couplings (Majumder et al., 1997). In light of the observation that presence of a substituent like methoxyl group at C-1 in 9,10-dihydrophenenthrene derivatives causes an upfield shift of C10 by ca 7 ppm as in coelogin (1a), coeloginin diacetate and coeloginin dimethyl ether (Majumder et al., 1982), the upfield benzylic methylene carbon at C 21.3 of 2a having a methoxyl group at C-1 must be assigned to C10 which showed 1-bond correlation to the protons at H 2.62. The signal at H 2.62 of 2a must then be assigned to its H2-10 which showed 3-bond correlations with the carbons resonating at C 142.0, 149.4 and 128.7, which may, therefore, be assigned to C-8a, C-1 and C-4a, respectively, of 2a. The signal at H 2.62 (H210) also exhibited a 2-bond correlation with the carbon at C 123.6 which has already been assigned to C-10a. Since both 2a and 2e have identical substituents in their ring C (OH at C-5 and C-7), the confirmation of the assignment of the carbon resonance at C 142.0 of 2a to its C-8a implies that the signal at C 142.0 of 2e (earlier attributed to its C-3) may, by analogy, be also assigned to its C-8a. The signal at C 131.4 of 2e (earlier assigned to its C-8a) would correspond to its C-3. The proton at H 7.52 corresponding to H-4 showed 3-bond correlations with the carbons at C 114.4 (C-4b), 123.6 (C-10a) and 137.8 which must correspond to C-2. H-4 (H 7.52) also exhibited a 2-bond correlation with the carbon at C 147.1 (C-3) and a weak 4-bond correlation with the carbon at C 149.4 (C-1). The proton at H 6.29 assigned to H-6 was found to exhibit 3-bond correlations with the carbons at C 107.7 and 114.4 which were already assigned to C-8 and C-4b, respectively. H-6 (H 6.29) also showed 2-bond correlations with the nonprotonated carbons at C 153.7 and 155.0, which corresponded to C-5 and C-7, respectively. Similarly, the proton at H 6.33 assigned to H-8 showed 3-bond cor-
relations with the carbons at C 102.1 (C-6), 114.4 (C4b) and 30.1 (C-9) and a weak 2-bond correlation with the carbon at C 155.0 (C-7). The methoxyl protons at H 3.83 and 3.95, which were associated with methoxyl carbons at C 60.7 and 60.9, respectively, as evident from HMQC spectrum of 2a, showed 3-bond correlations with the carbons at C 149.4 (C-1) and 137.8 (C2), respectively. All the above 1H–13C correlations observed in the HMBC spectra of 2a are depicted in its structural diagram. The above spectral data thus unequivocally established the structure of coeloginanthridin (2a). Although similar spectral analysis could not be made with coeloginanthrin (2c) due to paucity of material, the structure of the compound was confirmed by the conversion of coeloginanthridin triacetate (2b) to coeloginanthrin triacetate (2d) by dehydrogenation of the former to the latter with 2,3-dichloro-5,6-dicyano-1,4benzoquinone (DDQ) in dry benzene. Acid-hydrolysis of the dehydrogenation product finally afforded coeloginanthrin (2c). Coeloginanthridin (2a) and coeloginanthrin (2c) are thus two new additions to the growing lists of the naturally occurring 9,10-dihydrophenanthrenes and phenanthrenes respectively. In the light of the earlier reports that a variety of phenanthrene derivatives were found to be potent phytoalexins (Gaumann and Kern, 1959a,b; Gorham, 1980; Purkayastha, 1985, 1995), while several others were shown to act as endogenous plant growth regulators (Gorham, 1980), it would be interesting to study 2a and 2c for similar biological activities.
3. Experimental Melting points: uncorr. CC: Silica gel (100–200 mesh). MPLC: silica gel (230–400 mesh). TLC: silica gel G. UV: 95% aldehyde-free EtOH. IR: KBr discs. 1H and 13 C NMR: 300 and 75 MHz, respectively (for 2b, 2c and 2d); 500 MHz (for 1H, HMBC and HMQC of 2a), 125 MHz (for 13C NMR of 2a). NMR spectra were recorded in CDCl3 using TMS as internal standard and chemical shifts are expressed in (ppm). MS: direct inlet system, 70 eV. All analytical samples were routinely dried over P2O5 for 24 h in vacuo and were tested for purity by TLC and MS. The petrol used had bp 60– 80 C. 3.1. Plant materials Coelogyne cristata was collected from Kalimpong (Darjeeling), India in October, 1998. A voucher specimen (Majumder s.n.) was deposited in the Herbarium of the Department of Botany, University of Calcutta (CUH).
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3.2. Isolation of coeloginanthridin (2a) and coeloginanthrin (2c) Air-dried finely powdered whole plant of Coelogyne cristata (5 kg) was kept soaked in MeOH (10 l) for 3 weeks. The MeOH extract was concentrated to ca. 100 ml, diluted with H2O (750 ml) and exhaustively extracted with Et2O. The Et2O extract was fractionated into acidic and nonacidic fractions with 2 M NaOH. The aqueous alkaline solution was acidified in the cold with conc. HCl and the liberated solids were extracted with Et2O, washed with H2O, dried and the solvent removed. The residue was subjected to CC. The petrol–EtOAc (7:1) eluate gave a mixture of 1a and 1b. The above mixture of 1a and 1b was then subjected to CC and the early fractions of the petrol–EtOAc (7:1) eluate also gave a mixture of 1a and 1b containing predominantly 1b. The later fractions of the same eluate also afforded a mixture of 1a and 1b containing mostly 1a. The fractions containing mostly 1a and those containing mostly 1b were separately mixed together and evaporated. The two residues enriched with respect to 1a and 1b were then separately subjected to MPLC using petrol–EtOAc (5:1) as the solvent system. The early fractions of the residue containing mostly 1b gave pure 1b, while the later fractions of the same eluate gave pure 1a. Similarly, MPLC of the residue containing mostly 1a gave in the early fractions pure 1b, while the later fractions afforded pure 1a. The fractions containing pure 1b and 1a were separately mixed together and the solvent removed to give pure 1b (0.3 g), crystallized from petrol–EtOAc in yellowish needles, mp 198 C and pure 1a (0.25 g), also crystallized from the same solvent mixture in fine colourless needles, mp 151 C. Washing the main column with petrol–EtOAc (3:1) gave a gummy residue comprising mainly coeloginanthridin (2a) and coeloginanthrin (2c). The above mixture was subjected to MPLC using petrol–EtOAc (2:1) as the solvent system. The early fractions gave a residue which contained coeloginanthridin (2a) as the major constituent, while the later fractions gave a mixture containing mostly coeloginanthrin (2c). The above two mixtures were again separately subjected to MPLC using the same solvent system to give finally pure coeloginanthridin (2a) (0.10 g) and coeloginanthrin (2c) (0.02 g), both as semisolid mass. Coeloginanthridin (2a) (found: C, 66.61; H, 5.48; C16H16O5 requires: C, 66.67; H, 5.60%). UV 0:1 M NaOH lEtOH nm: 212.5, 286.0 and 311.5 (log " 4.51, max 3.98 and 4.00); IR max cm 1: 3240–3480 (OH), 1610, 1550, 1480, 880 and 840 (aromatic nucleus); MS m/z (rel. int.): 288 [M+] (100), 273 (25), 258 (15) and 230 (10); 1H NMR: 7.52 (1H, s; H-4), 6.33 and 6.29 (each 1H, d, J=2.0 Hz, H-8 and H-6), 5.76 (3H, br. s, 3xArOH), 3.95 and 3.83 (each 3H, s, 2xArOMe), 2.62 and 2.57 (each 2H, m, H2-10 and H2-9). Acetylation of
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coeloginanthridin (2a) with Ac2O and pyridine in the usual manner gave coeloginanthridin triacetate (2b), also as a semisolid mass. (found: C, 63.71; H, 5.25; C22H22O8 requires: C, 63.77; H, 5.35%). IR: max cm 1: 1215, 1220, 1225, 1745, 1755 and 1770 (OAc), 1615, 1600, 1580, 1475, 820, 775, 735, 715, 690 and 595 (aromatic nucleus); 1H NMR: 7.43 (1H, s, H-4), 6.94 and 6.81 (each 1H, d, J=2.34 Hz, H-8 and H-6), 3.89 and 3.84 (each 3H, s, 2xArOMe), 2.80 and 2.77 (each 2H, m, H2-10 and H2-9), 2.35, 2.29 and 2.27 (each 3H, s, 3xOAc); MS m/z (rel. int.): 414 [M+] (25), 372 (30), 330 (35), 288 (100), 273 (20), 258 (10) and 230 (10). Coeloginanthrin (2c): (found: C, 67.02, H, 4.83. C16H14O5 requires: C, 67.13; H, 4.93%). UV 0:1 M NaOH lEtOH nm: 212, 228.5, 277, 307 and 377 (log " max 4.47, 4.45, 4.44, 4.15 and 3.69); IR max cm 1: 3220– 3490 (OH), 1625, 1615, 1585, 1480, 885, 840 and 650 (aromatic nucleus); MS m/z (rel. int.) 286 [M+] (100), 271 (30), 256 (20) and 228 (10); 1H NMR: 8.85 (1H, s, H-4), 7.85 and 7.35 (each 1H, d, J=8.7 Hz, H-10 and H-9), 6.75 and 6.55 (each 1H, ill-resolved meta-coupled d, H-8 and H-6), 5.70, 5.65 and 4.99 (each 1H, br.s, 3xArOH), 4.10 and 4.00 (each 3H, s, 2xArOMe). Acetylation of coeloginanthrin (2c) with Ac2O and pyridine in the usual manner gave coeloginanthrin triacetate (2d), also as a semisolid mass. (found: C, 63.97; H, 4.78. C22H20O8 requires: C, 64.07; H, 4.89%). IR nmax cm 1: 1205, 1220, 1230, 1775, 1780 and 1785 (OAc), 1615, 1585, 1555, 1485, 845, 820, 810, 765 and 740 (aromatic nucleus); 1H NMR: 8.57 (1H, s, H-4), 8.08 and 7.67 (each 1H, d, J=9.2 Hz, H-10 and H-9), 7.57 and 7.20 (each 1H, ill-resolved meta-coupled d, H-8 and H-6), 4.07 and 4.03 (each 3H, s, 2xArOMe), 2.49, 2.42 and 2.36 (each 3H, s, 3xOAc); MS m/z (rel. int.) 412 [M+] (35), 370 (30), 328 (20), 286 (100), 271 (25), 256 (10) and 228 (8). 3.3. Conversion of coeloginanthridin triacetate (2b) to coeloginanthrin triacetate (2d) by dehydrogenation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and acid-hydrolysis of coeloginanthrin triacetate (2d) to coeloginanthrin (2c) To a soln. of coeloginanthridin triacetate (2b) (0.03 g) in dry C6H6 (25 ml) was added 0.07 g of 2,3-dichloro5,6-dicyano-1,4-benzoquinone. The resultant mixture was heated under reflux for 16 h. The solvent was removed under red. pres. to give a solid which was extracted with Et2O, washed three times with 2 M aq. NaOH and then with H2O, dried and the solvent removed. The residue was chromatographed. The petrol-EtOAc (10:1) eluate gave coeloginanthrin triacetate (2d) (0.018 g) as a semi-solid mass. To a soln. of 0.01 g of coeloginanthrin triacetate (2d) was added 5 ml of 2 M aq. methanolic HCl and the mixture was heated under reflux for 5 h. MeOH was removed under red. pres. and
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the residue was extracted with Et2O, washed with H2O, dried and the solvent removed. The residue was chromatographed. The petrol–EtOAc (3:1) eluate gave coeloginanthrin (2c) (0.008 g) as a semi-solid mass.
Acknowledgements The work was supported by C.S.I.R., New Delhi, India.
References Gaumann, E., Kern, H., 1959a. U¨ber die Isolierung and den Chemischen Nachweis des Orchinobs. Phytopathologische Zeitschrift 35, 347–356. Gaumann, E., Kern, H., 1959b. U¨ber chemische Abwehrreaktionen bei Orchideen. Phytopathologische Zeitschrift 36, 1–26. Gorham, J., 1980. The Stilbenoids. Progress in Phytochemistry 6, 203– 252. Letcher, R.M., Nhamo, L.R.M., 1971. Chemical constituents of the combretaceae Part 1. Substituted phenanthrenes and 9,10-dihydrophenanthrenes from the heartwood of Combretum apiculatum. Journal of the Chemical Society (C) 3070–3076. Letcher, R.M., Wong, K.M., 1978. Structure and synthesis of the phenanthrenes TaI and TaV from Tamus communis. Journal of the Chemical Society Perkin Trans. I 739–742. Majumder, P.L., Bandyopadhyay, D., Joardar, S., 1982. Coelogin and coeloginin: two novel 9,10-dihydrophenanthrene derivatives from the orchid Coelogyne cristata. Journal of the Chemical Society Perkin Trans I 1131–1136. Majumder, P.L., Banerjee, S., 1988. Structure of flavanthrin, the first dimeric 9,10-dihydrophenanthrene derivative from the orchid Eria flava. Tetrahedron 44, 7303–7308. Majumder, P.L., Banerjee, S., Lahiri, S., Mukhoti, N., Sen, S., 1998a. Dimeric phenanthrenes from two Agrostophyllum species. Phytochemistry 47, 855–860. Majumder, P.L., Banerjee, S., Sen, S., 1996. Three stilbenoids from the orchid Agrostophyllum callosum. Phytochemistry 42, 847–852. Majumder, P.L., Chakraborty, J., 1989. Chemical constituents of the orchid Dendrobium farmerii: Further evidence for the revised structure of dengibsin. Journal of the Indian Chemical Society 66, 834– 837. Majumder, P.L., Ghosal, S., 1991. Arundinol, a new triterpene from the orchid Arundina bambusifolia. Journal of the Indian Chemical Society 68, 88–91. Majumder, P.L., Guha, S., Sen, S., 1999a. Bibenzyl derivatives from the orchid Dendrobium amoenum. Phytochemistry 52, 1365–1369.
Majumder, P.L., Kar, A., 1987. Confusarin and confusaridin, two phenanthrene derivatives of the orchid Eria confusa. Phytochemistry 26, 1127–1129. Majumder, P.L., Kar, A., Banerjee, S., Pal, A., Joardar, M., 1997. Application of 2D NMR spectroscopy in structural elucidation of some phenanthrene derivatives from Indian Orchidaceae plants. Journal of the Indian Chemical Society 74, 908–916. Majumder, P.L., Lahiri, S., 1989. Chemical constituents of the orchid Lusia indivisa. Indian Journal of Chemistry 28B, 771–774. Majumder, P.L., Lahiri, S., 1990. Lusianthrin and lusianthridin, two stilbenoids from the orchid Lusia indivisa. Phytochemistry 29, 621– 624. Majumder, P.L., Lahiri, S., Mukhoti, N., 1995. Chalcone and dihydrochalcone derivatives from the orchid Lusia volucris. Phytochemistry 40, 271–274. Majumder, P.L., Lahiri, S., Pal, S., 1994. Occurrence of lignans in the Orchidaceae plants Lusia volucris and Bulbophyllum triste. Journal of the Indian Chemical Society 71, 645–647. Majumder, P.L., Maiti, D.C., 1991. Isoflaccidinin and isooxoflaccidinin, stilbenoids from Coelogyne flaccida. Phytochemistry 30, 971–974. Majumder, P.L., Pal, S., 1990. A steroidal ester from Coelogyne uniflora. Phytochemistry 29, 2717–2720. Majumder, P.L., Pal, S., Majumder, S., 1999b. Dimeric phenanthrenes from the orchid Bulbophyllum reptans. Phytochemistry 50, 891– 897. Majumder, P.L., Roychowdhury, M., Chakraborty, S., 1998b. Thunalbene, a stilbene derivative from the orchid Thunia alba. Phytochemistry 49, 2375–2378. Majumder, P.L., Sabzabadi, E., 1988. Agrostophyllin, a naturally occurring phenanthropyran derivative from Agrostophyllum khasiyanum. Phytochemistry 27, 1899–1901. Majumder, P.L., Sen, R.C., 1991. Pendulin, A polyoxygenated phenanthrene derivative from the orchid Cymbidium pendulum. Phytochemistry 30, 2432–2434. Majumder, P.L., Sen, S., Banerjee, S., 1999c. Agrostophyllol and isoagrostophyllol, two novel diastereomeric 9,10-dihydrophenanthropyran derivatives from the orchid Agrostophyllum callosum. Tetrahedron 55, 6691–6702. Purkayastha, R.P., 1985. Phytoalexins. In: Mukerji, K.G., Pathak, N.C., Singh, V.P. (Eds.), Frontiers in Applied Microbiology. Vol. 1. Print House (India), Lucknow, pp. 363–407. Purkayastha, R.P., 1995. Progress in Phytoalexin Research during the Past 50 Years. In: Daniel, M., Purkayastha, R.P. (Eds.), Handbook of Phytoalexin Metabolism and Action. Marcel Dekker, New York, pp. 1–39. Yoshikawa, M., Murakami, T., Kishi, A., Sakurama, T., Matsuda, H., Nomura, M., Matsuda, H., Kubo, M., 1998. Novel indole S,Obisdesmoside, calanthoside, the precursor glycoside of tryptanthrin, indirubin, and isatin, with increasing skin blood flow promoting effects, from two Calanthe species (Orchidaceae). Chemical and Pharmaceutical Bulletin 46, 886–888.