Indole alkaloids from two species of Ophiorrhiza

Article abstract of DOI:10.1071/CH99112

Extraction of the aerial parts of Ophiorrhiza rosacea Ridley (Rubiaceae) has yielded harman-2-oxide (1). Extraction of the aerial parts of O. kunstleri King yielded the alkaloids palicoside (3) and 3,14-didehydro-19-methylnormalindine (8). The structures

Full text of DOI:10.1071/CH99112

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Volume 53, 2000
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Aust. J. Chem., 2000, 53, 221–224
Indole Alkaloids from
Two Species of Ophiorrhiza
Dachriyanus,A,B Dayar Arbain,B Deddi Prima Putra,B
Melvyn V. Sargent,A Revi SusilaB and Fatma Sri WahyuniA,B
A Department of Chemistry, University of Western Australia, Nedlands, W.A. 6907.
B Jurusan Farmasi, FMIPA, Universitas Andalas, Kampus Limau Manis,
P.O. Box 143, Padang 25163, Indonesia.
Extraction of the aerial parts of Ophiorrhiza rosacea Ridley (Rubiaceae) has yielded harman-2-oxide (1).
Extraction of the aerial parts of O. kunstleri King yielded the alkaloids palicoside (3) and 3,14-didehydro-19-
methylnormalindine (8). The structures of these compounds followed from their spectroscopic properties.
Keywords. Alkaloids; Ophiorrhiza; indole; natural products.
In continuation of our studies of the alkaloids of Sumatran
members of the genus Ophiorrhiza (Rubiaceae)1 we have
examined the alkaloid content of O. rosacea and O. kunst-
leri. O. rosacea is a small herbaceous shrub, which attains a
height of c. 30 cm. It was collected at Gunung Tujuh,
National Park Kerinci Seblat, Jambi, Indonesia, c. 2000 m
above sea level. This plant exhibits a characteristic red
colour on maceration with methanol. No traditional medici-
nal value has been reported for this plant.
The methanolic extract of the plant yielded after chro-
matography a crystalline alkaloid which was subsequently
identified as harman-2-oxide (1). This compound was previ-
ously isolated from Calligonum minimum2 and it also has
been found in wine.3,4 This non-crystalline compound was
identified on the basis of reduction to harman (2) and oxida-
tion of harman to the N-oxide.2 The n.m.r. spectra for this
compound were not reported.
oxide the M – 17 peak had a relative abundance of 30% com-
pared with 24% for the M – 1 peak of harman (2). Direct
comparison of the infrared spectrum of this compound with
that of harman showed that they were different, and the N-
oxide contained a band at 1379 cm–1 attributed to the N–O
stretching mode.6
1
The H n.m.r. spectrum of the N-oxide recorded in
(CD3)2SO was markedly different to that of harman (2)
recorded under the same conditions. The spectrum of
harman was interpreted by the assistance of double irradia-
tion and by the n.O.e. difference technique. The pertinent
data are shown in Table 1. The shielding effect of the N–O
group is evident. In pyridine and its benzannelated deriva-
tives N-oxidation causes a downfield shift at the ␣-position
and usually a slight upfield shift at the ␤-position.7 The
effect at the 4-position of the ␤-carboline system is thus dif-
ferent. N-Oxidation also causes an increase in the magnitude
of the pyridine 2,3-coupling constant which is also mani-
fested in the present case.
The e.i. mass spectrum of the compound did not exhibit a
molecular ion which is common behaviour for N-oxides. The
The 13C n.m.r. spectrum determined in (CD3)2SO was
compared with that of harman, determined in the same
solvent.8 The chemical shifts of the benzenoid carbons were
little changed but those of the pyridine ring were shifted: ␦
Me 16.05 (20.56), C 1 143.14 (142.28), C 3 115.33 (137.63),
C 4 128.48 (112.64), C 4a 133.88 (127.06) and C 8b 131.62
(134.72), where values for harman (2) are in parentheses.
base peak was at m/z 182 which by high resolution corre-
sponded to C12H10N2 and therefore to the loss of oxygen
from the molecular ion. Aromatic N-oxides often show an
M – 17 peak which is of slightly greater intensity than the
M – 1 peak in the parent heterocycle.5 Such was the case with
harman-2-oxide when the spectrum was compared with that
of harman determined under the same conditions. For the N-
1
Table 1. Comparison of pertinent H n.m.r. signals of (1) and
(2) in (CD3)2SO
Proton
(1)
(2)
␦H, mult., coupling constants
␦H, mult., coupling constants
H3
H4
Me
8.61, d, J3,4 6.3 Hz
8.43, d, J4,3 6.3 Hz
3.06, s
8.19, d, J3,4 5.3 Hz
7.91, d, J4,3 5.3 Hz
2.75, s
Manuscript received 8 September 1999
© CSIRO 2000
10.1071/CH99112
0004-9425/00/030221

222
Short Communications
Confirmation of the structure of harman-2-oxide (1) came
m/z 286 which was attributed to the loss of a methyl radical
from the molecular ion. High resolution established the
molecular formula C20H19N3 so that it was isomeric with the
pyridinoindole alkaloid decussine (5).12 The 13C n.m.r. spec-
trum (Table 2) supported the molecular formula and applica-
tion of the DEPT technique revealed that there were two
methyl, two methylene, eight methine and eight quaternary
carbons. It thus differed from decussine (5), normalindine
(6),13,14 and isonormalindine (7).14 The electronic spectrum,
like that of decussine, showed evidence of conjugation
between the indole and pyridine entities of the molecule.
from its reduction with zinc in acetic acid to harman (2)
which exhibited identical properties with those recorded in
the literature.8,9
In order to unambiguously assign both the proton and
carbon spectra, it was found that the proton spectrum of
compound (1) in CD3OD was much better resolved than that
in (CD3)2SO. The assignments of these spectra, recorded in
the experimental section, were assisted by the DEPT, HMQC
and HMBC techniques.
Ophiorrhiza kunstleri King was collected on W mountain,
Pariaman, West Sumatra, at an altitude of 600 m, and is a
small herb which attains a height of about 30 cm. The stems
and leaves are characterized by the presence of tiny yellow
hairs.
1
Table 2. 13C and H n.m.r. data for 3,14-didehydro-19-methyl-
normalindine (8) in CD3OD
Carbon
␦
␦H, mult., coupling constants
C
The methanolic extract of the aerial parts of the plant gave
a positive test for alkaloids and chromatography afforded a
new tertiary alkaloid from the less polar fractions. The more
polar yielded the known N-methylstrictosidinic acid, palico-
side (3), the stereochemistry at C 3 of which followed from
its conversion into strictosamide.10 Palicoside has been iso-
lated previously from the Brazilian species Palicourea marc-
gravii (Rubiaceae)10 and from O. acuminata.11
2
3
5
130.16A
155.83
49.83
3.25, ddd, J5eq,5ax 17.9, J5eq,6ax 5.0, J5eq,6eq 2.2 Hz
4.15, ddd, J5ax,5eq 17.9, J5ax,6ax 12.9, J5ax,6eq 3.8 Hz
3.02, ddd, J6eq,6ax 17.1, J6eq,5ax 3.8, J6eq,5eq 2.2 Hz
3.38, ddd, J6ax,6eq 17.1, J6ax,5ax 12.9, J6ax,5eq 5.0 Hz
6
23.06
7
8
9
117.30
130.30A
119.89
120.27
124.63
112.00
139.17
121.90
151.78
117.94
149.62
26.98
7.50, dd, J9,10 8.0, J9,11 1.0 Hz
10
11
12
13
14
15
16
17
18
7.03, ddd, J10,9 8.0, J10,11 7.5, J10,12 1.0 Hz
7.17, ddd, J11,12 8.1, J11,10 7.5, J11,9 1.0 Hz
7.32, dd, J12,11 8.1, J12,10 1.0 Hz
The new crystalline alkaloid was devoid of optical activ-
ity as shown by its lack of a c.d. spectrum. The e.i. mass
spectrum showed a molecular ion at m/z 301 and an ion at
6.83, s
7.35, d, J16,17 4.9 Hz
8.37, d, J17,16 4.9 Hz
1.66, s (H 18␤)
35.43
2.33, s (H 18␣)
19
20
21
73.95
146.93
142.68
8.50, s
A Assignments may be reversed.
These data are best accommodated by the structure (8)
and the results of HMQC and HMBC (see Table 3) experiments
are in agreement. The location of the methyl groups was con-
firmed by the results of n.O.e. difference experiments which
are shown in Fig. 1.
Normalindine (6) and isonormalindine (7) are formally
derived from strictosidine (4) by junction between N 4 and
C 19. This is probably achieved biosynthetically by inter-
vention of an 18,19-epoxide of strictosidine as suggested by
Table 3. 1H and 13C n.m.r. coupling determined by
HMBC experiments
Proton
Carbon
Two-bond
Three-bond
5eq
6eq
6ax
9
10
11
12
14
16
3, 19
2
5, 7
5, 7
7, 8, 11, 13
8, 12
9, 13
8, 10
2
11
12
3, 15
17
20
17
16
15, 21
18␣
18␤
21
19
19
20
20
3, 15

Short Communications
223
Reduction of Harman-2-oxide
the isolation of isodihydrocadambine (9) from
Anthocephalus cadamba.15 The isolation of 19␣- and 19␤-
cadamine (10) from the same source, although they have
been assigned the 3␤ stereochemistry,16 points to the likely
origin of normalindine (6) and isonormalindine (7). The
mode of introduction of the extra methyl group into the new
alkaloid, for which we suggest the trivial name 3,14-didehy-
dro-19-methylnormalindine (8), is obscure.
An excess of activated zinc dust was added to a stirred solution of
harman-2-oxide (1) (25 mg) in acetic acid (1 ml) and the mixture was
stirred for 2 h. The solution was poured into saturated sodium hydrogen
carbonate and extracted with ethyl acetate, washed with water and sat-
urated brine and dried over sodium sulfate. The solvent was removed
under reduced pressure to give harman (2) (20 mg, 87%) which crys-
tallized from methanol/ethyl acetate as needles, m.p. 235–237° (lit.9
228°).
Extraction of Ophiorrhiza kunstleri Ridley
The botanical material was collected at W mountain, Pariaman,
West Sumatra, in April 1997. A voucher specimen is lodged in the
herbarium at Andalas University.
Finely chopped fresh aerial parts (5 kg) were extracted for 5 days
with methanol (18 litre) and the process was repeated twice more. The
combined extracts were reduced in volume under diminished pressure
to 400 ml and acetic acid (20 ml) was added. The solution was set aside
and the next day it was decanted and extracted with ethyl acetate
(4×250 ml). Excess sodium hydrogen carbonate was added and the
solution was extracted with butanol (5×400 ml). The combined butanol
extracts were evaporated under reduced pressure to dryness which gave
a dark gum (15.14 g). A portion of this gum (10.0 g) was preadsorbed
on silica gel and chromatographed over a column of silica gel with
increasing amounts of methanol in ethyl acetate as eluent. The less polar
fractions were further purified by radial chromatography with the same
eluent. Those fractions which showed one spot on t.l.c. were combined
and the residue was crystallized from ethyl acetate/light petroleum to
give 3,14-didehydro-19-methylnormalindine (8) as yellow needles (20
Fig. 1. N.O.e. difference data for 3,14-didehydro-19-methyl-
normalindine (8).
Experimental
mg), m.p. 128–130° (dec.) (Found [f.a.b.]: M++H, 302.1647.
General directions have been given before.17 N.m.r. spectra were
recorded on a Bruker ARX-500 spectrometer unless otherwise stated.
12
C
20
1H2014N3 requires M++H, 302.1657). m/z 301 (M+, 20%), 286 (12),
259 (21), 258 (100), 257 (37), 256 (22), 255 (12), 243 (12). ␭max/nm
(MeOH) 217, 254, 368 (⑀ 18300, 7800, 16200).
Extraction of Ophiorrhiza rosacea Ridley
The more polar fractions were rechromatographed over silica gel
with 0–5% acetic acid/butanol as eluent. Fractions which showed one
spot on t.l.c. were combined and the residue was crystallized from
methanol to give palicoside (3) as needles (200 mg), m.p. 206–207°
(lit.10 206–208°), which exhibited identical spectra (electronic, 1H and
13C n.m.r.) to those described in the literature.
The botanical material was collected at Gunung Tujuh c. 2000 m
above sea level, in Kerinci Setlat National Park, Jambi, Indonesia, in
December 1997. A voucher specimen (DR-158) was identified by Dr
Rusjdi Tamin and is held at Herbarium Biology, Andalas University.
Finely chopped fresh aerial parts (2.1 kg) were extracted for 3 days
with methanol (5 litre). This was repeated twice more and the combined
extracts were evaporated under reduced pressure to c. 250 ml and then
extracted with light petroleum (3×100 ml) and ethyl acetate (4×250 ml).
The ethyl acetate was washed with saturated brine, dried over anhy-
drous sodium sulfate and evaporated to dryness to give a dark red gum
(850 mg). The crude extract gave one major spot on t.l.c. after develop-
ment with ethyl acetate/methanol (1 : 1) and spraying with Dragendorff
reagent.
Acknowledgments
AusAID is thanked for a scholarship (Dachriyanus) and
Dr Rusjdi Tamin kindly identified the plant material.
References
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14