New anthraquinone glycosides from the roots of Morinda citrifolia

Article abstract of DOI:10.1016/j.fitote.2009.01.014

Six new anthraquinone glycosides: digiferruginol-1-methylether-11-O-β-gentiobioside (1); digiferruginol-11-O-β-primeveroside (2); damnacanthol-11-O-β-primeveroside (3); 1-methoxy-2-primeverosyloxymethyl-anthraquinone-3-olate (4); 1-hydroxy-2-primeverosylo

Full text of DOI:10.1016/j.fitote.2009.01.014

Fitoterapia 80 (2009) 196–199
Contents lists available at ScienceDirect
Fitoterapia
journal homepage: www.elsevier.com/locate/fitote
New anthraquinone glycosides from the roots of Morinda citrifolia
⁎
Kohei Kamiya , Wakako Hamabe, Shogo Tokuyama, Toshiko Satake
Faculty of Pharmaceutical Sciences, Kobe Gakuin University, 1-1-3 Minatojima, Chuo-ku, Kobe 650-8586, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Six new anthraquinone glycosides: digiferruginol-1-methylether-11-O-β-gentiobioside
(1); digiferruginol-11-O-β-primeveroside (2); damnacanthol-11-O-β-primeveroside (3);
1-methoxy-2-primeverosyloxymethyl-anthraquinone-3-olate (4); 1-hydroxy-2-
primeverosyloxymethyl-anthraquinone-3-olate (5); and 1-hydroxy-5,6-dimethoxy-2-methyl-
7-primeverosyloxyanthraquinone (6) were isolated from Morinda citrifolia (Rubiaceae) roots
together with four known anthraquinone glycosides. The structures of the new compounds
were established using spectral methods. For five of the new compounds, the sugar is
attached via the hydroxymethyl group of the anthraquinone C-2 carbon. This type of bond is
rarely found for anthraquinone glycosides isolated from natural sources.
Received 22 September 2008
Accepted in revised form 15 January 2009
Available online 21 February 2009
Keywords:
Morinda citrifolia
Rubiaceae
Noni
Anthraquinone glycoside
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
report herein the structures of the new anthraquinone glyco-
sides (Fig. 1).
The bush Morinda citrifolia L. is a member of the Rubiaceous
family and is widely distributed throughout tropical Asia, India,
and the Pacific islands. In Japan, it is called “Yaeyama-aoki” and
grows in Okinawa Prefecture. Preparations of its roots, stems,
bark, leaves, and fruits have been used as antibacterial, antiviral,
antifungal, antitumor, antihelmin, analgesic, hypotensive, anti-
inflammatory, and immune-enhancing agents; although, their
efficacies are unproven [1]. Noni juice, which is made from M.
citrifolia fruit, has become a popular tonic in recent years since it
is reputed to prevent lifestyle-related diseases, such as diabetes,
hypertension [2], and arteriosclerosis. In Japan and the USA,
noni juice is widely distributed by several companies. As part
of our studies on bioactive natural products, we are very
interested in those compounds that might prevent or amelio-
rate lifestyle-related diseases. Previously, we isolated six
lignanes from the fruit of M. citrifolia that inhibit LDL oxidation
[3], which is part of the process leading to arteriosclerosis. We
also isolated two anthraquinone glycosides from its roots that
decrease blood sugar levels in diabetic mice [4]. Our on-going
investigation of the natural products of M. citrifolia roots has
resulted in the isolation of six new anthraquinone glycosides
and the isolation of four known anthraquinone glycosides. We
2. Experimental
2.1. General procedures and plant material
General experimental procedures and plant material have
been described [4]. HR-ESI-MS were performed with a Bruker
micrOTOF-Q instrument.
2.2. Extraction and Isolation
The extraction and partition processes have been published
[4]. The n-BuOH soluble phase was chromatographed over
Sephadex LH-20 with MeOH as the mobile phase to obtain a
mixture of anthraquinone glycosides. This mixture was then
repeatedly chromatographed over a SiO2 column using EtOAc-
MeOH-H2O (90:7:3–85:10:5–77:13:10) and an Rp-18 column
using MeOH–H2O (1:3–1:2) to separate 1 (32 mg), 2 (20 mg),
3 (48 mg), 4 (10 mg), 5 (14 mg), 6 (3 mg) and the four previously
characterized anthraquinone glycosides.
Digiferruginol-1-methylether-11-O-β-gentiobioside (1): Yel-
low amorphous powder, [α]2D 3 −54.8° (c=0.25, MeOH); HR-
ESI-MS: m/z [M-H]− 591.1710 (calcd for C28H31O14: 591.1719);
IR νmax cm−1: 3391, 2920, 1674, 1553, 1450, 1331, 1275, 1075,
⁎
Corresponding author. Tel.: +81 78 974 1551; fax: +81 78 974 5689.
MeOH
E-mail address: kamiya@pharm.kobegakuin.ac.jp (K. Kamiya).
1050, 1028, 968; UV λ
nm (log ε): 275 (4.12), 255 (4.53),
max
0367-326X/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.fitote.2009.01.014

K. Kamiya et al. / Fitoterapia 80 (2009) 196–199
197
Fig. 1. Chemical structures of compounds 1–6.
209 (4.41); 1H and 13C NMR (400 MHz and 100 MHz, DMSO-d6):
see Tables 1 and 2.
Acid hydrolysis of 4: Compound 4 (6 mg) was treated with
5% HCl (6 ml) under reflux for 6 h. The residue was purified by
column chromatography on Sephadex LH-20 using MeOH to
yield damnacanthol (4a) (2 mg). The solution was neutra-
lized with Amberlite IRA and the resin was filtered off. The
filtrate was purified by column chromatography on SiO2 using
EtOAc-MeOH–H2O (90:7:3–85:10:5) to afford D-glucose
(1 mg) and D-xylose (1 mg).
Digiferruginol-11-O-β-primeveroside (2): Yellow amor-
3
phous powder, [α]2D −23.2° (c=0.03, MeOH); HR-ESI-MS:
m/z [M-H]− 547.1451 (calcd for C26H27O13: 547.1457); IR νmax
cm−1: 3395, 1670, 1632, 1590, 1472, 1425, 1353, 1290, 1079,
MeOH
1048, 1014; UV λ
nm (log ε): 406 (3.56), 254 (4.32), 223
max
(4.19), 202 (4.28); 1H and 13C NMR (400 MHz and 100 MHz,
DMSO-d6): see Table 1 and 2.
Damnacanthol (4a): 1H NMR (400 MHz, DMSO-d6) δ: 3.87
(3H, s, 1-OCH3), 4.57 (2H, s, H-11), 7.52 (1H, s, H-4), 7.84
(1H, td, J 7.5, 1.5 Hz, H-6), 7.90 (1H, td, J 7.5, 1.5 Hz, H-7), 8.11
(1H, dd, J 7.5, 1.5 Hz, H-8), 8.16 (1H, dd, J 7.5, 1.5 Hz); 13C NMR
(100 MHz, DMSO-d6) δ: 182.56 (C-10), 179.98 (C-9), 162.17
(C-3), 161.66 (C-1), 135.37 (C-10a), 134.60 (C-8a), 134.56 (C-7),
133.36 (C-6),132.09 (C-4a),128.80 (C-2),126.67 (C-8),126.10 (C-
5), 117.85 (C-9a), 109.80 (C-4), 62.40 (C-11), 52.15 (1-OCH3). 1-
Hydroxy-2-primeverosyloxymethyl-anthraquinone-3-
olate (5): Yellow amorphous powder, [α]2D3 −64.3°(c=0.1,
MeOH/H2O=1:1); HR-ESI-MS: m/z [M]− 563.1390 (calcd
Damnacanthol-11-O-β-primeveroside (3): Yellow amor-
phous powder, [α]2D 3 −46.2° (c=0.1, MeOH); HR-ESI-MS: m/z
[M-H]− 577.1550 (calcd for C27H29O14: 577.1563); IR νmax cm−1
:
MeOH
3345, 2915, 1667, 1576, 1333, 1281, 1076, 1041, 977; UV λ
max
nm (log ε): 275 (4.32), 245 (4.28), 203 (4.37); 1H and 13C NMR
(400 MHz and 100 MHz, DMSO-d6): see Tables 1 and 2.
1-Methoxy-2-primeverosyloxymethyl-anthraquinone-3-
olate (4): Yellow amorphous powder, [α]2D −35.4° (c=0.1,
3
MeOH/H2O=1:1); HR-ESI-MS: m/z [M]− 577.1548 (calcd for
C
27H29O14: 577.1563); IR νmax cm−1: 3427, 2920,1666,1651,1574,
MeOH
1558, 1330, 1276,1076,1041,1028, 977; UV λ
nm (log ε): 278
for C26H27O14: 563.1406); IR νmax cm− 1: 3429, 1668, 1618,
max
(4.44), 245 (4.36), 203 (4.43); 1H and 13C NMR (400 MHz and
1588, 1454, 1338, 1308, 1282, 1078, 1041, 964; UV λ
nm
MeOH
max
100 MHz, DMSO-d6): see Tables 1 and 2.
(log ε): 418 (3.56), 280 (4.13), 246 (4.22), 203 (4.29); 1H
Table 1
1H NMR spectral data for compounds 1–6 in DMSO-d6.
No.
1
2
3
4
5
6
1-OH
12.78 (s)
14.08 (s)
12.70 (s)
3
4
5
6
7
8
11
8.10 (d, 8.0)
8.03 (d, 8.0)
8.00 (d, 8.0)
7.74 (d, 8.0)
8.20 (m)
7.95 (m)
7.95 (m)
8.25 (m)
4.93 (d, 14.8)
4.76 (d, 14.8)
7.67 (d, 7.7)
7.56 (d, 7.7)
7.49 (s)
6.77 (s)
6.58 (s)
8.15 (dd, 7.5, 1.5)
7.89 (td, 7.5, 1.4)
7.93 (td, 7.5, 1.5)
8.17 (dd, 7.5, 1.4)
5.00 (d, 14.1)
4.82 (d, 14.1)
3.89 (s)
8.08 (dd, 7.5, 1.4)
7.81 (td, 7.5, 1.3)
7.87 (td, 7.5, 1.4)
8.14 (dd, 7.5, 1.3)
4.90 (d, 9.9)
4.56 (d, 9.9)
3.87 (s)
8.01 (dd, 7.5, 1.2)
7.68 (td, 7.5, 1.1)
7.79 (td, 7.6, 1.2)
8.11 (dd, 7.6, 1.1)
4.74 (d, 9.5)
8.04 (dd, 7.6, 1.3)
7.70 (td, 7.6, 1.3)
7.80 (td, 7.6, 1.3)
8.11 (dd, 7.6, 1.3)
4.70 (d, 9.8)
7.81 (s)
2.29 (s)
4.53 (d, 9.5)
3.75 (s)
4.48 (d, 9.8)
1-OCH3
5-OCH3
6-OCH3
1′
2′–6′
1″
3.84 (s)
3.94 (s)
5.17 (d, 7.3)
2.98–3.94 (m)
4.13 (d, 7.1)
2.98–3.94 (m)
4.35 (d, 7.6)
3.01–4.05 (m)
4.31 (d, 7.8)
3.01–4.05 (m)
4.32 (d, 7.6)
3.06–3.95 (m)
4.22 (d, 7.6)
3.06–3.95 (m)
4.32 (d, 7.8)
2.97–3.95 (m)
4.31 (d, 7.5)
2.97–3.95 (m)
4.27 (d, 7.4)
2.99–3.98 (m)
4.38 (d, 7.6)
2.99–3.98 (m)
4.25 (d, 7.7)
2.97–3.96 (m)
4.33 (d, 7.6)
2.97–3.96 (m)
2″–6″ (5″)
Coupling patterns and coupling constants ( J ) in Hz are given in parentheses.

198
K. Kamiya et al. / Fitoterapia 80 (2009) 196–199
skeleton. The 1H NMR spectrum of 1 shows an AA′BB′ coupling
Table 2
13C NMR spectral data for compounds 1–6 in DMSO-d6.
system [δ 8.17 (dd, J=7.5, 1.4 Hz), 8.15 (dd, J=7.5, 1.5 Hz),
7.93 (td, J=7.5, 1.5 Hz), and 7.89 (td, J=7.5, 1.4 Hz)] assigned
to a 1,2-disubstituted phenyl group and ortho-coupled proton
signals [δ 8.10 and 8.03 (d, J=8.0 Hz for both signals)]. These
observations suggest that one of the aromatic rings of the
anthraquinone is unsubstituted and that the other ring
possesses two substituents. Oxygen-bearing methylene proton
signals at δ 5.00 and 4.82 (d, J=14.1 Hz for both signals), a
methoxy proton signal at δ 3.89 (s), and two anomeric proton
signals at δ 4.35 (d, J=7.6 Hz) and δ 4.31 (d, J=7.8 Hz) are
also present. Carbon signals with chemical shifts of 103.36,
102.36, 76.86, 76.76, 76.60, 76.00, 73.56, 73.48, 70.10, 70.07,
68.47, and 61.06 ppm are present in the 13C NMR spectrum of 1
and correspond to those expected for two glucosyl moieties. In
its HMBC spectrum, there is a cross peak between the anomeric
proton signal at δ 4.31 and the C-6′ carbon signal at δ 68.47
(Fig. 2), which is consistent with the presence of a gentiobiosyl
group [5]. The regiochemistry of the oxygen-bearing methy-
lene, methoxy, and gentiobiosyl groups were determined using
the connectivities of the HMBC spectrum. The anomeric proton
signal of the gentiobiose at δ 4.35 correlates with the oxygen-
bearing methylene carbon signal at δ 64.57. Another con-
nectivity is observed between the methoxy proton signal at δ
3.89 and the aromatic carbon signal of C-1 at δ 157.88. Based on
the aforementioned evidence, 1 is digiferruginol-1-methy-
lether-11-O-β-gentiobioside. Compound 2 was also isolated as
yellow amorphous powder and has a molecular formula of
1
2
3
4
5
6
1
2
3
4
4a
5
6
7
8
8a
9
9a
10
10a
11
1-OCH3
5-OCH3
6-OCH3
1′
2′
3′
4′
5′
6′
1″
2″
3″
157.88
140.41
134.36
122.67
134.24 1)
126.21
133.92
134.60
126.75
132.12
181.60
124.97
182.40
134.321)
64.57
158.72
133.87
135.17
118.59
131.91
126.86
134.60
135.08
126.60
133.19
188.58
115.22
181.77
132.79
64.04
162.58
125.82
163.42
110.11
133.95
126.09
133.30
134.652)
126.67
134.612)
179.68
117.24
182.69
132.04
59.35
163.26
125.38
177.89
116.25
135.57
126.14
131.50
133.84
125.11
136.33
176.07
107.71
185.07
132.32
59.92
165.29
115.16
179.33
117.24
133.12
126.12
132.03
134.07
125.43
135.33
178.47
102.00
184.21
132.74
59.62
159.42
132.67
137.45
118.35
132.48
154.51
149.07
154.90
100.05
129.59
187.45
114.44
179.82
121.71
15.58
61.88
62.58
61.31
61.37
61.37
102.36
73.56
76.86
70.10
76.00
68.47
103.36
73.48
76.76
70.07
76.60
61.06
102.53
73.41
76.50
69.86
75.93
68.23
103.90
73.30
76.53
69.52
65.52
102.97
73.38
76.52
69.97
76.07
68.15
103.95
73.38
76.74
69.59
65.66
102.32
72.82
76.60
70.20
76.44
68.44
103.81
73.41
76.60
69.61
65.62
102.33
72.99
76.53
70.20
76.26
68.47
103.82
73.40
76.53
69.60
65.61
100.48
73.203)
76.264)
69.495)
75.85
68.11
104.03
73.163)
76.414)
69.325)
65.55
4″
5″
6″
C
26H28O13 as deduced from the data of its HR-negative-ESI-MS.
1)–5)
Symbols
in each column may be interchanged.
Its 1H NMR spectrum is similar to that of 1, except that the
signal corresponding to the methoxyl group at δ 3.89 is absent;
instead, a signal assignable to a chelated hydroxyl group at δ
12.78 is observed. The NMR data also support the presence of
one unsubstituted anthraquinone aromatic ring and one
disubstituted anthraquinone ring. The 13C NMR spectrum of 2
differs somewhat from that of 1. Signals at 102.53, 73.41, 76.50,
69.86, 75.39, and 68.23 ppm could be assigned to glucosyl
carbons; whereas the 13C signals at 103.90, 73.30, 76.53, 69.52,
and 65.52 ppm are consistent with those expected for a xylosyl
moiety. In the HMBC spectrum of 2, there is a crosspeak in-
volving the xylosyl anomeric proton signal at δ 4.22 and the
glucosyl C-6′ carbon signal at δ 68.23; therefore, a primeverosyl
group exists [6]. The glycosyl–anthraquinone linkage is
β(1′→11) as a cross peak between the anomeric proton at δ
4.32 and the C-11 oxygen-bearing methylene carbon at δ 64.04
exists. Consequently, 2 is digiferruginol-11-O-β-primeveroside.
The molecular formula of compound 3 is C27H30O14 as
determined from the data of its HR-negative-ESI-MS (m/z
577.1550 [M-H]−). For the 1H NMR spectrum of 3, the presence
of an AA′BB′ coupling system and an isolated aromatic proton
and 13C NMR (400 MHz and 100 MHz, DMSO-d6): see
Tables and 2. 1-Hydroxy-5,6-dimethoxy-2-methyl-7-
1
primeverosyloxyanthraquinone (6): Yellow amorphous
powder, [α]2D 3 −131.65° (c=0.1, MeOH); HR-ESI-MS: m/z
[M-H]− 607.1645 (calcd for C28H31O15: 607.1668); IR νmax cm−1
:
3395, 2915, 1653, 1628, 1576, 1569, 1480, 1457, 1358, 1278, 1072,
MeOH
1043, 985; UV λ
nm (log ε): 409 (3.91), 270 (4.56), 219
max
(4.45); 1H and 13C NMR (400 MHz and 100 MHz, DMSO-d6): see
Tables 1 and 2.
3. Results and discussion
Compound 1 was obtained as yellow amorphous powder.
Its methanolic solution has a negative optical rotation ([α]D2 3
−54.8°). The molecular formula of 1 is C28H32O14 as deduced
from the data of its HR-negative-ESI-MS. Its IR spectrum has
absorption bands arising from a hydroxyl group (3391 cm−1),
a conjugated carbonyl group (1674 cm−1), and an aromatic
ring (1553, 1450 cm−1). Its UV spectrum contains absorption
maxima corresponding to those expected for an anthraquinone
Fig. 2. Main HMBC (Arrows) correlations of 1.

K. Kamiya et al. / Fitoterapia 80 (2009) 196–199
199
Fig. 3. Key HMBC (Arrows) correlations of 3 and 4.
signal at δ 7.49 indicates that one anthraquinone aromatic is
unsubstituted and the other ring is trisubstituted. A methoxy
proton signal at δ 3.87 and oxygen-bearing methylene proton
signals at δ 4.90 and 4.56 are also present. That two sugars exist
is indicated by the presence of two anomeric proton signals at δ
4.32 and 4.31. The sugar carbon signals of 3 resemble those of 2,
indicating that the sugar is a primeverosyl moiety. The regio-
chemistry of each functional group was determined using the
data of a HMBC experiment (Fig. 3). The isolated aromatic
proton signal at δ 7.49 correlates with a carbonyl carbon at δ
182.69, proving that the anthraquinone is 1,2,3-trisubstituted.
The anomeric proton signal of the premeverosyl group at δ 4.32
correlates with the oxygen-bearing methylene carbon signal at
δ 59.35. The methoxy proton signal at δ 3.87 correlates with the
aromatic carbon signal at δ 162.58 (C-1). Other HMBC cor-
relations are shown in Fig. 3. From these observations, 3 is
damnacanthol-11-O-β-primeveroside.
As found in the HR-negative-ESI-MS of compound 4, the
parent m/z value is 577.1548 (calcd for C27H29O14: 577.1563),
which is almost the same value as that found for the parent peak
of 3. The 1H NMR spectrum of 4 is similar to that of 3, indi-
cating that the anthraquinone is also 1,2,3-trisubstituted, with a
methoxy group, a primeverosyloxymethyl group, and an ionized
hydroxyl (see below). The types of long-range connectivities
found in the HMBC spectrum of 4 (Fig. 3) are almost identical to
thosefoundin the spectrum of 3; however, theC-3 carbon signal
of 4 is shifted downfield (177.89 ppm) of the corresponding
signal (163.42 ppm) for 3. This downfield shift indicates that
the C-3 hydroxyl is ionized. The presence of ionized hydroxyl
group was supported from obtaining damnacanthol (4a) as
aglycone by the acid hydrolysis of 4. Therefore, 4 is 1-methoxy-
2-primeverosyloxymethyl-anthraquinone-3-olate.
group. An HMBC spectrum of 6 was used to determine the posi-
tions of the aforementioned functional groups on the anthra-
quinone skeleton. The ortho-coupled proton signal at δ 7.67
connects to the signal of a sp2-type carbon that is attached to a
hydroxyl group (δ 159.42) and also connects to a methyl carbon
signal at δ 15.58. The other ortho-coupled proton signal at δ 7.56
correlates with the signal of the carbonyl carbon at δ 179.82.
Connectivities exist between the isolated aryl proton signal at
δ 7.81 and carbon signals at δ 187.45, 154.90, 149.07, 129.59,
and 121.71. The two methoxy proton signals at δ 3.94 and
3.84 correlate with carbon signals at δ 149.07 and 154.51, res-
pectively. The primeverosyl group is linked to the anthraqui-
none C-7 as shown by the existence of a cross peak between the
anomeric proton signal at δ 5.17 and the aromatic carbon signal
at δ 154.90. Therefore, 6 is 1-hydroxy-5,6-dimethoxy-2-methyl-
7-primeverosyloxyanthraquinone. Most interestingly, for the
new anthraquinone glycosides, the sugar moieties of com-
pounds 1–5 are attached to the hydroxymethyl group at the C-2
position of the anthraquinone. This type of glycosidic linkage to
an anthraquinone is rarely found naturally.
The other four anthraquinone glycosides were identified
as lucidin-3-O-β-D-glucoside [7], damnacanthol-3-O-β-D-
glucoside [8], rubiadin-1-methyether-3-O-β-primeveroside
[9], and soranjidiol-6-O-β-primeveroside [10] by comparison
of their spectral data with data obtained from the literature.
Acknowledgments
We are grateful to Misses S. Harada, Y. Okada, A. Takashima,
and Y. Kuroda of Kobe Gakuin University for their excellent
technical assistance.
The m/z of the parent MS peak of 5 is 563.1390 (calcd for
References
C
26H27O14: 563.1406), which is 14 mass units less than that of 4.
The 1H and 13C NMR spectra of 5 mimic those of 4, except that
signals for the methoxy group of 4 (δ 3.75 for the 1H resonance
and δ 61.31 for the 13C resonance) are absent and a signal
for a chelated hydroxyl group (δ 14.08) is present. Thus, 5 is
1-hydroxy-2-primeverosyloxymethyl-anthraquinone-3-olate.
Compound 6 has the molecular formula C28H32O15 as deduced
from the data of its HR-negative-ESI-MS. Ortho-coupled proton
signals at δ 7.67 and 7.56 (d, J=7.7 Hz for both signals) and an
isolated aryl proton signal at δ 7.81 (s) are present in the 1H
NMR spectrum of 6. There are also singlets at δ 3.94, 3.84, 2,29,
and 12.70 corresponding to two methoxy groups, a methyl
group, and a chelated hydroxyl respectively. The 13C-resonances
of the sugar moiety of 6 are those expected for a primeverosyl
[1] Wang MY, West BJ, Jensen CJ, Nowicki D, et al. Acta Pharmacol Sin
2002;23:1127.
[2] Youngken HW, Jenkis HJ, Butler CL. J Am Pharm Assoc 1960;40:271.
[3] Kamiya K, Tanaka Y, Endang H, Umar M, Satake T. J Agric Food Chem
2004;52:5843.
[4] Kamiya K, Hamabe W, Harada S, Murakami R, et al. Biol Pharm Bull
2008;31:935.
[5] Moriyama H, Iizuka T, Nagai M. Yakugaku Zasshi 2001;121:817.
[6] Inoshiri S, Sasaki M, Kohda H, Otsuka H, Yamasaki K. Phytochemistry
1987;26:2811.
[7] El-Emary NA, Backheet EY. Phytochemistry 1998;49:277.
[8] Li B, Zhang DM, Lou YM, Chen XG. Chem Pharm Bull 2006;54:297.
[9] Lu Y, Xu PJ, Chen ZN, Lui GM. Phytochemistry 1998;49:1135.
[10] Demagos GP, Baltus W, Hoefle G, Zeitschrift fuer Naturforschung, Teil B:
Anorganische Chemie, Organische Chemie 1981;36B:1180.