Two novel tetracycles, cassibiphenols A and B from the flowers of Cassia siamea

Article abstract of DOI:10.1016/j.tetlet.2014.01.023

Chemical investigation of the flowers of Cassia siamea (Leguminosae), resulted in the isolation of two novel tetracycles connecting 5-(2-hydroxypropyl)benzene-1,3-diol, cassibiphenols A (1) and B (2). The structures were elucidated by analysis of the 1D, 2D NMR, and HRMS spectra. Synthesis of a tetracyclic core of 1 and 2 led to determine the absolute configuration of 1 and C-12 of 2.

Full text of DOI:10.1016/j.tetlet.2014.01.023

Tetrahedron Letters 55 (2014) 1362–1365
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Two novel tetracycles, cassibiphenols A and B from the flowers
of Cassia siamea
Jun Deguchi a, Tadahiro Sasaki a, Yusuke Hirasawa a, Toshio Kaneda a, Idha Kusumawati b, Osamu Shirota c,
Hiroshi Morita a,
⇑
a Faculty of Pharmaceutical Sciences, Hoshi University, Ebara 2-4-41, Shinagawa-ku, Tokyo 142-8501, Japan
b Faculty of Pharmacy, Airlangga University, Jalan Dharmawangsa Dalam, Surabaya 60286, Indonesia
c Faculty of Pharmaceutical Sciences at Kagawa Campus, Tokushima Bunri University, 1314-1 Shido, Sanuki City, Kagawa 769-2193, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Chemical investigation of the flowers of Cassia siamea (Leguminosae), resulted in the isolation of two
novel tetracycles connecting 5-(2-hydroxypropyl)benzene-1,3-diol, cassibiphenols A (1) and B (2). The
structures were elucidated by analysis of the 1D, 2D NMR, and HRMS spectra. Synthesis of a tetracyclic
core of 1 and 2 led to determine the absolute configuration of 1 and C-12 of 2.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 25 November 2013
Revised 26 December 2013
Accepted 7 January 2014
Available online 13 January 2014
Keywords:
Cassia siamea
Cassibiphenols A and B
Biphenyl
Structure elucidation
Partial synthesis
In our screening study on new antiplasmodial agents from plant
resources, we have succeeded in the isolation of new various
alkaloids,1 sesquiterpenes,2 and limonoids.3 Cassiarin A,4 an
unprecedented tricyclic alkaloid exhibiting potent antimalarial
activity against Plasmodium falciparum in vitro as well as Plasmo-
dium berghei in vivo,5 was isolated from the leaves of Cassia siamea
and has attracted attention of synthetic organic chemists6 as well
as pharmacologists.7 Cassia siamea Lam. (Leguminosae), has been
used widely in traditional medicine, particularly for treatment of
periodic fever and malaria in Indonesia.8 Recently we isolated cas-
siarins G, H, J, and K,9 showing antiplasmodial activity from leaves
of C. siamea, and achieved total synthesis of a novel tetracyclic
alkaloid, cassiarin F isolated from flowers of C. siamea.10 Further
isolation work on extracts from the flowers of C. siamea has led
to purification and structure elucidation of two novel tetracyclic
cassibiphenols A (1) and B (2). In this Letter, we would like to
report the structure elucidation based on spectroscopic analyses
and partial synthesis of 1 and 2. The absolute configuration at
C-12 was established by the comparison of the CD spectra of
tetracyclic core in 1 and 2.
subjected to a silica gel column, an ODS column, an LH-20 column,
and ODS HPLC to give cassibiphenols A (1, 0.00002%)11 and B (2,
0.00002%).12
Cassibiphenol A (1) was obtained as yellowish amorphous
solids. 1 showed the molecular formula C26H24O6, which was
determined by HRESIMS [m/z 433.1655 (M+H)+, +0.4 mmu]. The
optical activity of 1 was confirmed by the positive and negative
Cotton effects at 209 nm and 231 nm of the CD spectrum. IR
absorptions implied the presence of hydroxyl (3742 cmꢀ1) and
carbonyl (1678 cmꢀ1) functionalities. 1H and 13C NMR spectra are
presented in Table 1. The 13C NMR spectrum revealed 26 carbon
signals due to one carbonyl carbon, twelve sp2 quaternary carbons,
six sp2 and two sp3 methines, two sp3 methylenes, and three
methyl groups. Among them, five sp2 quaternary carbons (dC
156.7, 158.1, 159.0, 161.5, and 166.8) and two sp3 methine carbons
(dC 68.6 and 79.0) were ascribed to those bearing an oxygen atom.
The flowers of C. siamea (1.0 kg) were extracted with MeOH, and
the extract was partitioned between EtOAc and 3% aqueous tartaric
acid. The aqueous layer was adjusted at pH 9 with saturated Na2-
CO3 aq and extracted with CHCl3. CHCl3-soluble materials were
⇑
Corresponding author. Tel./fax: +81 354985778.
E-mail address: moritah@hoshi.ac.jp (H. Morita).
0040-4039/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2014.01.023

J. Deguchi et al. / Tetrahedron Letters 55 (2014) 1362–1365
1363
Table 1
166.8. The presence of 5-(2-hydroxypropyl)benzene-1,3-diol was
supported by 1H and 13C NMR data (dH 3.58, 6.26, and 6.34; dC
68.6, 101.6, 109.4, 122.0, 138.3, 156.7, and 159.0) and HMBC corre-
lations of H-110a (dH 2.21) to C-50 (dC 138.3), C-60 (dC 109.4), and
C-100 (dC 122.0) indicating the connection of partial structure c
and C-50. The connectivity of C-40 and C-100 was assigned by a
HMBC correlation of H-30 (dH 7.08) to C-100.
The diastereotopic methylene protons (H2-11) at dH 2.82 and
3.01 were vicinally coupled with H-12 with respective J values of
H-11a (dd, J = 16.3, 16.3 Hz) and H-11b (d, J = 16.3 Hz), which indi-
cated that the former proton was pseudoaxial and the latter was
assignable to pseudoequatorial position. The relative configuration
of 1 was deduced through inspection of a molecular model as well
as the ROESY spectrum (Fig. 2), with ROESY correlations between
H-30/H-110b, H-110a/H3-130, and H3-130/H3-13 indicating a biphenyl
bond at C-40 and C-100 was S⁄-configuration and a secondary hy-
droxyl group at C-120 was R⁄-configuration, and a methyl group
at C-12 was S⁄-configuration as shown in Figure 2. Therefore, Cas-
sibiphenol A (1) was concluded to be a novel tetracycle connecting
5-(2-hydroxypropyl)benzene-1,3-diol.
Cassibiphenol B (2) was obtained as yellowish amorphous solid.
2 showed the similar CD curve and the same molecular formula as
1, C26H24O6, which was determined by HRESIMS [m/z 433.1653
(M+H)+, +0.2 mmu]. 1H and 13C NMR spectra are presented in Ta-
ble 1. The gross structure of 2 was deduced from extensive analysis
of the HMBC spectrum in CD3OD and revealed 2 had the same pla-
nar structure as 1 (Fig. 1). According to the similar CD data of 1 and
2, they were deduced to have the same absolute configuration at
C-12 and/or biphenyl configuration. The two possible relative con-
figurations of 2 were deduced through the difference of the 1H
NMR chemical shifts from H-110 to H-130 of 2 and 1 and ROESY cor-
relations between H-30/H-110a, and H-110a/H3-130 indicating 2 was
a steroisomer at C-120 or a rotational isomer of 1 (Fig. 3). The ste-
reochemistry of the secondary hydroxyl group of 1 and 2 could be
assigned by applying the Mosher method.13 However, the limited
amount of 1 and 2 prohibited the further investigation by the
chemical means.
1H NMR data [dH (J, Hz)] and 13C NMR Data [dC] of cassibiphenols A (1) and B (2) in
CD3OD at 300 K
Cassibiphenol A (1)
Cassibiphenol B (2)
Position dH
dC
dH
dC
2
3
4
5
6
7
158.1
113.5
166.8
137.5
123.1 6.41 (1H, s)
186.0
103.3 6.32 (1H, d, 1.4)
161.5
103.3
34.3 2.79 (1H, dd, 16.6, 13.4)
3.01 (1H, dd, 16.6, 2.9)
79.0 4.45 (1H, m)
20.2 1.16 (3H, d, 6.3)
117.7 7.44 (1H, s)
148.5
131.9 7.06 (1H, s)
139.0
138.3
157.7
113.3
166.8
137.7
123.2
185.9
103.3
161.7
103.3
34.5
6.41 (1H, s)
6.32 (1H, s)
8
9
10
11a
11b
12
13
14
20
2.82 (1H, dd, 16.3, 16.3)
3.01 (1H, d, 16.3)
4.41 (1H, m)
1.16 (3H, d, 6.1)
7.44 (1H, s)
79.5
20.2
117.7
148.2
132.4
139.2
139.3
109.1
158.9
101.3
156.5
121.8
44.1
30
7.08 (1H, s)
40
50
60
6.34 (1H, s)
6.26 (1H, s)
109.4 6.34 (1H, d, 2.0)
159.0
101.6 6.28 (1H, d, 2.0)
156.7
70
80
90
100
110a
110b
120
130
140
122.0
2.21 (1H, dd, 13.0, 5.4)
2.71 (1H, dd, 13.0, 5.4)
3.58 (1H, m)
1.01 (3H, d, 5.9)
2.55 (3H, s)
44.3 2.30 (1H, dd, 13.6, 6.2)
2.49 (1H, dd, 13.6, 6.2)
68.6 3.76 (1H, m)
22.9 0.95 (3H, d, 6.1)
22.0 2.54 (3H, s)
68.9
22.9
22.0
The gross structure of 1 was classified into two units, 3-methyl-
3H-isochromen-6(4H)-one (C-4 to C-13) and a biphenyl unit (C-2,
C-3, C-14, and C-20 to C-140), which were deduced from extensive
analysis of HMBC spectrum in CD3OD (Fig. 1).
Three partial structures, a (C-11 to C-13), b (C-14, C-20, C-30, and
C-140), and c (C-110 to C-130) were deduced from analysis of the
1H–1H COSY spectrum. Connection between partial structure a
and the dienone ring, which form 3-methyl-3H-isochromen-
6(4H)-one, could be assigned by HMBC correlations of H-6 (dH
6.41) to C-8 (dC 103.3), C-10 (dC 103.3), and C-11 (dC 34.3) and
H-8 (dH 6.32) to C-7 (dC 186.0), C-9 (dC 161.5), and C-10, and a four
bond HMBC correlation to C-4 (dC 166.8), and H-11a (dH 3.01) to
C-5 (dC 137.5) and C-10 (dC 103.3). Partial structure b in the
benzene ring, which connected with C-2 and C-3 through C-40
was indicated by HMBC correlations of H-14 (dH 7.44) to C-2 (dC
158.1) and C-3 (dC 113.5) and H-30 (dH 7.08) to C-3 and C-40 (dC
139.0). The ether linkage between C-2 and C-9 could be assigned
by the both down-field shifts of 13C NMR data; dC 161.5 and dC
158.1. The connection of C-3 and C-4 was deduced by the 13C
NMR chemical data of quaternary sp2 carbons; dC 113.5 and dC
Plausible biogenetic pathways for 1 and 2 were proposed as
shown in Scheme 1. 1 and 2 might be derived through a Michael
addition of 5-acetonyl-7-hydroxy-2-methylchromone14 producing
chrobisiamone A,15 followed by cleavage of an ether bond, cycliza-
tion with the
a,b-unsaturated ketone, and finally acid-promoted
ring disclosure16 to produce an isochromen part as shown in
Scheme 1.
Figure 1. Selected 2D NMR correlations for cassibiphenol A (1) and B (2).
Figure 2. Selected ROESY correlations for cassibiphenol A (1).

1364
J. Deguchi et al. / Tetrahedron Letters 55 (2014) 1362–1365
Figure 3. Selected ROESY correlations for cassibiphenol B (2).
Scheme 3. Synthesis of enantiopure 3,4-dihydroisocoumarins, 5a and 5b.
nol 6) needed for assembly of 4a and 4b. Enantiomerically pure
5a and 5b were obtained from 9 by inserting proper stereochemis-
try from the chiral reagent S- or R-propylene oxide (8a and 8b). In
addition, the d-valerolactone could be synthesized through oxa-
Pictet–Spengler cyclization18 and Jones oxidation.19
Our synthesis began with preparation of two enantiomerically
pure MOM-protected S- or R-6-hydroxymellein (5a and 5b), which
was readily accessible from a known bromobenzene 920 synthe-
sized from commercially available 1-bromo-3,5-difluorobenzene.
Regioselective boron trifluoride diethyl etherate promoted ring
opening of 8a and 8b with an aryl anion of 9 to afford the
phenyl-2-propanol 7a and 7b in 70% and 67% yields, respectively.
Condensation of 7a and 7b with trimethyl orthoformate under
p-TsOH condition yielded the isochroman acetal 10a and 10b in
76% and 80% yields, respectively, which were characterized to be
single diastereomers of trans-pyran.21 Jones oxidation of 10a and
10b led to the formation of lactones 11a and 11b, which were
converted to the natural product, S- or R-6-hydroxymellein 12a
and 12b by removal of both aromatic benzyl ether functions under
hydrogenation. Finally, 5a and 5b were obtained by re-protection
of two phenolic alcohols as MOM ethers in 86% and 80% yields,
respectively (Scheme 3).
Scheme 1. Plausible biogenetic pathway for cassibiphenols A (1) and B (2).
Assessment of the enantiomeric excess by HPLC analysis of 11a
and 11b confirmed >99% enantiomeric excess (see the Supplemen-
tary data), and the each positive and negative specific optical rota-
tion of 12a (½a 2D0
ꢁ
+54 (c 0.12, MeOH)) and 12b (½a D20
ꢀ53 (c 0.35,
ꢁ
MeOH) {lit.22
½
a 1D8
ꢁ
ꢀ51 (c 0.10, MeOH)}) and the characteristic Cot-
Scheme 2. Retrosynthetic analysis for tetracyclic core of 1 and 2.
ton effects of S-6-hydroxymellein, 12a (CD (MeOH) kmax (nm)/
De:
234 (+9.5) and 268 (+7.1) {lit.23 CD (MeOH) kmax (nm)/
D
e
: 233
(+10.4) and 268 (+6.7)}, and R-6-hydroxymellein, 12b (CD (MeOH)
In order to determine the absolute stereochemistry of C-12, a
synthesis of a tetracyclic core of cassibiphenols A (1) and B (2),
3a and 3b was undertaken. Our retrosynthetic analysis of 3 is out-
lined in Scheme 2. To construct the 3-methyl-3H-isochromen-
6(4H)-one, left half of the tetracyclic core, 4a and 4b were regarded
as synthetic precursors, which converted to 3 through dehydration
of a tertially alcohol as the similar conversion of barakol to
anhydrobarakol.16 Making the two disconnections generated the
three building blocks (methoxymethyl (MOM)-protected S- or R-
6-hydroxymellein17 5a and 5b, and MOM protected 2-bromophe-
kmax (nm)/
D
e
: 233 (ꢀ9.7) and 268 (ꢀ7.0) {lit.23 CD (MeOH) kmax
(nm)/
configuration.
D
e
: 233 (ꢀ9.0) and 268 (ꢀ8.1)) confirmed the absolute
MOM-protected 2-bromophenol 6, which was produced from
commercially available 2-bromophenol in quantitative yield, was
treated with n-BuLi and allowed to react with 5a and 5b to give
hemiketals 13a and 13b as a mixture of two diastereomers. The
phenolic MOM groups of 13a and 13b were removed by 3 M HCl
aq in MeOH, which occurred through a dehydration of tertially

J. Deguchi et al. / Tetrahedron Letters 55 (2014) 1362–1365
1365
Supplementary data
Supplementary data (experimental details, scanned copies of
NMR spectra including 1H NMR, 13C NMR, 1H–1H COSY, HSQC,
HMBC, ROESY, and chiral HPLC spectra) associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2014.01.023.
References and notes
1. (a) Astulla, A.; Zaima, K.; Matsuno, Y.; Hirasawa, Y.; Ekasari, W.;
Widyawaruyanti, A.; Zaini, N. C.; Morita, H. J. Nat. Med. 2008, 62, 470–472;
(b) Nugroho, A. E.; Hirasawa, Y.; Piow, W. C.; Kaneda, T.; Hadi, A. H. A.; Shirota,
O.; Ekasari, W.; Widyawaruyanti, Y.; Morita, H. J. Nat. Med. 2012, 66, 350–353.
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2009, 17, 727–730.
Scheme 4. Construction of the tetracyclic core, 3a and 3b.
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11. Cassibiphenol A (1): yellow amorphous solid; IR (Zn-Se)
m
max 3742, 3366, 2923,
1678, 1604, and 1558 cmꢀ1; UV (MeOH) kmax 204 (
e
30860), 231 (
e
23410), 268
Figure 4. Comparison of CD spectra of 1, 2, 3a, and 3b.
(
e 12220), 366 (e 6050), and 423 (e 11240) nm; CD (MeOH, 0.00046 M) kmax
e
(D
) 202 (ꢀ2.7), 209 (+10.3), 231 (ꢀ11.5), and 257 (+3.4) nm; 1H and 13C NMR
(Table 1); ESIMS (pos.) m/z 433 (M+H)+; HRESIMS m/z 433.1655 (M+H)+, calcd
for C26H25O6 433.1651.
alcohol forming isochromen skeleton and an intramolecular
dehydrate cyclization to give desired 3a and 3b in 40% and 43%
yields, respectively (Scheme 4).
12. Cassibiphenol B (2): yellow amorphous solid; IR (Zn-Se)
m
max 3730, 3260, 2952,
23117), 270
1683, 1597, and 1544 cmꢀ1; UV (MeOH) kmax 203 (
e
31688), 231 (
e
(
e 12554), 359 (e 5931), and 423 (e 12943) nm; CD (MeOH, 0.00046 M) kmax
e
In the CD spectra of 3a and 3b, two Cotton effects were
observed in methanol. The bands of negative and positive sign
(D
) 203 (ꢀ0.7), 214 (+8.8), 228 (ꢀ6.1), and 242 (+3.7) nm; 1H and 13C NMR
(Table 1); ESIMS (pos.) m/z 433 (M+H)+; HRESIMS m/z 433.1653 (M+H)+, calcd
for C26H25O6 433.1651.
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p⁄ transition
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of 3H-xanthen-3-one chromophore and the helicity of the pyran
ring. Comparison of the CD spectra for 1, 2, 3a, and 3b indicated
the absolute configuration at C-12 of 1 and 2 were S configuration
as in 3a (Fig. 4). According to the relative configuration of 1, we
came to the conclusion of the absolute configuration of 1 as
12S,40S,120R.
In conclusion, structure elucidation of cassibiphenols A (1) and
B (2) consisting of a novel tetracyclic skeleton and a biphenyl unit
was reported and synthesis of a tetracyclic core of 1 and 2 (11%
overall yield for 3a and 3b in 8 steps) for determination of the
absolute configuration of 1 and C-12 of 2 was described. The strat-
egy with two sequences of dehydrate aromatic ring construction
seems to be an innovative approach to the synthesis of the tetracy-
clic core of 1 and 2, which would be potentially useful in the total
synthesis and determination of the absolute configuration of 2.
Studies in this direction are underway.
21. Zheng, H.; Zhao, C.; Fang, B.; Jing, P.; Yang, J.; Xie, X.; She, X. J. Org. Chem. 2012,
77, 5656–5663.
Acknowledgments
22. Islam, M. S.; Ishigami, K.; Watanabe, H. Tetrahedron 2007, 63, 1074–1079.
23. Schlingmann, G.; Roll, D. M. Chirality 2005, 17, S48–S51.
This work was supported in part by Grants-in-Aid for Scientific
Research from Japan Society for the Promotion of Science (JSPS).