Hypercohin A, a new polycyclic polyprenylated acylphloroglucinol possessing an unusual bicyclo[5.3.1]hendecane core from Hypericum cohaerens

Article abstract of DOI:10.1039/c2cc32352d

Hypercohin A (1), an unprecedented polycyclic polyprenylated acylphloroglucinol featuring with an unusual bicyclo[5.3.1]hendecane core, was isolated from Hypericum cohaerens. Its structure and absolute configurations were elucidated by extensive NMR methods and crystal X-ray diffractions of its p-bromobenzoate ester (2).

Full text of DOI:10.1039/c2cc32352d

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COMMUNICATION
Hypercohin A, a new polycyclic polyprenylated acylphloroglucinol
possessing an unusual bicyclo[5.3.1]hendecane core from
Hypericum cohaerensw
Xing-Wei Yang,ab Xu Deng,ab Xia Liu,ab Chun-Yan Wu,a Xiao-Nian Li,a Bin Wu,a
Huai-Rong Luo,a Yan Li,a Hong-Xi Xu,c Qin-Shi Zhaoa and Gang Xu*a
Received 1st April 2012, Accepted 18th April 2012
DOI: 10.1039/c2cc32352d
Hypercohin A (1), an unprecedented polycyclic polyprenylated
acylphloroglucinol featuring with an unusual bicyclo[5.3.1]hendecane
core, was isolated from Hypericum cohaerens. Its structure and
absolute configurations were elucidated by extensive NMR methods
and crystal X-ray diffractions of its p-bromobenzoate ester (2).
Natural polycyclic polyprenylated acylphloroglucinols (PPAPs),
with highly oxygenated and densely substituted bicyclo[3.3.1]-
nonane-2,4,9-trione or other related core structures decorated
with prenyl or geranyl side chains, are a special class of complex
natural products that have been only isolated from plants of the
family Guttiferae so far.1 This kind of metabolites show a wide
variety of biological activities such as antitumor, antimicrobial,
Fig. 1 Structures of hypercohin A (1) and its derivative (2).
anti-HIV, antioxidant, and especially in the CNS as modulators
of neurotransmitters associated with neuronal damage and
depression.1,2 In recent years, the fascinating chemical structures
and intriguing biological activities of PPAPs have attracted
crystal X-ray diffractions of its p-bromobenzoate ester (2). In
the bioactive studies, both 1 and 2 showed moderate inhibitory
activities on acetylcholinesterase (AChE). In addition, compound
1 displayed moderate cytotoxic activities against five human
widespread attention from phytochemical, organic synthetic,
and pharmacological endeavors.3
cancer cell lines in vitro (IC50 10.4–24.7 mM).
The plants of genus Hypericum (Guttiferae), occurring
widely in temperate regions, have been used as traditional
medicine in many countries all over the world.4 H. cohaerens
N. Robson is an endemic plant distributed in Guizhou and
Yunnan provinces, P. R. China,5 and has not been phytochemically
studied so far. As a part of our systematic search for new and
bioactive natural PPAPs from Guttiferae plants,6 the chemical
constituents of this plant were studied and hypercohin A (1), an
unprecedented PPAP featured with an unusual bicyclo[5.3.1]-
hendecane core, was isolated (Fig. 1). The structure of 1 was
elucidated using a combination of NMR spectroscopy and the
Hypercohin A (1) was obtained as colorless oil. Its molecular
formula C38H48O5 was established by the positive HRESIMS
(m/z 585.3578, [M + H]+), indicating 15 degrees of unsaturation.
The IR spectrum showed obvious absorption bands for hydroxyl
(3436 cmꢀ1), carbonyl (1716, 1686, and 1651 cmꢀ1), and
olefinic (1618 cmꢀ1) groups, respectively. The 1H NMR spectrum
revealed the presence of a monosubstituted benzene ring
(dH 7.74, 2H, d, J = 8.3 Hz; 7.52, 1H, t, J = 7.5 Hz; and
7.34, 2H, dd, J = 8.3, 7.5 Hz), four olefinic protons (dH 5.89,
1H, d, J = 9.8 Hz; 5.13, 1H, t, J = 7.5 Hz; 4.99, 1H, t, J = 6.6 Hz;
4.73, 1H, t, J = 7.9 Hz), and nine methyls (dH 1.28–1.73, s). The
13C NMR and DEPT spectra (Table 1) resolved 38 carbon signals
due to eight quaternary carbons (including two ketones, three
olefinic, and an oxygenated one), four methines (one oxygenated
and one olefinic), one methylene, three methyls, and 22 other
signals assignable to a benzoyl and three isoprenyl groups.
Careful analysis of these data indicated that the characteristic
signals for a PPAP type metabolite of a nonconjugated
carbonyl (dC 206.2, C-9), an enolized 1,3-diketone group
(dC 168.3, C-2; 125.9, C-3; 201.2, C-4), two quaternary carbons
at dC 75.9 (C-1) and 63.9 (C-5), a methine at dC 40.2 (C-7), and
a State Key Laboratory of Phytochemistry and Plant Resources in
West China, Kunming Institute of Botany, Chinese Academy
of Sciences, Kunming 650204, P. R. China.
E-mail: xugang008@mail.kib.ac.cn; Tel: +86-871-5217971
b Graduate University of the Chinese Academy of Sciences,
Beijing 100049, P. R. China
c Shanghai University of Traditional Chinese Medicine,
Shanghai 201203, P. R. China
w Electronic supplementary information (ESI) available: 1D, 2D
NMR spectra, detailed experimental procedures of 1. CCDC 872701
(2). For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2cc32352d
c
5998 Chem. Commun., 2012, 48, 5998–6000
This journal is The Royal Society of Chemistry 2012

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Table 1 1H and 13C NMR data for 1 in CD3ODa
No. dH (mult, J in Hz) dC No. dH (mult, J in Hz) dC
1
2
3
4
5
6
75.9 20
1.73 (s)
1.64 (s)
2.69 (dd, 13.2, 7.9)
2.36 (overlapped)
4.73 (t, 7.9)
26.3
18.2
40.9
168.3 21
125.9 22
201.2
63.9 23
44.3 24
25
40.2 26
144.4 27
206.2
196.4 28
138.3 29
129.6 30
129.4 31
133.9 32
129.4 33
129.6 34
23.4 35
36
119.4
137.0
26.6
17.9
33.8
2.35 (br d, 12.8)
1.40 (t, 12.8)
2.61 (m)
Fig. 2 Key HMBC (arrows) and 1H–1H COSY (bold) correlations of 1.
1.57 (s)
1.39 (s)
2.08 (m)
2.01 (m)
4.99 (t, 6.6)
7
8
9
The relative configurations of 1 were elucidated by analysis
of the ROESY spectrum. Diagnostic cross peaks between
H-16 and H-23 indicated that the benzoyl and isoprenyl
groups at C-5 were both located on the same face of the
molecule, which permitted the b-orientations of both C-10 and
C-22. In addition, the ROESY correlations of H-17/Me-37,
Me-37/H-34, and H-34/H-7 indicated that H-7, H-34, and
Me-37 were a-oriented. Then, H-35 was determined to be
b-oriented by its ROESY correlations with Me-38.
To determine the absolute configurations and confirm the
unique structure of 1 furthermore, other solid evidence such as
X-ray diffraction analysis was necessary. Since 1 was obtained
as oil, the p-bromobenzoate derivative (2) was prepared by its
esterification.3 The single-crystal X-ray study of 2z unambiguously
confirmed the structure of 1 and clarified the absolute configura-
tions of 1 as 1S, 5R, 7S, 34S, and 35R (Fig. 3).
10
11
12
13
14
15
16
17
123.5
134.3
26.0
18.2
19.2
127.2
52.5
73.9
92.8
20.6
28.7
7.74 (d, 8.3)
7.34 (dd, 8.3, 7.5)
7.52 (t, 7.5)
7.34 (dd, 8.3, 7.5)
7.74 (d, 8.3)
3.22 (dd, 13.6, 7.5)
3.18 (dd, 13.6, 7.5)
5.13 (t, 7.5)
1.71 (s)
1.62 (s)
1.63 (s)
5.89 (d, 9.8)
3.67 (dd, 10.9, 9.8)
4.28 (d, 10.9)
18
19
121.9 37
134.1 38
1.28 (s)
1.29 (s)
a
Data were recorded on a Bruker AM-600 spectrometer.
a methylene at dC 44.3 (C-6) can be easily distinguished.6,7
These observations, conjugated with the fact that a number of
PPAPs have been isolated from Hypericum species, indicated
that 1 could be ascribed as being a PPAP type derivative.
Comparative analysis of its 1D NMR spectral data with those of
the normal PPAPs with a bicyclo[3.3.1]nonane core displayed some
significant differences. It is obvious that the characteristic signals for
A
plausible biogenetic pathway for 1 was proposed
(Scheme 1). Biogenetically, 1 should be derived from the
normal PPAPs such as uralodins B and C which have been
also isolated in this study.7c An intermediate (M) was derived
from oxidation and dehydration of uralodin B or C, which was
further proceeded by a key C[1, 3]s migration rearrangement
reaction to obtain an expanded octatomic ring B, and then
followed by dehydration, keto–enol tautomerism, and further
cyclization to afford 1.
a normal PPAP at C-8 (usually present between dC 47–52 in the 13
NMR spectrum) as well as the angular methyl (Me-32, dC 12–16,
H 0.9–1.2, s), the methylene (C-33, dC 35–38, dH 1.3–2.1), and the
C
d
isoprenyl side chain substituted at C-8 were absent in 1.6,7 Instead,
eight unusual signals including two quaternary carbons (dC 144.4
and 92.8), three methines (dC 127.2, dH 5.89, d, J = 9.8 Hz; dC 52.5,
dH 3.67, dd, J = 10.9, 9.8 Hz; and dC 73.9, dH 4.28, d, J = 10.9 Hz),
and three methyls (dC 19.2, dH 1.63; dC 20.6, dH 1.28; and dC 28.7,
dH 1.29) were present. These observations indicated that the carbon
skeleton of ring B should be different.
The 2D NMR data of 1 deduced the linkage of C-6/C-7/C-8/
C-33/C-34/C-35/C-36/C-37(C-38) by the HMBC correlations
from Me-32 to C-7, C-8 and C-33, from Me-37 and Me-38 to
1
C-35 and C-36, and the H–1H COSY correlations of H-6/H-7
and H-33/H-34/H-35. The evidence mentioned above, conjugated
with the HMBC correlations of H-34 with C-1, C-2, and C-9; H-6
with C-4, C-5, and C-9; as well as H-7 with C-5 and C-6
established the unique eight-membered ring B of 1.
Fig. 3 X-ray crystallographic structure of 2 (displacement ellipsoids
are drawn at the 30% probability level).
The ether ring C was formed by the connection of C-2 and
C-36 through an oxygen atom, which was elucidated by the
weak HMBC correlations from both Me-37 and Me-38 to C-2
coupled with the degrees of the unsaturation. In addition, the
HMBC correlations of H-17 with C-2, C-3, and C-4; H-22 with
C-4, C-5, C-6, and C-9; H-27 with C-6, C-7, and C-8, as well as
H-34 with C-10, coupled with the proton spin systems H-17/
H-18, H-22/H-23, and H-6/H-7/H-27/H-28 can all be found,
which not only established the connection of the benzoyl and
three isoprenyl groups at C-1, C-3, C-5, and C-7, respectively,
but also confirmed the core structure of 1 furthermore. Then, the
planar structure of 1 was elucidated as shown (Fig. 2).
Scheme 1 Plausible biogenetic pathway of 1.
Chem. Commun., 2012, 48, 5998–6000 5999
c
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Table 2 Cytotoxicity of 1 against human cancer and normal cell lines
Raising Foundation of Yunnan Province (No. 2009CI073),
and the foundation from CAS to Dr G. Xu.
IC50/mM
HL-60 SMMC-7721 A-549 MCF-7 SW480 Beas-2B
Notes and references
1
2
13.4
>40
13.8
>40
9.6
10.4
>40
10.0
24.1
>40
24.6
24.7
>40
23.5
17.8
—
8.6
z Crystal data for 2: C45H51BrO6, M = 767.77, triclinic, a =
9.0921(12) A, b = 9.5542(12) A, c = 12.8573(17) A, a = 72.794(2)1,
b = 70.787(2)1, g = 78.211(2)1, V = 1000.4(2) A3, T = 100(2) K,
space group P1, Z = 1, m(MoKa) = 1.074 mmꢀ1, 13 227 reflections
measured, 9544 independent reflections (Rint = 0.0217). The final R1
values were 0.0357 (I > 2s(I)). The final wR(F2) values were 0.1030
(I > 2s(I)). The final R1 values were 0.0391 (all data). The final wR(F2)
values were 0.1042 (all data). The goodness of fit on F2 was 1.037.
Flack parameter = 0.013(5). CCDC 872701.
DDPa 3.3
a
Positive control.
The AChE inhibitory activity of 1 and 2 was assayed using
the Ellman method.8 Both 1 and 2 showed moderate inhibitory
activities (inhibition percentages were 41.7% and 23.0%,
respectively) at the concentration of 50 mM. In addition,
compounds 1 and 2 were also tested for their cytotoxic effects
against five human cancer cell lines, HL-60, A-549, SMMC-7721,
MCF-7, SW480, and the human normal bronchial epithelial cell
line (Beas-2B), using the MTT method described previously.9
Interestingly, 1 showed moderate toxicities, while 2 exhibited no
inhibitory activity (Table 2).
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To the best of our knowledge, compound 1 can be seen as
the first PPAP featured with an eight-membered ring B. The
core structure of 1 is an unusual bicyclo[5.3.1]hendecane
accompanied by a six-membered ether ring, which is really rare
in natural products. Only two classes of famous diterpenoids,
i.e., taxol and pleuromutilin, together with their analogues have
been reported to possess a similar bicyclo[5.3.1]hendecane core
so far.10 As for the oxygen-bridge between C-2 and C-36, it has
been also never reported to be present in natural PPAPs before.
In the structural elucidation of natural PPAPs, determining the
absolute configurations was always a tough task to complete.
Biosynthetically, PPAPs were derived from monocyclic polypreny-
lated acylphloroglucinols (MPAPs) by the cyclization of the prenyl
group at C-5 to C-1, which permitted the different configurations
of C-1 and C-5.1 In addition, natural PPAPs were usually obtained
as oil or gum since polyprenyl substitutions existed in the
molecules, which led to the difficulties in the crystallization of
these metabolites. Based on the detailed investigation, the
appropriate single crystals of less than ten PPAPs for X-ray
diffraction analysis have been reported to be achieved, and
only one of these X-ray studies has determined the absolute
configurations of the target molecule.6c,11 Despite the absolute
configurations of two normal PPAPs have been determined by
comparison of their electronic circular dichroism measurements
with those values predicted by DFT calculations,2d X-ray
analysis is still the most effective and powerful method. For
the compounds obtained as oil or gum, preparing derivatives
has been proved to be a useful method to make the products or
intermediates more crystallizable.12 In 2010, the absolute
configurations of two adamantane derivatives, which can be
derived by further cyclization of PPAPs, have been published
in ‘‘J. Am. Chem. Soc.’’ by preparing their p-bromobenzoate
ester.3 This method was also used in this study, which can be
seen as the first use of this method in the determination of
absolute configurations of natural PPAPs.
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This work was supported financially by the foundations
from NSFC (20972167 and 81173485), the 973 programs
(2009CB522300 and 2011CB915503), the Young Academic Leader
12 E. A. Couladouros, M. Dakanali, K. D. Demadis and V. P. Vidali,
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c
6000 Chem. Commun., 2012, 48, 5998–6000
This journal is The Royal Society of Chemistry 2012