Cnidimonins A-C, Three Types of Hybrid Dimer from Cnidium monnieri: Structural Elucidation and Semisynthesis

Article abstract of DOI:10.1021/acs.orglett.7b02290

Three pairs of racemic dimers, (±)-cnidimonins A-C (1-3), were isolated from the fruits of Cnidium monnieri. They represent novel hybrid-dimerization patterns of coumarin skeleton with structurally diverse units (flavonol, benzofuran, and chromone) via an

Full text of DOI:10.1021/acs.orglett.7b02290

Letter
pubs.acs.org/OrgLett
Cnidimonins A−C, Three Types of Hybrid Dimer from Cnidium
monnieri: Structural Elucidation and Semisynthesis
Fangyi Su, Zheng Zhao, Shuanggang Ma, Rubing Wang, Yong Li, Yunbao Liu, Yuhuan Li, Li Li,†
†
†
†
†
†
†
‡
,†
,†
Jing Qu,* and Shishan Yu*
†State Key Laboratory of Bioactive Substance and Function of Natural Medicines, Institute of Materia Medica, Chinese Academy of
Medical Sciences and Peking Union Medical College, Beijing 100050, China
‡
Institute of Medicinal Biotechnology, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing 100050,
China
*
S Supporting Information
ABSTRACT: Three pairs of racemic dimers, (±)-cnidimonins A−C (1−3), were isolated from the fruits of Cnidium monnieri.
They represent novel hybrid-dimerization patterns of coumarin skeleton with structurally diverse units (flavonol, benzofuran, and
chromone) via an unprecedented terminal chiral carbon of prenyl. The absolute configurations of the enantiomers were
determined by electronic circular dichroism (ECD). To investigate their bioactivities in depth, (±)-cnidimonins A−C (1−3)
were synthesized. The racemic mixture (±)-1 exhibited stronger antiviral activity against HSV-1 (IC : 1.23 μM) than its
50
corresponding optically pure enantiomers.
Cnidium monnieri (L.) Cusson (Umbelliferae) is an annual
gregarious herb widely grown in Vietnam, China, Japan, and
Russia. Its fruits have been used as a well-known traditional
Chinese medicine for the treatment of asthma, cough, fungal
Herein, we describe the isolation, structural elucidation,
chiral separation, semisynthesis, and antiviral activity of
(
±)-cnidimonins A−C (1−3, Figure 1).
Cnidimonin A (1) was obtained as a yellow powder. The
+
1
−5
HRESIMS of 1 ([M + H] , m/z 529.1499) combined with the
infection, and arrhythmia.
the isolated constituents from this plant exhibit diverse
Recent studies have shown that
1
13
H NMR and C NMR data indicated a molecular formula of
2
−8
C H O (19 degrees of unsaturation). The IR spectrum
3
0
24
9
biological activities.
For example, osthol, a coumarin
−1
showed the presence of hydroxyl (3262 cm ), carbonyl (1717,
1649 cm ), and aromatic groups (1601, 1512 cm ). The H
NMR spectrum (Table S1) exhibited signals of two pairs of AB-
compound isolated from the fruits of C. monnieri, was found
−
1
−1
1
9
10
to have antihepatitis B virus, 12anti-inflammatory, antidia-
1
1
betic, and antitumor activities.
type doublets δ 7.94, 6.21 (each 1H, J = 9.6 Hz, H-4″, H-3″);
H
To find new bioactive constituents from this plant, three
pairs of enantiomeric natural dimers, (±)-cnidimonins A−C
δ 7.51, 6.97 (each 1H, J = 8.4 Hz, H-5″, H-6″) as well as one
H
methoxyl (δH 3.62, 3H, s), suggesting the presence of a 7-
(1−3), which represent three novel types of hybrid dimers,
13
methoxy-8-substituted coumarin nuclei. The presence of the
coumarin−flavonol, coumarin−benzofuran, and coumarin−
chromone, featuring an unprecedented terminal chiral-carbon
linkage of prenyl, were isolated. Structurally, cnidimonins A−C
possess the same coumarin moiety and only one chiral center at
1
prenyl chain was determined on the basis of the H NMR
signals at δ 6.01, 6.10 (each 1H, d, J = 9.6 Hz, H-11″, 12″) and
H
two methyls (δ 1.63, 1.57, each 3H, s), corresponding to the
H
13
C NMR signals at δ 31.0, 125.3, 129.3, 17.5, 25.4, which was
C
(
C-11″), indicating that they were derived from a common
verified by the HMBC correlations of H -15″/C-12″, C-13″;
3
biosynthesis precursor of osthol. Although their unprecedented
structure piqued our interest, it was difficult to investigate their
bioactivities in depth due to the scarcity of material. Thus, it
was necessary to develop a synthetic strategy for the
cnidimonins.
H -14″/C-13″; H-11″/C-12″, and C-13″. In addition, the
3
remaining aromatic singlet at δ
6.12 (1H, s) as well as four
H
Received: August 2, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02290
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Organic Letters
Letter
Figure 1. Structures of (±)-cnidimonins A−C.
protons belonging to a para-substituted phenyl were assigned to
Figure 3. Experimental ECD spectra of (+)-1/(−)-1, (+)-2/(−)-2,and
(+)-3/(−)-3 with calculated ECD spectra of 1a-(11″S)/1b-(11″R),
14,15
4′,5,7-trihydroxyflavonol (kaempferol)
substituted at C-6
2
a-(11″R)/2b-(11″S), and 3a-(11″S)/3b-(11″R).
or C-8. In the HMBC experiment (Figure 2), the attachment of
showed a total of 27 carbon resonances. Except for an osthol
moiety, the 13C NMR data showed the presence of one
carbonyl carbon, six aromatic carbons from a benzene ring, two
olefinic carbons, two methylenes, and one methoxy. The above
data, combined with the degrees of unsaturation, indicated the
presence of a benzofuran moiety, which was confirmed by the
HMBC correlations of H-1′/C-4, C-5; H-6/C-1′. Additionally,
the coupling system detected on the COSY and HSQC spectra
1
1
Figure 2. Selected H− H COSY and HMBC correlations for 1.
consisted of two methylenes at δ 2.87 (2H, t, J = 7.5 Hz, 15.0
H
Hz, H-7) and δ 2.57 (2H, t, J = 7.5 Hz, 15.0 Hz, H-8). The
H
presence of the C7−C9 unit was verified by the HMBC
the prenyl group to the kaempferol moiety was deduced to be
via C-11″−C-8 based on HMBC correlations of H-11″with C-7
correlations of H -7/C-9. Moreover, the C7−C9 unit was
2
determined to be attached to C-1 through C-7 by the HMBC
(
δ 161.7), C-8 (δ 107.3), and C-9 (δ 154.5). Additionally,
C C C
correlations from H-7 to C-1 (δ 123.8), C-2 (δ 153.2), and
C
C
the linkage of the prenyl group to coumarin moiety through C-
1″−C-8″ was further supported by the observed HMBC
C-6 (δ 121.2) (Figure 4). The linkage of the osthol moiety
C
1
correlations of H-11″ to C-7″ (δ 160.1), C-8″ (δ 119.4), and
C
C
C-9″ (δ 152.5). Thus, the planar structure of 1 was
C
determined as depicted (Figure 1). Despite the presence of a
stereogenic center at C-11″, measurement of the optical
2
5
rotations of 1 gave a [α]
value near zero, and no Cotton
D
effects were observed in its ECD spectra, indicating that 1 was a
racemic mixture. Subsequently, chiral resolution of 1 was
performed on chiral column to yield (+)-cnidimonin A [(+)-1]
and (−)-cnidimonin A [(−)-1] in equal amounts (see Figure
S57). The CD spectra of (+)-1 and (−)-1 revealed opposite
Cotton effects, confirming their enantiomeric relationship. The
absolute configurations of (+)-1 and (−)-1 were determined by
comparison of the experimental and calculated ECD spectra. As
shown in Figure 3, the calculated ECD spectra for (11″R)-1b
and (11″S)-1a were consistent with the experimental ECD
spectra for (−)-1 and (+)-1, respectively. Accordingly, the
absolute configuration of the enantiomers were determined as
1
1
Figure 4. Selected H− H COSY and HMBC correlations for 2.
and the benzofuran moiety via C-11″-C-2′ was established by
the HMBC correlations of a methine proton at δ 5.73 (H-
H
11″) with C-1′ (δ 102.7), C-2′ (δ 158.0), and C-8″ (δ
C
C
C
117.3). Thus, the planar structure of 2 was established as shown
in Figure 1. In contrast to 1, 2 also possessed a stereogenic
center at C-11″. (+)-Cnidimonin B [(+)-2] and (−)-cnidimo-
nin B [(−)-2] were successfully obtained in a ratio of
approximately 1:1 by using HPLC on a chiral column (see
Figure S57). The absolute configuration of each enantiomer
was determined by calculation of the ECD spectrum. As shown
in Figure 3, the absolute configuration of (−)-2 was assigned as
11″R and that of (+)-2 as 11″S.
(
+)-(11″S)-1 and (−)-(11″R)-1.
Cnidimonin B (2) was isolated as a white, amorphous
powder. Its molecular formula was established as C H O by
2
7
26
7
+
HRESIMS ([M + H] , m/z 463.1751), corresponding to 15
degrees of unsaturation. The IR spectrum of 2 showed the
presence of hydroxyl (3329 cm ), carbonyl (1732 cm ),
Cnidimonin C (3) was purified as a white, amorphous
powder. It possessed a molecular formula of C H O with 16
degrees of unsaturation, as deduced from HRESIMS ([M +
H] , m/z 519.2016). The IR spectrum of 3 showed the
presence of hydroxyl (3401 cm ), carbonyl (1729 cm ),
−
1
−1
30
30
8
−1
aromatic groups (1605, 1460 cm ). The comparison of NMR
+
data (see Table S1) of 1 and 2 suggested that the structure of 2
1
3
−1
−1
should possess an osthol moiety. The C NMR spectrum of 2
B
DOI: 10.1021/acs.orglett.7b02290
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
aromatic groups (1604, 1495 cm ). Analysis of its NMR data
Letter
−1
Scheme 1. Synthetic Route of 7
(
see Table S1) suggested 3 possessed the same osthol moiety
1
3
as 1 and 2. Except for the osthol moiety, the C NMR
spectrum revealed 15 carbon resonances ascribed to one
carbonyl carbon, six aromatic carbons from a benzene ring, four
olefinic carbons, one methylene, two methyls, and one
hydroxymethyl. The NMR data of 3 exhibited signals
corresponding to a 6-substituted 5,7-dihydroxy-2-methylchro-
1
6
mone ring, which was confirmed by comparison with
literature data of related structures. Additionally, the HMBC
correlations of H -5′/C-2′, C-3′, C-4′; H -4′/C-2′, and H -1′/
3
2
2
C-5, C-6, C-7 verified the presence of the prenyl chain and its
Scheme 2. Synthetic Route to 11
location at C-6 (Figure 5). The key HMBC correlations of a
Alkylation of 12 with prenyl bromide led to the known
20
compound 13, which was oxidized with selenium dioxide in
5% ethanol solution to afford 14. The key intermediate 15 can
9
be constructed from 14 over two steps via acetylation with
1
1
Figure 5. Selected H− H COSY, HMBC, and NOE correlations for
acetic anhydride in pyridine and aldol reaction by NaH
21
3.
(
Scheme 3).
Scheme 3. Synthetic Route to 15
methine proton at δ 6.05 (H-11″) with C-7 (δ 162.1), C-8
H
C
(
δ 108.3), and C-9 (δ 155.7) suggested that the osthol
C C
moiety was linked to chromone moiety via C-11″−C-8.
Meanwhile, the C-2′/C-3′ olefin was assigned Z-geometry
based on NOE correlations of H-2′ with H -5′ and H-1′a, and
3
H -4′ with H-1b (Figure 5). Thus, the planar structure of 3 was
2
established as shown in Figure 1. Three was a racemate based
on the lack of optical activity. Subsequently, 3 was separated
into two equal amounts of optically pure enantiomers,
(
+)-cnidimonin C [(+)-3] and (−)-cnidimonin C [(−)-3]
using HPLC on a chiral column (see Figure S57). The absolute
configurations of (+)-3 and (−)-3 were established by
comparing of the calculated ECD spectra with experimental
spectra. From the above evidence, the absolute stereochemistry
for (+)-3 (11″R) and (−)-3 (11″S) were unambiguously
determined as shown in Figure 3.
With 7, 8, 11, and 15 in hand, we next focused on their
coupling. Condensation of 7 with 8, 11, and 15 in the presence
of Nafion 50 was accomplished in methanol solution at room
temperature for 12 h to afford the racemic dimers (±)-1−3 in
22
1
5%, 12%, and 10% yield, respectively (Scheme 4). Spectral
The novel structures of cnidimonins A−C were intriguing.
However, it is difficult to further explore their bioactivities due
to the scarcity of material, which makes the synthesis of
cnidimonins extraordinarily meaningful. In this paper, the
synthesis of cnidimonins A−C was efficiently achieved. One
common characteristic of the structures is that each cnidimonin
derivative possesses the same moiety 7, which simplifies the
retrosynthetic analysis (see Figure S56) and makes it
reasonable to propose the synthetic strategy of connecting 7
with 8, 11, and 15, especially since 8 (kaempferol) is
commercially available.
data for 1−3 were identical in all respects to the natural
Scheme 4. Synthesis of (± )-cnidimonins A-C (1-3)
Osthol 4 was isolated from C. monnieri. Following the
17
Sharpless dihydroxylation of 4 with an AD-mix-β, diol 5 was
obtained in high yield at room temperature for 12 h.
Subsequent reaction of 5 with N,N′-carbonyldiimidazole
(
CDI) afforded the cyclic carbonate 6, which was converted
into the key intermediate 7 by reaction with DBU in
methylbenzene at 120 °C for 12 h (Scheme 1).
18
Starting from commercially available psoralen 9, 11 can be
accomplished in 41% yield over two steps by sequences of
hydrolyzation with MeONa in dry methanol and reduction by
19
magnesium powder (Scheme 2).
C
DOI: 10.1021/acs.orglett.7b02290
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
cnidimonins A−C, respectively (see Figures S47−S55).
Synthetic racemates 1−3 were successfully separated by
HPLC using a CHIRALPAK AD-H column to obtain the
optically pure enantiomers.
The synthetic racemic mixtures of cnidimonins A−C as well
as their enantiomers were tested for antiviral (HSV-1) activity
*E-mail: yushishan@imm.ac.cn.
ORCID
Yunbao Liu: 0000-0002-1338-0271
Shishan Yu: 0000-0003-4608-1486
Notes
(see Table 1). Each enantiomer was obtained with enantio-
The authors declare no competing financial interest.
Table 1. Antiviral Activity against HSV-1 and Cytotoxicity of
Compounds in Vero Cells
ACKNOWLEDGMENTS
■
a
This work was supported by grants from the CAMS Innovation
Fund for Medical Science (CIFMS: 2016-I2M-1-010) and the
National Natural Science Foundation of China (No.
1573321). We are grateful to the Department of Instrumental
Analysis of Peking Union Medical College for the spectroscopic
measurements.
b
c
compd
TC50 (μM)
IC50 (μM)
SI
(
(
(
(
(
(
(
(
(
±)-1
+)-1
−)-1
±)-2
+)-2
−)-2
±)-3
+)-3
−)-3
2.14
19.25
16.02
23.11
23.11
23.11
100
1.23
6.41
1.7
3.0
4.3
8
3.70
>11.11
11.11
>11.11
>33.33
>33.33
>100
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: qujing@imm.ac.cn.
D
DOI: 10.1021/acs.orglett.7b02290
Org. Lett. XXXX, XXX, XXX−XXX