Total synthesis of Palmarumycin BGs, C1 and Guignardin e

Article abstract of DOI:10.1039/c9ra10316c

The first total synthesis of Palmarumycin BG1-3, BG5-6, C₁ and Guignardin E (1-7) were achieved by the same intermediate Palmarumycin C₂ through a N-benzyl cinchoninium chloride-catalyzed epoxidation, an organoselenium-mediated reduction, and a cerium(iii) chloride hydrate-promoted regioselective ring-opening and elimination of cyclic α,β-epoxy ketone as the key steps via6-7 step routes using 1,8-dihydroxynaphthalene (DHN) and 5-methoxytetralone as the starting materials in overall yields of 1.0-17.4%, respectively. Their structures and absolute configurations were characterized and determined by ¹H, ¹³C NMR, IR, HR-ESI-MS and X-ray diffraction data. These compounds displayed significant inhibition activities against HCT116, U87-MG, HepG2, BGC823 and PC9 cell lines.

Full text of DOI:10.1039/c9ra10316c

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Total synthesis of Palmarumycin BGs, C1 and
Guignardin E†
Cite this: RSC Adv., 2020, 10, 1588
a
Xinlei Liu,
Shuyi Li,a Xinyu Wei,a Yu Zhao,a Daowan Lai,b Ligang Zhoub
a
*
and Mingan Wang
The first total synthesis of Palmarumycin BG1–3, BG5–6, C1 and Guignardin E (1–7) were achieved by the
same intermediate Palmarumycin C2 through a N-benzyl cinchoninium chloride-catalyzed epoxidation, an
organoselenium-mediated reduction, and a cerium(III) chloride hydrate-promoted regioselective ring-
opening and elimination of cyclic a,b-epoxy ketone as the key steps via 6–7 step routes using 1,8-
dihydroxynaphthalene (DHN) and 5-methoxytetralone as the starting materials in overall yields of 1.0–
17.4%, respectively. Their structures and absolute configurations were characterized and determined by
1H, 13C NMR, IR, HR-ESI-MS and X-ray diffraction data. These compounds displayed significant inhibition
activities against HCT116, U87-MG, HepG2, BGC823 and PC9 cell lines.
Received 9th December 2019
Accepted 19th December 2019
DOI: 10.1039/c9ra10316c
rsc.li/rsc-advances
its limited access in the fermentation extract of the endophytic
fungus Berkleasmium sp., and the importance of the halogen
atom in the A-ring.22,23 the total synthesis and structure revision
Introduction
The members of the spirobisnaphthalene family showed broad
bioactivities, such as antifungal, antimicrobial, antitumor,
anticancer, antiparasitic, anti-inammatory, and cytotoxic
activities.1–4 Palmarumycin BG1–3, and BG5–6 (Fig. 1) were
previously isolated from the leaves and stems of the Bruguiera
gymnorrhiza plant, which were collected from the Zhanjiang
mangrove national nature protection area.5 Guignardins A–F
and Palmarumycin C1 were isolated from the endophytic fungus
Guignardia sp. KcF8, and the mangrove unidentied fungus
BCC 25903, respectively.6,7 Among these compounds, Palmar-
umycin BG5 showed excellent cytotoxicities against human
breast carcinoma MCF-7 with IC50 7.6 mM and promyelocytic
leukemia HL60 with IC50 1.9 mM, Guignardins E, F and Pal-
marumycin C1 exhibited signicant cytotoxicities against 10
human tumor cell lines, such as MCF-7 with LD50 8.79, 6.48 and
3.08 mM, HL60 with LD50 2.94, 3.06 and 2.90 mM, and HeLa with
LD50 1.32, 0.38 and 1.24 mM et al.6 Many reports related to the
total synthesis, structure modication, and biological activity
evaluation of the spirobisnaphthalene natural products have
appeared in recent years.8–20 The synthesis of Palmarumycin
CP17 and its analogues were completed in our laboratory, and
the bioassay results showed that they have antifungal activity
against several phytopathogens.21 Because of the interesting
larvicidal activity of Palmarumycin B6 against Aedes albopictus,
of Palmarumycin B6 were achieved, and found that the location
of the chlorine atom plays a crucial role for the larvicidal activity
against A. albopictus in the previous report.24 The antitumor
activities of Palmarumycin BG5, Guignardin E and Palmar-
umycin C1 aroused our enthusiasm to gain insights into the
structure–activity relationships of these spirobisnaphthalene
compounds and explore their mode of action. As such, the rst
total synthesis of Palmarumycin BGs, Guignardin E and Pal-
marumycin C1 (1–7) were carried out, and the results would be
presented in this paper.
Results and discussion
The retro-synthetic analysis of Palmarumycin BGs and
Guignardin E (1–6) is shown in Scheme 1. Palmarumycin BG2,
BG3, BG5 and BG6 can be obtained through reduction of Pal-
marumycin BG1 and Guignardin E. Palmarumycin BG1 and
Guignardin E can be synthesized by the selective reduction and
ring-opening of Palmarumycin C2, which was easily prepared by
the stereoselectivity epoxidation of Palmarumycin CP1. Pal-
marumycin CP1 could be obtained from a ketalization of 5-
methoxytetralone with 1,8-dihydroxynaphthalene (DHN) in
a similar process, such as that used in the synthesis of Pal-
marumycins CP17 and B6.21,24
The synthesis of Palmarumycin BG1–6 and Guignardin E (1–
6) started with 5-methoxy-3,4-dihydronaphthalen-1(2H)-one
and 1,8-dihydroxynaphthalene (DHN), as shown in Scheme 2.
Compound 8 was obtained via the direct ketalization of 5-
methoxytetralone with 1,8-dihydroxynaphthalene (DHN).
Oxidation of compound 8 with pyridinium dichromate (PDC)
aDepartment of Applied Chemistry, College of Sciences, China Agricultural University,
Beijing 100193, People's Republic of China. E-mail: wangma@cau.edu.cn
bDepartment of Plant Pathology, College of Plant Protection, China Agricultural
University, Beijing 100193, People's Republic of China
† Electronic supplementary information (ESI) available. CCDC 1947664. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c9ra10316c
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Fig. 1 Structures of Palmarumycin BG1–3, BG5–6, Guignardin E, and Palmarumycin C1 (1–7).
and t-BuOOH in benzene gave compound 9, which was further 78% yields, respectively. The NaBH4 reduction of the mixture of
oxidized with DDQ to produce 10. Then, 10 was demethylated Guignardin E (6) and its C-3 epimer (13) afforded Palmarumycin
with B-bromocatecholborane to afford the target compound BG5 (4), BG6 (5) and the C-4 epimer (14) of 4 in 23%, 9% and
Palmarumycin CP1 (11) following the literature procedures.24–27 60% yields, respectively, but the C-4 epimer (15) of 5 was not
Next, the stereoselective epoxidation with t-BuOOH of Palmar- obtained because of the limited quantity of 13 in the mixture
umycin CP1 catalyzed with N-benzylcinchoninium chloride at and the hindrance effect in the NaBH4 reduction. So we
ꢀ
25 C yielded Palmarumycin C2 (12) in 90.7% ee value. The ee deduced rationally that Guignardin E (6) should be the
value was improved to be 97.9% at 0 ꢀC.28 The organoselenium- biosynthetic precursor of Palmarumycin BG5 (4) and BG6 (5).
mediated reduction of Palmarumycin C2 afforded Palmar- Treatment of Palmarumycin C2 (11) with cerium(III) chloride
umycin BG1 (1) as the sole product in 92% yield, and the hydrate at reux temperature afforded Palmarumycin C1 (7) in
conguration of C-2 did not change.29–31 The NaBH4 reduction 79% yield.34 One possible mechanism is that cerium(III) chloride
of 1 resulted in a mixture of Palmarumycin BG2 (2) and BG3 (3), is hydrolyzed to produce hydrochloride in situ. Then, the
which were isolated in silica gel to give Palmarumycin BG2 (2) regioselective epoxide ring-opening can occur between Pal-
and BG3 (3) in 56% and 30% yields, respectively. Treatment of marumycin C2 and hydrochloride to give the mixture of chlo-
Palmarumycin C2 (12) with the diluted hydrogen chloride at rohydrins Guignardin E (6) and its C-3 epimer (13). A
25 ꢀC afforded a mixture of Guignardin E (6) and its C-3 epimer subsequent loss of water could then lead to obtain Palmar-
(13) in 95% yield (dr value: 5.7 : 1).32 When this transformation umycin C1 at reux temperature. This approach avoided the use
was performed at 10 ꢀC, the dr value was improved to 25 : 1. of the strong acid, which removes the possibility of breaking the
Recrystallization of this mixture yielded pure Guignardin E (6). ketal bond.20 When the mixture of 6 and 13 was treated with
Guignardin E as the unnatural product was mentioned in diluted hydrochloride in methanol or methanol/water at the
mixture form of Guignardin E and its epimer.32,33 but it was rst reux temperature, a complex mixture of products were ob-
isolated as the natural product from the fungus Guignardia sp. tained that could not be puried. Palmarumycin C1, which was
KcF8.6 The NaBH4 reduction of Guignardin E (6) afforded Pal- reported to exhibit antifungal activity against phytopathogen
marumycin BG5 (4) and the C-4 epimer (14) of 4 in 22% and Fusarium oxysporum and cytotoxicities against 10 human tumor
Scheme 1 Retro-synthetic analysis of Palmarumycin BG1–6 and Guignardin E.
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Scheme 2 Synthesis of Palmarumycin BG1–6, Guignardin E (1–6) and Palmarumycin C1 (7). Reagents and conditions: (a) TsOH, toluene, reflux,
72 h, 57%; (b) PDC, Celite, t-BuOOH, benzene, rt, 24 h, 74%; (c) DDQ, benzene, reflux, 82%; (d) B-bromocatecholborane, DCM, 0 ꢀC, 75%; (e) N-
benzylcinchoninium chloride, t-BuOOH, NaOH (0.1 M), toluene/H2O, 0 ꢀC, 73%; (f) (PhSe)2, NaBH4, AcOH, EtOH, 0 ꢀC, 92%; (g) HCl (1 M), THF/
H2O, 10 ꢀC, 3 day, 96% for 6 + 13, dr ¼ 25 : 1; (h) NaBH4, MeOH, rt, 56% for 2 and 30% for 3, dr ¼ 2 : 1; (i) NaBH4, MeOH, rt, 23% for 4, 9% for 5, and
60% for 14; (j) CeCl3$7H2O, MeOH/H2O, reflux, 79%.
cell lines with LD50 values in the range of 1.24–39.2 mM, was those of C-2, C-3 and C-4 of Palmarumycin BG5 (4) were 2S, 3R
isolated from the fungus Guignardia sp. KcF86, and Con- and 4S, those of C-2, C-3 and C-4 of Palmarumycin BG6 (5) were
iothyrium sp.32
2S, 3S and 4R, and those of C-2 and C-3 of Guignardin E (6) were
Aer nishing the synthesis of the desired compounds, all 2S and 3S, all of these absolute congurations were consistent
structures were characterized with the 1H, 13C NMR, IR and HR- with those of the natural products.5,6 Because the
ESI-MS data. In order to conrm the absolute conguration, the organoselenium-mediated reduction of the a,b-epoxy ketone
X-ray diffraction analysis of 14 was performed using Cu Ka did not change the conguration of the b-carbon,29–31 so the
radiation and its structure was depicted in Fig. 2, which absolute congurations of C-2 and C-3 of 12 would be 2R and 3S,
unambiguously showed that the absolute conguration of C-2, which are in agreement with those of natural Palmarumycin
C-3 and C-4 were 2S, 3R and 4R in Flack parameter 0.006(16) C2,32,33 and Guignardin F,6 implies that Palmarumycin C2 and
(CCDC ID 1947664). Based on this result, the C-2 absolute Guignardin F are the same compound.
conguration of Palmarumycin BG1 (1) was 2R, the congura-
The cytotoxic activities of 1–7 and 14 against the growth of
tion of C-2 and C-4 of Palmarumycin BG2 (2) were 2R and 4S, tumor cell lines (HCT116, U87-MG, HepG2, BGC823 and PC9)
those of C-2 and C-4 of Palmarumycin BG3 (3) were 2R and 4R, were evaluated using a MTT assay22 and the results were shown
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Compounds 1–4, 6–7 and 14 displayed signicant inhibition
activity against HCT116, U87-MG, HepG2, BGC823 and PC9 cell
lines.
Experimental procedures
General experimental procedures
Melting points (uncorrected) were measured on a WRX-4
microscopic melting point apparatus (Shanghai Yi Ce Instru-
1
ment Factory). All H and 13C NMR spectra were obtained on
a Bruker DPX 300 spectrometer with CDCl3 and CD3OD as
solvents and TMS as an internal standard. HR-ESI-MS spectra
were analyzed on a Bruker Apex II mass spectrometer. IR spectra
were recorded with a Shimadzu IRTracer 100 FT-IR spectrom-
eter (KBr plate). Optical rotation values were measured with
a JASCO P-2000 Polarimeter. The crystal structure was analyzed
with a Thermo Fisher ESCALAB 250 four-circle X-ray diffrac-
tometer (Xcalibur, Eos, Gemini). Unless otherwise stated, all
reagents were purchased from commercial suppliers and used
without further purication. Organic solutions were concen-
trated under reduced pressure using a rotary evaporator or oil
pump. Flash column chromatography was performed using
Qingdao Haiyang silica gel (200–300 mesh). HPLC analyses were
performed on an Agilent 1100 instruments, UV detection was
monitored at 254 and 220 nm, an IB N-5 column (5 mm, 4.6 ꢁ
250 mm) was used as the chiral stationary phase, and hexane/i-
PrOH (90 : 10) was used as the mobile phase at a ow rate of 1.0
Fig. 2 ORTEP drawing of Palmarumycin BG5 C-4 epimer (14).
Table 1 Cytotoxic activities of compounds 1–7 and 14 (IC50, mM)
Compounds HCT116
U87-MG
HepG2
BGC823
PC9
1
2
3
4
9.14
30.08
29.12
6.71
>50
19.40
>50
26.10
9.62
>50
20.36
35.56
28.81
4.21
>50
19.15
>50
24.55
6.25
14.87
30.50
22.10
5.58
5
>50
>50
6
7
14
3.70
2.63
6.62
2.45
2.60
19.64
4.06
6.05
9.64
4.36
14.08
12.06
7.42
15.39
14.37
Taxol
0.000616 0.000245 0.00475 0.000642 0.000823
mL minꢂ1
.
Synthesis of intermediates 8–12
in Table 1. These results indicated that compounds 1–4, 6–7 and
14 exhibited signicant cytotoxicity against several tumor cell
lines with an IC50 in the range of 2.45–30.50 mM, while
compound 5 was inactive against above mentioned cancer cells
(IC50 > 50 mM). When comparison the IC50 data of compounds
1–3 with compounds 4, 6–7 and 14, we found that the
compounds bearing a chlorine atom at the C-3 position
exhibited stronger cytotoxic activity than those compounds
without the chlorine atom at C-3 position. In the other aspect,
the reduction products of carbonyl group at C-4 such as 2 and 3,
4, 5 and 14 exhibited weaker cytotoxic activity than those
retaining C-4 carbonyl compounds 1 and 6. These results indi-
cated that the chlorine atom at C-3 and the carbonyl at C-4 play
a critical role for cytotoxicity.23,24
The intermediates 8–11 were synthesized following the litera-
ture procedures, and their analytical data were consistent with
the reported data.25–27
Palmarumycin CP1 (11, 120 mg, 0.38 mmol) was added into
a toluene (10 mL) solution of N-benzylcinchoninium chloride
(48 mg, 0.114 mmol, 0.3 eq.) in a 50 mL round-bottom ask.
NaOH solution (0.1 M; 5.7 mL, 1.5 eq.) was added dropwise in
the mixture, followed by the addition of t-BuOOH (0.524 mL,
7.2 M, 3.77 mmol) at 0 ꢀC (the ice-water bath) and stirred for 6 h.
Aer completion of the reaction, the solution was diluted with
EtOAc (50 mL), washed with 0.2 M HCl solution (2 ꢁ 20 mL),
brine, and the organic phase was dried over anhydrous Na2SO4.
The solvent was removed and the crude product was puried by
ash column chromatography on silica gel (petroleum ether/
EtOAc, 20 : 1) to give Palmarumycin C2 (12, 100 mg, 79%) as
a yellow solid.28 The ee value of the key intermediate 12 was
Conclusions
In summary, Palmarumycin BG1–3, BG5–6, Guignardin E and analyzed by HPLC to be 97.9%. Compound 12, a yellow solid,
Palmarumycin C1 (1–7) were rst synthesized in overall yields of mp 218–220 ꢀC (lit. 11 mp 219–221 ꢀC; lit. 28 mp 225 ꢀC; lit. 32
1.0–17.4% through a N-benzylcinchoninium chloride-catalyzed mp 228 ꢀC); [a]D20 ¼ ꢂ335 (c ¼ 0.66, CHCl3), (lit. 11 [a]D20 ¼ ꢂ340,
epoxidation, an organoselenium-mediated reduction of the c ¼ 1.00, CHCl3; lit. 28 [a]2D0 ¼ ꢂ300, c ¼ 1.00, CH2Cl2; lit. 32
1
a,b-epoxy ketone, and a cerium(III) chloride hydrate-promoted [a]2D0 ¼ ꢂ341, c ¼ 1.00, CHCl3); H NMR (300 MHz, CDCl3) d:
ring-opening and elimination as the key steps via 6–7 step 11.37 (1H, s), 7.64 (1H, dd, J ¼ 8.4, 8.0 Hz), 7.62–7.50 (3H, m),
routes using 1,8-dihydroxynaphthalene (DHN) and 5-methox- 7.45 (2H, dd, J ¼ 8.0, 7.5 Hz), 7.19 (1H, d, J ¼ 7.5 Hz), 7.14 (1H, d,
ytetralone as the starting materials. Their structures and abso- J ¼ 8.4 Hz), 6.92 (1H, d, J ¼ 7.5 Hz), 4.09 (1H, d, J ¼ 4.0 Hz), 3.68
lute congurations were characterized by 1H, 13C NMR, IR, HR- (1H, d, J ¼ 4.0 Hz); 13C NMR (75 MHz, CDCl3) d: 196.65, 162.00,
ESI-MS data and X-ray crystallographic data (CCDC 1947664). 147.07, 146.81, 137.77, 137.02, 134.31, 127.90, 127.76, 121.56,
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121.46, 120.17, 119.18, 112.92, 112.41, 110.28, 109.45, 96.12, ꢂ108 (c ¼ 0.64, CHCl3), (lit. 5 [a]1D6 ¼ ꢂ40, c ¼ 0.055, CHCl3); 1H
53.40, 53.36; IR nmax 3426, 3051, 1651, 1612, 1454, 1412, 1381, NMR (300 MHz, CDCl3) d: 7.55 (1H, d, J ¼ 8.4 Hz), 7.52 (1H, d, J
1269, 1238, 1177, 1115, 1065, 968, 876, 806, 752 cmꢂ1; ESI-MS, ¼ 8.4 Hz), 7.48 (1H, d, J ¼ 7.4 Hz), 7.45 (1H, dd, J ¼ 7.6, 1.5 Hz),
m/z: 333 [M + H]+. These data were identical with the published 7.42 (1H, dd, J ¼ 8.4, 7.5 Hz), 7.40 (1H, dd, J ¼ 8.0, 7.6 Hz), 7.09
data.11,28,32
(1H, dd, J ¼ 7.6, 1.5 Hz), 7.04 (1H, d, J ¼ 7.0 Hz), 6.90 (1H, s),
6.87 (1H, dd, J ¼ 7.5, 0.3 Hz), 5.0 (1H, dd, J ¼ 10.7, 5.3 Hz), 4.43
(1H, d, J ¼ 3.7 Hz), 3.40 (1H, d, J ¼ 11.2 Hz), 2.55 (1H, dd, J ¼
15.3, 5.6 Hz), 2.51 (1H, s), 2.40 (1H, ddd, J ¼ 15.3, 4.5, 1.7 Hz);
13C NMR (75 MHz, CDCl3) d: 155.65, 147.33, 146.99, 134.34,
132.08, 130.30, 127.83, 127.67, 124.88, 121.40, 120.95, 119.65,
118.24, 113.47, 109.43, 109.10, 99.68, 65.86, 63.41, 31.89; IR nmax
3210, 3059, 2928, 1636, 1609, 1474, 1412, 1377, 1339, 1269,
1119, 1042, 972, 891, 822, 790, 756 cmꢂ1; HR-ESI-MS, m/z:
Synthesis of (2R)-2,5-dihydroxy-2,3-dihydro-4H-spiro
[naphthalene-1,20-naphtho[1,8-de] [1,3]dioxin]-4-one
(Palmarumycin BG1, 1)
The reduction reagent (Na[PhSeB(OEt)3]) was prepared
following the protocol in the literature.29,30 (PhSe)2 (245 mg, 0.79
mmol) and EtOH (15 mL) were added into a 100 mL round-
bottom ask at 0 ꢀC, then NaBH4 (60 mg, 1.58 mmol) was
gradually added into the mixture. Aer vigorous evolution of
hydrogen ceased and NaBH4 was thoroughly consumed, AcOH
(0.108 mL, 1.89 mmol) was added and the mixture was stirred
for 10 min to obtain the reduction reagent solution. Then
a solution of Palmarumycin C2 (12, 131 mg, 0.39 mmol) in
EtOH/AcOH (1 : 1, 10 mL) was added into the reduction reagent
solution, stirred for 10 min and removed from the ice-water
bath. Aer completion of the reaction, the solution was
diluted with EtOAc (100 mL), washed with brine, and the
organic phase was dried over anhydrous Na2SO4. The solvent
was removed under reduced pressure, and the crude product
was puried by ash column chromatography on silica gel
(petroleum ether/EtOAc, 5 : 1) to give Palmarumycin BG1 (1,
121 mg, 92%) as a white solid. mp 73–75 ꢀC; [a]D20 ¼ ꢂ152 (c ¼
0.60, CHCl3); (lit. 5 [a]D17 ¼ ꢂ151, c ¼ 0.50, CHCl3); 1H NMR (300
MHz, CDCl3) d: 12.35 (1H, s), 7.57 (1H, d, J ¼ 8.0 Hz), 7.54 (2H, d,
J ¼ 8.4 Hz), 7.49 (1H, dd, J ¼ 8.0, 7.6 Hz), 7.44 (1H, dd, J ¼ 8.4,
7.6 Hz), 7.34 (1H, dd, J ¼ 8.0, 1.1 Hz), 7.08 (2H, d, J ¼ 8.4 Hz),
6.91 (1H, d, J ¼ 7.6 Hz), 4.59 (1H, s), 3.24 (1H, dd, J ¼ 17.8, 4.0
Hz), 2.94 (1H, dd, J ¼ 17.8, 4.0 Hz), 2.38 (1H, s); 13C NMR (75
MHz, CDCl3) d: 201.12, 162.21, 147.24, 146.42, 138.03, 137.16,
134.30, 127.82, 127.78, 121.52, 121.22, 119.94, 118.10, 115.46,
113.26, 109.66, 108.97, 98.83, 67.36, 41.40; IR nmax 3433, 3059,
2924, 1643, 1608, 1585, 1454, 1412, 1381, 1346, 1269, 1234,
1165, 1119, 1069, 976, 891, 822, 756 cmꢂ1; HR-ESI-MS, m/z:
C
20H16O5 [M–H]ꢂ, calcd. 335.0925, found: 335.0954. Palmar-
umycin BG3 (3), mp 145–147 ꢀC; [a]D20 ¼ ꢂ262 (c ¼ 0.56, MeOH);
(lit. 5 [a]1D7 ¼ ꢂ261, c ¼ 0.14, acetone); 1H NMR (300 MHz,
CD3OD) d: 7.51 (1H, dd, J ¼ 8.4, 1.2 Hz), 7.49 (1H, dd, J ¼ 8.4, 1.2
Hz), 7.45 (1H, dd, J ¼ 8.4, 7.3 Hz), 7.42 (1H, dd, J ¼ 8.4, 7.3 Hz),
7.20 (2H, d, J ¼ 4.2 Hz), 7.01 (1H, dd, J ¼ 7.2, 1.3 Hz), 6.92–6.85
(2H, m), 5.30 (1H, dd, J ¼ 8.8, 6.2 Hz), 4.31 (1H, dd, J ¼ 5.8, 2.2
Hz), 2.43 (1H, ddd, J ¼ 13.4, 6.1, 6.1 Hz), 2.28 (1H, ddd, J ¼ 13.5,
8.8, 2.3 Hz); 13C NMR (75 MHz, CD3OD) d: 157.16, 149.40,
148.74, 135.89, 135.54, 129.94, 128.68, 128.42, 126.38, 121.38,
121.30, 119.77, 117.52, 114.69, 110.47, 109.22, 100.75, 67.62,
65.21, 36.18; IR nmax 3152, 2924, 2851, 1636, 1605, 1474, 1412,
1377, 1315, 1269, 1115, 1072, 972, 883, 822, 802, 756 cmꢂ1; HR-
ESI-MS, m/z:
C
20H16O5 [M–H]ꢂ, calcd. 335.0925, found:
335.0939. These data were identical with the published data.5
Synthesis of (2S,3S)-3-chloro-2,5-dihydroxy-2,3-dihydro-4H-
spiro [naphthalene-1,20-naphtho[1,8-de] [1,3]dioxin]-4-one
(Guignaridin E, 6)
A solution of HCl (1 M, 10 mL) was added into a mixture of
Palmarumycin C2 (12, 32 mg 0.10 mmol) and THF (12 mL) in
a 50 mL round-bottom ask at 25 ꢀC, and the mixture was
stirred at room temperature for 24 h. The solution was diluted
with EtOAc (50 mL), washed with brine, and the organic phase
was dried over anhydrous Na2SO4. The solvent was removed,
and the crude product was puried by ash column chroma-
tography on silica gel (petroleum ether/EtOAc, 2 : 1) to give the
mixture of Guignardin E and its C-3 epimer (6 and 13, 34 mg,
95%) as a yellow solid (dr ¼ 5.7 : 1).
C
20H14O5 [M–H]ꢂ, calcd. 333.0768, found: 333.0791. These data
were consistent with the published data.5,6
Synthesis of (2R,4S)-3,4-dihydro-2H-spiro[naphthalene-1,20-
naphtho[1,8-de][1,3]dioxine]-2,4,5-triol and (2R,4R)-2,3-
dihydro-2H-spiro[naphthalene-1,20-naphtho[1,8-de][1,3]
dioxine]-2,4,5-triol (Palmarumycin BG2 and BG3, 2 and 3)
A solution of HCl (1 M, 10 mL) was added into a palmar-
umycin C2 (12, 91 mg, 0.27 mmol) and THF (20 mL) in a 100 mL
round-bottom ask at 0 ꢀC, and the mixture was stirred at 10 ꢀC
NaBH4 (38 mg, 1.00 mmol) was added into a mixture of pal- for 72 h. Workup as routine, and the crude product was puried
marumycin BG1 (1, 107 mg, 0.32 mmol) and MeOH (10 mL) in by ash column chromatography on silica gel (petroleum ether/
a 25 mL round-bottom ask at 0 ꢀC. The mixture was stirred at EtOAc, 2 : 1) to afford the mixture of Guignardin E and its C-3
room temperature for 2 h. The solution was extracted with epimer (98 mg, 96%) as a yellow solid (dr ¼ 25 : 1). The
EtOAc (2 ꢁ 30 mL). The organic phase was washed with brine, mixture (dr ¼ 25 : 1) was recrystallized repeatedly with meth-
and dried over anhydrous Na2SO4. The solvent was removed anol to provide Guignardin E (6, 61 mg, 60%) as a yellow solid,
20
1
ꢀ
under reduced pressure, the residue was subjected to ash mp 178–180 C; [a]D ¼ ꢂ191 (c ¼ 0.53, CHCl3); H NMR (300
column chromatography on silica gel and eluted with petro- MHz, CDCl3) d: 11.84 (1H, s), 7.66 (1H, t, J ¼ 8.1 Hz), 7.61 (1H, d,
leum ether/EtOAc (3 : 1) to afford palmarumycin BG2 (2, 61 mg, J ¼ 8.1 Hz), 7.58 (1H, d, J ¼ 8.1 Hz), 7.52 (1H, dd, J ¼ 8.5, 7.4 Hz),
56%) as a white solid and palmarumycin BG3 (3, 31 mg, 30%) as 7.48 (1H, d, J ¼ 8.5 Hz), 7.46 (1H, dd, J ¼ 8.5, 8.4 Hz), 7.16 (1H,
20
ꢀ
a white solid. Palmarumycin BG2 (2), mp 189–191 C; [a]D
¼
dd, J ¼ 7.4, 1.0 Hz), 7.15 (1H, dd, J ¼ 7.4, 1.0 Hz), 6.93 (1H, dd, J
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¼ 7.5, 0.8 Hz), 5.31 (1H, d, J ¼ 2.2 Hz), 4.70 (1H, dd, J ¼ 2.8, 2.7 solution was extracted with EtOAc (2 ꢁ 30 mL). The organic
Hz), 2.68 (1H, d, J ¼ 2.7 Hz); 13C NMR (75 MHz, CDCl3) d: 194.14, phase was washed with brine and dried over anhydrous Na2SO4.
162.17, 146.74, 146.10, 138.03, 137.48, 134.34, 127.93, 127.84, The solvent was removed under reduced pressure, the residue
121.85, 121.70, 120.46, 118.83, 114.28, 113.06, 110.13, 109.24, was subjected to ash column chromatography on silica gel and
98.69, 72.34, 62.55; IR nmax 3471, 1663, 1609, 1585, 1458, 1412, eluted with petroleum ether/EtOAc (2 : 1) to afford Palmar-
1377, 1265, 1196, 1172, 1115, 1065, 987, 894, 818, 760, 694 cmꢂ1
;
umycin BG5 (4, 37 mg, 23%), the C-4 epimer of Palmarumycin
HR-ESI-MS, m/z: C20H13ClO5 [M–H]ꢂ, calcd. 367.0379, found: BG5 (14, 96 mg, 60%), and Palmarumycin BG6 (5, 15 mg, 9%) as
20
367.0397. These data were consistent with the published data.6 a yellow solid. Palmarumycin BG6 (5), mp 164–166 C; [a]D
¼
ꢀ
1
+56 (c ¼ 0.50, CHCl3), (lit. 5 [a]2D0 ¼ +60, c ¼ 0.04, CHCl3); H
NMR (300 MHz, CDCl3) d: 8.13 (1H, s), 7.51 (1H, d, J ¼ 8.1 Hz),
7.50 (1H, d, J ¼ 8.3 Hz), 7.45 (1H, dd, J ¼ 8.2, 7.5 Hz), 7.41 (1H,
dd, J ¼ 8.1, 7.7 Hz), 7.18 (1H, dd, J ¼ 8.0, 8.0 Hz), 7.03 (1H, d, J ¼
7.9 Hz), 6.96 (2H, d, J ¼ 8.0 Hz), 6.90 (1H, d, J ¼ 7.4 Hz), 5.32
(1H, d, J ¼ 7.7 Hz), 4.63 (1H, dd, J ¼ 8.7, 7.8 Hz), 4.38 (1H, d, J ¼
8.7 Hz), 3.51 (1H, brs), 2.64 (1H, brs); 13C NMR (75 MHz, CDCl3)
d: 156.04, 148.06, 147.12, 134.08, 134.04, 130.56, 127.68, 127.66,
120.93, 120.76, 119.21, 118.87, 118.53, 112.85, 108.72, 108.49,
Synthesis of (2S,3R,4S)-3-chloro-3,4-dihydro-2H-spiro
[naphthalene-1,20-naphtho[1,8-de] [1,3]dioxine]-2,4,5-triol,
(2S,3R,4R)-3-chloro-3,4-dihydro-2H-spiro[naphthalene-1,20-
naphtho [1,8-de][1,3]dioxine]-2,4,5-triol and (2S,3S,4R)-3-
chloro-3,4-dihydro-2H-spiro [naphthalene-1,20-naphtho[1,8-
de][1,3]dioxine]-2,4,5-triol (Palmarumycin BG5, its C-4 epimer
and BG6, 4, 14 and 5)
NaBH4 (7 mg, 0.18 mmol) was added into a solution of 99.32, 74.58, 73.73, 64.35; IR nmax 3319, 2933, 1609, 1585, 1465,
Guignardin E (6, 23 mg, 0.06 mmol) in MeOH (1.5 mL) in 1415, 1381, 1344, 1266, 1181, 1115, 1062, 986, 893, 796,
a 10 mL round-bottom ask. The mixture was stirred at room 756 cmꢂ1
;
HR-ESI-MS, m/z: C20H15ClO5 [M–H]ꢂ, calcd.
temperature for 2 h. The solution was extracted with EtOAc (2 ꢁ 369.0535, found: 369.0547. These data were in agreement with
10 mL). The organic phase was washed with brine, and dried the reported data.5
over anhydrous Na2SO4. The solvent was removed under
reduced pressure, the residue was subjected to ash column
chromatography on silica gel and eluted with petroleum ether/
EtOAc (2 : 1) to afford Palmarumycin BG5 (4, 5 mg, 22%) as
Synthesis of 3-chloro-5-hydroxy-4H-spiro[naphthalene-1,20-
naphtho[1,8-de][1,3]dioxin]-4-one (Palmarumycin C1, 7)
a white solid and its C-4 epimer (14, 18 mg, 78%) as a white CeCl3$7H2O (64 mg, 0.172 mmol), Palmarumycin C2 (12, 52 mg,
solid. Palmarumycin BG5 (4), mp 108–110 ꢀC; [a]D20 ¼ ꢂ316 (c ¼ 0.157 mmol), MeOH (9 mL), and H2O (3 mL) were added in
1
0.96, CHCl3), (lit. 5 [a]1D6 ¼ ꢂ314.7, c ¼ 0.75, CHCl3); H NMR a 25 mL round-bottom ask. The mixture was stirred at reux
(300 MHz, CDCl3) d: 8.38 (1H, s), 7.59 (1H, d, J ¼ 8.5 Hz), 7.55 temperature for 16 h, cooled to room temperature and the
(1H, dd, J ¼ 8.5, 0.9 Hz), 7.49 (1H, dd, J ¼ 8.3, 7.5 Hz), 7.45 (1H, mixture was diluted with EtOAc (50 mL). The organic phase was
dd, J ¼ 7.6, 7.6 Hz), 7.43–7.37 (2H, m), 7.09 (1H, dd, J ¼ 7.4, 0.9 washed with brine, and dried over anhydrous Na2SO4. The
Hz), 7.07 (1H, dd, J ¼ 7.0, 1.5 Hz), 6.92 (1H, dd, J ¼ 7.5, 1.0 Hz), solution was concentrated under vacuum. The crude product
5.47 (1H, d, J ¼ 9.1 Hz), 4.68 (1H, dd, J ¼ 9.1, 1.9 Hz), 4.47 (1H, d, was puried by ash column chromatography on silica gel
J ¼ 1.9 Hz), 3.23 (1H, s), 2.40 (1H, s); 13C NMR (75 MHz, CDCl3) (petroleum ether/EtOAc, 20 : 1) to give palmarumycin C1 (7,
d: 155.95, 147.08, 146.35, 134.36, 132.68, 130.90, 127.85, 127.80, 41 mg, 79%) as a yellow solid. mp 272–273 ꢀC (lit. 32 mp > 280
121.73, 121.25, 120.03, 119.88, 119.19, 113.19, 109.75, 109.33, ꢀC); 1H NMR (300 MHz, CDCl3) d: 11.87 (1H, s), 7.69 (1H, dd, J ¼
99.82, 71.77, 70.86, 63.95; IR nmax 3325, 2963, 2920, 2851, 1609, 8.4, 7.5 Hz), 7.61 (2H, dd, J ¼ 8.4, 1.0 Hz), 7.49 (2H, dd, J ¼ 8.4,
1462, 1412, 1381, 1342, 1265, 1107, 1022, 891, 799, 756 cmꢂ1
;
7.5 Hz), 7.47 (1H, dd, J ¼ 7.5, 1.0 Hz), 7.18 (1H, dd, J ¼ 8.4, 1.0
HR-ESI-MS, m/z: C20H15ClO5 [M–H]ꢂ, calcd. 369.0535, found: Hz), 7.17 (1H, s), 7.00 (2H, dd, J ¼ 7.5, 1.0 Hz); 13C NMR (75
369.0548. These data were consistent with the published data.5 MHz, CDCl3) d: 182.09, 162.32, 146.93, 138.63, 137.40, 135.84,
The C-4 epimer of Palmarumycin BG5 (14), mp 200–202 ꢀC; 135.18, 134.34, 127.84, 121.81, 120.28, 119.85, 113.30, 112.95,
[a]2D0 ¼ ꢂ96 (c ¼ 0.58, CHCl3); 1H NMR (300 MHz, CDCl3) d: 7.58 110.24, 93.70; IR nmax 3063, 2924, 1651, 1628, 1612, 1497, 1458,
(1H, d, J ¼ 7.6 Hz), 7.56 (1H, d, J ¼ 8.5 Hz), 7.51–7.42 (4H, m), 1412, 1381, 1269, 1231, 1169, 1119, 1069, 941, 903, 845, 826,
7.12 (1H, dd, J ¼ 7.3, 2.0 Hz), 7.07 (1H, dd, J ¼ 7.6, 0.9 Hz), 6.91 802, 756 cmꢂ1; HR-ESI-MS, m/z: C20H10ClO4 [M–H]ꢂ, cacld.
(1H, dd, J ¼ 7.5, 1.0 Hz), 6.38 (1H, s), 5.08 (1H, s), 4.77 (1H, dd, J 349.0273, found: 349.0276. These data were identical with the
¼ 4.8, 1.7 Hz), 4.48 (1H, s), 3.43 (1H, d, J ¼ 9.8 Hz), 2.86 (1H, s); published data.6,32,33
13C NMR (75 MHz, CDCl3) d: 155.22, 146.90, 146.61, 134.39,
131.59, 131.15, 127.90, 127.79, 123.53, 121.73, 121.37, 120.29,
118.79, 109.72, 109.32, 100.26, 100.17, 71.59, 66.25, 57.59; IR
X-ray diffraction analysis of Palmarumycin BG5 C-4 epimer
(14)
nmax 3464, 2928, 1609, 1585, 1466, 1416, 1377, 1323, 1180, 1115,
1057, 1038, 984, 887, 822, 791, 764, 691 cmꢂ1; HR-ESI-MS, m/z: Colorless plate-like crystals of compound 14 were obtained
C
20H15ClO5 [M–H]ꢂ, calcd. 369.0535, found: 369.0547.
from a slowly evaporating methanol solution. A 0.35 ꢁ 0.24 ꢁ
NaBH4 (50 mg, 1.32 mmol) was added into a mixture of 0.03 mm3 crystal was selected for analysis. The parameters and
Guignardin E and its C-3 epimer (6 and 13, 161 mg, 0.44 mmol, structure information for compound 14 have been deposited at
dr ¼ 5.7 : 1), and MeOH (10 mL) in a 25 mL round-bottom ask. the Cambridge Crystallographic Data Centre. CCDC ID 1947664
The mixture was stirred at room temperature for 2 h. The contains the supplementary crystallographic data for this paper.
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The cytotoxicity of the title compounds was evaluated against
HCT116, U87-MG, HepG2, BGC823 and PC9 cell lines using
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Conflicts of interest
19 J. Dong, H. Song, J. Li, Y. Tang, R. Sun and L. Wang, J. Nat.
Prod., 2008, 71, 952–956.
There are no conicts to declare.
´
´
20 M. L. Macıas-Rubalcava, B. E. Hernandez-Bautista,
M. Jemenez-Estrada, M. C. Gonzalez, A. E. Glenn,
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