Total Synthesis and Structure Revision of Palmarumycin B6

Article abstract of DOI:10.1021/acs.jnatprod.8b00258

Palmarumycin B₆ and its regioisomer were synthesized via 7- and 13-step routes using 2-chlorophenol and 4-chlorophenyl methyl ether as the starting materials in overall yields of 2.7% and 12%, respectively. Their structures were characterized b

Full text of DOI:10.1021/acs.jnatprod.8b00258

Article
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Cite This: J. Nat. Prod. XXXX, XXX, XXX−XXX
Total Synthesis and Structure Revision of Palmarumycin B6
Xinlei Liu,† Weiwei Wang,† Yu Zhao,† Daowan Lai,‡ Ligang Zhou,‡ Zhilong Liu,§
and Mingan Wang*,†
†Department of Applied Chemistry, College of Sciences, ‡Department of Plant Pathology, and §Department of Entomology, College
of Plant Protection, China Agricultural University, Beijing 100193, People’s Republic of China
S
* Supporting Information
ABSTRACT: Palmarumycin B6 and its regioisomer were synthesized via 7- and 13-step routes using 2-chlorophenol and 4-
chlorophenyl methyl ether as the starting materials in overall yields of 2.7% and 12%, respectively. Their structures were
characterized by 1H and 13C NMR, HRESIMS, and X-ray diffraction data. The structure of palmarumycin B6 was revised as 6-
chloropalmarumycin CP17. The bioassay results showed that the larvicidal activity of palmarumycin B6 with an LC50 value of
32.7 μM was significantly higher than that of its 8-chloro isomer, with an LC50 value of 227.3 μM.
he outbreak of the Zika fever epidemic in Brazil in 2015
has raised concerns that the mosquitoes of the genus
RESULTS AND DISCUSSION
■
T
The retro-synthetic analysis of palmarumycin B6 (2) is shown
in Scheme 1. Palmarumycin B6 can be synthesized from ketal
(9) by oxidation with pyridinium dichromate (PDC) and t-
BuOOH, followed by a demethylation of the methoxy group.
Ketal (9) could be obtained from a ketalization of 8-chloro-5-
methoxy-3,4-dihydronaphthalen-1(2H)-one (7) with 1,8-dihy-
droxynaphthalene in a similar process to that in the synthesis
of palmarumycin CP17.33 8-Chloro-5-methoxy-3,4-dihydro-
naphthalen-1(2H)-one (7) could be derived from 4-
chlorophenyl methyl ether, a commercially available starting
material, via Friedel−Crafts acylation, carbonyl reduction, and
acid-catalyzed cyclization.34
The synthesis of palmarumycin B6 (2) started with 4-
chlorophenyl methyl ether as shown in Scheme 2. Compound
5 was obtained via acylation reaction of 4-chlorophenyl methyl
ether using succinic anhydride and anhydrous AlCl3 in
dichloromethane (DCM) at room temperature in a 60%
yield. Reduction of the carbonyl group with trifluoroacetic acid
(TFA) and Et3SiH at room temperature afforded 6 in a 98%
yield. Compound 6 was subjected to an intramolecular
cyclization catalyzed by polyphosphoric acid (PPA) at 80 °C
to afford 8-chloro-5-methoxy-3,4-dihydronaphthalen-1(2H)-
one (7) in 74% yield. Although the core spiroketal compound
9 could be generated from a direct ketalization, the resulting
yield was too low. The yield was significantly improved by
forming enol ether 8 with trimethyl orthoformate. When 8
reacted with 1,8-dihydroxynaphthalene, compound 9 was
Aedes (such as A. aegypti and A. albopictus) might be the
vectors of the Zika virus. This issue has stimulated a renewed
interest to develop sustainable vector control systems.1,2 The
resistance of the mosquitoes to traditional insecticides has
developed quickly in recent years;3−6 therefore a search for
novel drugs, especially those derived from natural products, to
overcome this serious problem is important. Palmarumycins
CP17, B6, and C8 (Figure 1) were isolated from the endophytic
fungus Berkleasmium sp., which was purified from the
medicinal plant Dioscorea zingiberensis C. H. Wright. These
compounds exhibited excellent larvicidal activity against the
mosquito A. albopictus and antibacterial activity against several
bacteria.7,8 Palmarumycin B6, with an LC50 value of 32.7 μM
against A. albopictus, is a member of the spirobisnaphthalene
family that showed broad bioactivities, such as antifungal,
antimicrobial, antitumor, anticancer, antiparasitic, anti-inflam-
matory, and cytotoxic activity.9−12 Many reports related to the
total synthesis, structure modification, and biological activity
evaluation of the spirobisnaphthalene natural products have
appeared in recent years.13−32 The syntheses of palmarumycin
CP17 and its analogues were completed, and the bioassay
results showed that they have antifungal activity against several
phytopathogens.33 Because of the interesting larvicidal activity
of palmarumycin B6 against A. albopictus, its limited access in
the fermentation extract of the endophytic fungus Berkleas-
mium sp., and the importance of the halogen atom in the A-
ring,7,8 the total synthesis and structure confirmation of
palmarumycin B6 were assessed in order to gain insight into
the structure−activity relationships of spirobisnaphthalene
compounds and explore their modes of action.
Received: April 4, 2018
© XXXX American Chemical Society and
American Society of Pharmacognosy
A
DOI: 10.1021/acs.jnatprod.8b00258
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Journal of Natural Products
Article
Figure 1. Structures of palmarumycins 1−4.
shown in Scheme 4. Compound 12 was prepared via the Fries
rearrangement of 2-chlorophenyl acetate, following the
reported procedures.35,36 Compound 16 was obtained
following protection of 12, bromination of 13, and alkylation
of 14 with diethyl malonate, and hydrolysis and decarbox-
ylation of 15. The same protocol was utilized to accomplish
the total synthesis of 6-chloropalmarumycin CP17 (4) from 16
as was used to synthesize compound 8-chloropalmarumycin
CP17 (2). The 1H and 13C NMR data of 4 were identical with
the reported data of palmarumycin B6 (Table 1, Figure 2).7,8
The structure of 4 was also characterized by X-ray diffraction
analysis and is depicted in Figure 3. Therefore, the correct
structure of palmarumycin B6 is revised to 4, 6-chloropalmar-
umycin CP17.
The larvicidal activity of palmarumycin B6 (4) and its 8-
chloro isomer (2) was evaluated using the bioassay protocols
described in the previous report.8 The preliminary results
showed that the larvicidal activity of palmarumycin B6 (4) with
an LC50 value of 32.7 μM was significantly higher than that of
its 8-chloro isomer with an LC50 value of 227.3 μM, which
indicated that the location of the chlorine atom plays a crucial
role for the larvicidal activity against A. albopictus.
Scheme 1. Retrosynthetic Analysis of Palmarumycin B6 (2)
formed in an overall yield of 50% over the two steps. Oxidation
of compound 9 with PDC and t-BuOOH in benzene gave
compound 10 in 74% yield, which was demethylated with
Me3SiI to afford the target palmarumycin B6 (2) in 73% yield.
After completion of the synthesis of palmarumycin B6 (2), it
1
was found that its H and 13C NMR data were not consistent
with those of the reported palmarumycin B6.7,8 The H and
1
13C NMR chemical shifts at C-5, C-6, C-7, C-8, and C-8a
(Table 1, Figure 2) are different. It was hypothesized that the
correct structure of palmarumycin B6 was actually as depicted
in compound 4 (Figure 1).
Given that palmarumycin B6 may actually be compound 4,
the 6-chloro derivative of palmarumycin CP17, the synthesis of
compound 4 was undertaken. The retrosynthetic analysis of 4
is shown in Scheme 3, and it parallels the synthesis of
compound 2. However, the starting material should be 2-
chlorophenol or 2-chlorophenyl methyl ether based upon the
difference of the regiochemistry of the A-ring.35,36
The reaction of 2-chlorophenyl methyl ether with succinic
anhydride and anhydrous AlCl3 in DCM at room temperature
gave 4-(3-chloro-4-methoxyphenyl)-4-oxobutanoic acid in 98%
yield, an unwanted regioisomer.37 Therefore, 2-chlorophenol
was selected as the raw material. The synthetic route of 6-
chloropalmarumycin CP17 (4) started with 2-chlorophenol as
In summary, the 6- and 8-chloro derivatives of palmar-
umycin CP17 were synthesized. Their structures were
1
characterized by H and 13C NMR and HRESIMS data. The
revised structure of palmarumycin B6 was confirmed by X-ray
crystallographic data (CCDC 1834663). The larvicidal activity
of palmarumycin B6 (4) was significantly higher than that of its
8-chloro isomer (2).
EXPERIMENTAL SECTION
General Experimental Procedures. Melting points (uncor-
rected) were measured on a WRX-4 microscopic melting point
apparatus (Shanghai Yi Ce Instrument Factory). All 1H and 13C NMR
■
a
Scheme 2. Synthesis of 8-Chloropalmarumycin CP17 (2)
a
Reagents and conditions: (a) succinic anhydride, AlCl3, DCM, rt, 24 h, 60%; (b) Et3SiH, TFA, rt, 6 h, 98%; (c) PPA, 90 °C, 2 h, 74%; (d)
(CH3O)3CH, PPTs, MeOH, 60 °C, 12 h, 72%; (e) 1,8-dihydroxynaphthalene, TsOH, toluene, reflux, 72 h, 70%; (f) PDC, Celite, t-BuOOH,
benzene, rt, 24 h, 74%; (g) TMSI, CHCl3, 0 °C, 12 h, 73%.
B
DOI: 10.1021/acs.jnatprod.8b00258
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7,8
Table 1. NMR Data of 2, 4, and Palmarumycin B6 in CDCl3 (300 MHz)
1H NMR (δ, J in Hz)
13C NMR (δ)
7,8
7,8
position
palmarumycin B6
2
4
palmarumycin B6
2
4
1
2
3
98.2
29.5
34.3
99.1
30.3
34.3
98.2
29.5
34.3
2.49, t (6.5)
2.88, t (6.5)
2.54, t (6.3)
2.83, t (6.3)
2.49, t (6.2)
2.88, t (6.2)
4
4a
5
6
7
203.4
116.3
158.1
117.2
137.1
124.2
139.6
147.3
109.6
127.7
121.2
134.4
121.2
127.7
109.6
147.3
113.4
202.9
117.0
161.3
121.2
140.5
124.4
135.8
146.9
109.5
127.6
120.9
134.4
120.9
127.6
109.5
146.9
112.6
203.4
116.3
158.1
117.2
137.1
124.1
139.5
147.3
109.6
127.7
121.2
134.3
121.2
127.7
109.6
147.3
113.4
7.42, d (8.3)
7.70, d (8.3)
7.08, d (9.0)
7.63, d (9.0)
7.70, d (8.2)
7.42, d (8.2)
8
8a
1′
2′
3′
4′
4a′
5′
6′
7′
8′
8a′
OH
6.97, d (7.5)
7.46, dd (7.5, 8.3)
7.54, d (8.3)
6.99, d (7.4)
7.46, dd (7.4, 8.4)
7.54, d (8.4)
6.97, d (7.5)
7.46, dd (7.5, 8.4)
7.54, d (8.4)
7.54, d (8.3)
7.46, dd (7.5, 8.3)
6.97, d (7.5)
7.54, d (8.4)
7.46, dd (7.4, 8.4)
6.99, d (7.4)
7.54, d (8.4)
7.46, dd (7.5, 8.4)
6.97, d (7.5)
13.01, s
12.57, s
13.02, s
1
Figure 2. Comparison of H NMR spectra of compounds 2 and 4 in the low-field region.
Scheme 3. Retrosynthetic Analysis of Compound 4
spectra were obtained on a Bruker DPX 300 spectrometer with
CDCl3 and DMSO-d6 as solvents and tetramethylsilane (TMS) as an
internal standard. HRESIMS spectra were analyzed on a Bruker Apex
II mass spectrometer. All reactions were performed under a N2
atmosphere with magnetic stirring. Unless otherwise stated, all
reagents were purchased from commercial suppliers and used without
further purification. Organic solutions were concentrated under
reduced pressure using a rotary evaporator or oil pump. Flash
column chromatography was performed using Qingdao Haiyang silica
gel (200−300 mesh) and neutral Al2O3. The crystal structure was
analyzed with a Thermo Fisher ESCALAB 250 four-circle X-ray
diffractometer (Xcalibur, Eos, Gemini).
Synthesis of 8-Chloropalmarumycin CP17 (Palmarumycin
B6, 2). 4-(5-Chloro-2-methoxyphenyl)-4-oxobutanoic acid (5). To
succinic anhydride (200 mg, 2 mmol, 1 equiv), 4-chloroanisole (285
mg, 2 mmol), and DCM (10 mL) in a 25 mL round-bottom flask, was
C
DOI: 10.1021/acs.jnatprod.8b00258
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Article
a
Scheme 4. Synthesis of 6-Chloropalmarumycin CP17 (4)
a
Reagents and conditions: (a) acetyl chloride, Et3N, DCM, rt, 4 h, 97%; (b) AlCl3, 1,2-dichlorobenzene, 110 °C, 3 h, 32%; (c) CH3I, K2CO3,
acetone, reflux, 4 h, 94%; (d) Br2, Et2O, rt, 1 h, 78%; (e) diethyl malonate, NaH (60%), THF, rt, 1 h, 75%; (f) (i) NaOH, THF/H2O, 60 °C, 1 h;
(ii) HCl, dioxane/H2O, reflux, 8 h, 97%; (g) (i) Et3SiH, TFA, rt, 6 h, 96%; (ii) PdCl2, Et3SiH, THF, rt, 1 h, 82%; (h) PPA, 90 °C, 2 h, 78%; (i)
(CH3O)3CH, PPTs, MeOH, 60 °C, 12 h; (j) 1,8-dihydroxynaphthalene, TsOH, toluene, reflux, 72 h, 46%; (k) PDC, Celite, t-BuOOH, benzene, rt,
24 h, 73%; (l) TMSI, CHCl3, 0 °C, 12 h, 81%.
(2H, m); ESIMS, m/z 229 [M + H]+. The data were identical with
the published data.34
8-Chloro-5-methoxy-3,4-dihydronaphthalen-1(2H)-one (7). A
mixture of compound 6 (1.062 g, 4.64 mmol) was added to
polyphosphoric acid (30 mL) in a 100 mL round-bottom flask at 90
°C. The mixture was stirred for 2 h, quenched with ice/water, and
extracted with EtOAc (3 × 20 mL). The organic phase was combined,
washed with a saturated NaHCO3 solution (3 × 20 mL), and dried
over anhydrous Na2SO4. The solvent was removed, and the residue
was recrystallized from DCM to provide compound 7 (726 mg, 74%)
as a yellow solid: mp 47−49 °C; 1H NMR (300 MHz, CDCl3) δ 7.27
(1H, d, J = 8.7 Hz), 6.90 (1H, d, J = 8.7 Hz), 3.85 (3H, s), 2.89 (2H,
t, J = 6.3 Hz), 2.66 (2H, t, J = 6.3 Hz), 2.13−2.04 (2H, m); ESIMS,
m/z 211 [M + H]+. The data were identical with the published data.34
5-Chloro-4,8-dimethoxy-1,2-dihydronaphthalene (8). Com-
Figure 3. ORTEP drawing of palmarumycin B6 (4).
pound 7 (2.152 g, 10.2 mmol) and Pyridinium 4-toluenesulfonate
(PPTs) (513 mg, 2.05 mmol, 0.2 equiv) were added into a MeOH
(60 mL) solution of trimethyl orthoformate (30 mL) in a 250 mL
round-bottom flask at room temperature. The mixture was heated at
reflux for 12 h, Et3N (0.5 mL) was added, and the solvent was
removed under reduced pressure. The residue was diluted with
EtOAc (50 mL), the solution was washed with brine, and the organic
phase was dried over anhydrous Na2SO4. The solvent was removed,
and the crude product was purified by flash column chromatography
on neutral Al2O3 (petroleum ether/EtOAc, 20:1) to afford compound
added AlCl3 (373 mg, 2.8 mmol, 1.4 equiv) portionwise in an ice
bath. The mixture was stirred for 24 h at room temperature and
monitored by TLC. The reaction was quenched with crushed ice and
1 M HCl (5 mL), and the organic phase was washed with brine
solution. The organic phase was dried over anhydrous Na2SO4 and
concentrated under vacuum. The crude product was purified by flash
column chromatography on silica gel (petroleum ether/EtOAc, 3:1)
to give compound 5 (292 mg, 60%) as a white solid: mp 117−119
°C; 1H NMR (300 MHz, DMSO-d6) δ 12.13 (1H, brs), 7.60 (1H, dd,
J = 8.7, 2.7 Hz), 7.52 (1H, d, J = 2.7 Hz), 7.23 (1H, d, J = 8.7 Hz),
3.89 (3H, s), 3.14 (2H, t, J = 6.6 Hz), 2.53 (2H, t, J = 6.6 Hz);
ESIMS, m/z 243 [M + H]+. The data were consistent with the
reported data.34
4-(5-Chloro-2-methoxyphenyl)butanoic acid (6). Et3SiH (1.74 g,
15 mmol, 3 equiv) was added into a TFA (12 mL) solution of 5
(1.213 g, 5 mmol) in a 25 mL round-bottom flask at room
temperature. The mixture was stirred at room temperature for 6 h.
The solvent was removed under reduced pressure, diluted with EtOAc
(20 mL), and washed with water until the pH was >4. The solution
was treated with 1 M KOH (3 × 20 mL), the aqueous phase was
combined, and the pH was adjusted to 1 with 1 M HCl. The aqueous
phase was extracted with EtOAc (3 × 20 mL), and the organic phase
was dried over anhydrous Na2SO4. The solvent was removed, and the
residue was recrystallized from DCM to afford compound 6 (1.116 g,
98%) as a white solid: mp 80−82 °C; 1H NMR (300 MHz, CDCl3) δ
11.00 (1H, brs), 7.15−7.08 (2H, m), 6.75 (1H, d, J = 8.7 Hz), 3.79
(3H, s), 2.64 (2H, t, J = 7.2 Hz), 2.37 (2H, t, J = 7.2 Hz), 1.97−1.80
1
8 (1.655 g, 72%) as a colorless, viscous liquid: H NMR (300 MHz,
CDCl3) δ 7.17 (d, J = 8.7 Hz), 6.71 (1H, d, J = 8.7 Hz), 5.20 (1H, t, J
= 5.4 Hz), 3.80 (3H, s), 3.68 (3H, s), 2.68 (2H, t, J = 6.3 Hz), 2.20−
2.14 (2H, m); ESIMS, m/z 225 [M + H]+.
8-Chloro-5-methoxy-3,4-dihydro-2H-spiro[naphthalene-1,2′-
naphtho[1,8-de][1,3]dioxine] (9). Compound 8 (1.655 g, 7.39
mmol), 1,8-dihydroxynaphthalene (1.317 g, 8.23 mmol, 1.11 equiv),
p-toluenesulfonic acid (254 mg, 1.34 mmol, 0.18 equiv), and toluene
(80 mL) were added into a 250 mL round-bottom flask, and the
mixture was stirred and heated at reflux under a N2 atmosphere for 3
days. After completion of the reaction, Et3N (0.5 mL) was added, the
solution was diluted with EtOAc (100 mL) and washed with brine,
and the organic phase was dried over anhydrous Na2SO4. The solvent
was removed, and the crude product was purified by flash column
chromatography on silica gel (petroleum ether/EtOAc, 20:1) to give
1
compound 9 (1.81 g, 70%) as a yellow solid: mp 144−146 °C; H
NMR (300 MHz, CDCl3) δ 7.49−7.39 (4H, m), 7.32 (1H, d, J = 8.7
Hz), 6.92 (2H, dd, J = 7.2, 1.0 Hz), 6.81 (1H, d, J = 8.7 Hz), 3.84
(3H, s), 2.78 (2H, t, J = 6.0 Hz), 2.19−2.15 (2H, m), 1.89−1.80 (2H,
D
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m); 13C NMR (75 MHz, CDCl3) δ 155.59, 147.66, 134.35, 132.42,
130.76, 129.94, 127.50, 126.20, 120.21, 112.98, 111.31, 109.14,
100.67, 55.89, 33.01, 24.39, 18.85; HRESIMS, m/z C21H18ClO3 [M +
H]+, calcd 353.0939, found 353.0936.
(1H, t, J = 7.8 Hz), 2.66 (3H, s); ESIMS, m/z 171 [M + H]+. These
data were consistent with the published data.35,36
2-Chloro-6-acetylphenyl methyl ether (13). Compound 12 (4.671
g, 27.4 mmol) was dissolved in acetone (60 mL) in a 200 mL round-
bottom flask. K2CO3 (7.575 g, 54.79 mmol, 2 equiv) was added to the
solution, and the mixture was stirred. CH3I (5.56 mL, 4 equiv) was
added to the stirred mixture, and the reaction mixture heated at reflux
for 3 h. The mixture was cooled to room temperature, and the solid
was removed. The solvent was evaporated under vacuum, and the
residue was diluted with EtOAc (50 mL). The crude product was
washed with saturated aqueous Na2S2O3 (2 × 30 mL) solution and
brine, and the organic phase dried over anhydrous Na2SO4. The
organic solution was concentrated under reduced pressure to give
8-Chloro-5-methoxy-2,3-dihydro-4H-spiro[naphthalene-1,2′-
naphtho[1,8-de][1,3]dioxin]-4-one (10). Compound 9 (348 mg, 0.99
mmol) and PDC (1.123 g, 3 mmol, 3 equiv) were added to a 100 mL
round-bottom flask containing Celite (2.8 g) and benzene (30 mL) in
an ice−water bath, the mixture was stirred, and 5−6 M t-BuOOH (8
mL) was injected into the mixture with a syringe over 15 min. The
ice−water bath was removed, and the mixture was stirred at room
temperature for 24 h. The mixture was diluted with EtOAc (50 mL),
and the solids were filtered off. The organic phase was washed with
0.2 M HCl (2 × 50 mL) solution and brine and dried over anhydrous
Na2SO4. The organic phase was concentrated under reduced pressure,
and the residue was purified by flash column chromatography on silica
gel (petroleum ether/EtOAc, 10:1) to afford compound 10 (266 mg,
74%) as a white solid: mp 196−198 °C; 1H NMR (300 MHz, CDCl3)
δ 7.65 (1H, d, J = 8.7 Hz), 7.51 (2H, dd, J = 7.2, 1.0 Hz), 7.45 (2H, t,
J = 7.8 Hz), 7.07 (2H, d, J = 8.7 Hz), 6.96 (2H, dd, J = 7.5, 1.0 Hz),
3.95 (3H, s), 2.79−2.74 (2H, m), 2.56−2.51 (2H, m); 13C NMR (75
MHz, CDCl3) δ 195.02, 158.08, 146.92, 137.84, 137.64, 134.33,
127.59, 125.98, 123.63, 120.79, 115.05, 112.64, 109.41, 99.57, 56.79,
36.97, 31.50; HRESIMS, m/z C21H16ClO4 [M + H]+, calcd 367.0732,
found 367.0735.
1
compound 13 (4.728 g, 94%) as a colorless liquid: H NMR (300
MHz, CDCl3) δ 7.53 (1H, dd, J = 7.8, 1.8 Hz), 7.51 (1H, dd, J = 7.8,
1.8 Hz), 7.12 (1H, t, J = 7.8 Hz), 3.89 (3H, s), 2.63 (3H, s); ESIMS,
m/z 185 [M + H]+. These data were identical with the reported
data.35,36
2-Chloro-6-bromoacetylphenyl methyl ether (14). Compound 13
(7.544 g, 40.89 mmol) was added to a 250 mL round-bottom flask
containing Et2O (80 mL). Bromine (2.1 mL, 1 equiv) was added
dropwise, and the solution was stirred for 8 h. The reaction was
quenched with water (60 mL) and extracted with Et2O (2 × 30 mL).
The organic phase was washed with saturated aqueous Na2CO3 (20
mL) solution and brine and dried over anhydrous Na2SO4. The
solvent was removed, and the crude product was purified by column
chromatography on silica gel (DCM/petroleum ether, 1:15) to afford
compound 14 (8.304 g, 78%) as a white wax solid: mp 40−42 °C; 1H
NMR (300 MHz, CDCl3) δ 7.57 (2H, d, J = 8.1 Hz), 7.16 (1H, t, J =
8.1 Hz), 4.56 (2H, s), 3.95 (3H, s); ESIMS, m/z 263 [M + H]+.
Diethyl 2-[2-(3-chloro-2-methoxyphenyl)-2-oxoethyl]malonate
(15). NaH (60%) (586 mg, 14.65 mmol, 1.05 equiv) was suspended
in dry tetrahydrofuran (THF) (10 mL) in a 100 mL round-bottom
flask. A solution of diethyl malonate (2.273 g, 14.2 mmol, 1 equiv) in
dry THF (10 mL) was added dropwise at 0 °C under a N2
atmosphere. The solution was stirred at 0 °C for 10 min until it
turned clear. Then a solution of compound 14 (3.743 g, 14.2 mmol)
in dry THF (20 mL) was added. The mixture was warmed to room
temperature and stirred for 2 h under a N2 atmosphere. The reaction
was quenched with a saturated aqueous NH4Cl (20 mL) solution at 0
°C, and the THF was removed under reduced pressure. The reaction
mixture was poured into water and extracted with EtOAc (3 × 20
mL). The organic phase was washed with brine and dried over
anhydrous Na2SO4. The solvent was removed under vacuum, and the
crude product was purified by flash column chromatography on silica
gel (petroleum ether/EtOAc, 10:1) to give compound 15 (3.662 g,
75%) as a colorless liquid: 1H NMR (300 MHz, CDCl3) δ 7.56−7.52
(2H, m), 7.12 (1H, t, J = 8.1 Hz), 4.21 (4H, t, J = 7.5 Hz), 4.02 (1H,
t, J = 7.2 Hz), 3.93 (3H, s), 3.61 (2H, d, J = 7.2 Hz), 1.28 (6H, t, J =
7.5 Hz); 13C NMR (75 MHz, CDCl3) δ 197.73, 168.54, 154.80,
133.97, 133.60, 128.54, 128.00, 124.60, 61.91, 61.36, 47.07, 41.50,
13.68; HRESIMS, m/z C16H20ClO6 [M + H]+, calcd 343.0943, found
343.0946.
4-(3-Chloro-2-methoxyphenyl)-4-oxobutanoic acid (16). Com-
pound 15 (2.232 g, 6.5 mmol) was added to a 100 mL round-bottom
flask with a LiOH solution (1.642 g, 39.1 mmol, 6 equiv) in THF (30
mL) and water (30 mL), and the mixture stirred and heated at 60 °C
for 1 h. The THF was removed under reduced pressure, and the
aqueous solution was acidified with HCl (1 M) to pH = 1. The
aqueous solution was extracted with EtOAc (3 × 20 mL), and the
organic phase was combined and washed with brine twice, dried over
anhydrous Na2SO4, and concentrated under vacuum to afford the
intermediate diacid. This intermediate was dissolved in 1,4-dioxane
(30 mL), and 2 M HCl (30 mL) was added. The mixture was heated
at reflux for 8 h, the solution cooled, and 1,4-dioxane removed under
reduced pressure. The solution was extracted with EtOAc (3 × 20
mL), and the organic phase was washed with brine, dried over
anhydrous Na2SO4, and evaporated to afford compound 16 (1.533 g,
97%) as a yellow solid: mp 40−42 °C; 1H NMR (300 MHz, CDCl3)
δ 7.68 (1H, dd, J = 8.1, 1.8 Hz), 7.56 (1H, dd, J = 7.8, 1.8 Hz), 7.25
8-Chloro-5-hydroxy-2,3-dihydro-4H-spiro[naphthalene-1,2′-
naphtho[1,8-de][1,3]dioxin]-4-one (2) (Palmarumycin B6, 8-chlor-
opalmarumycin CP17). TMSI (0.3 mL, 2.1 mmol) was added into a
CHCl3 (2 mL) solution of compound 10 (78 mg, 0.21 mmol) in a 10
mL round-bottom flask in an ice−water bath. The mixture was stirred
at 0 °C for 12 h. The solution was extracted with saturated aqueous
Na2S2O3 (2 × 30 mL) solution. The organic phase was washed with
brine and dried over anhydrous Na2SO4. The solvent was removed
under reduced pressure, and the residue was subjected to flash
column chromatography on silica gel and eluted with petroleum/
EtOAc(20:1) to afford compound 2 (55 mg, 73%) as a light yellow
1
solid: mp 129−131 °C; H NMR (300 MHz, CDCl3) δ 12.57 (1H,
s), 7.63 (1H, d, J = 8.7 Hz), 7.56−7.43 (3H, m), 7.07 (1H, d, J = 8.7
Hz), 6.98 (2H, d, J = 7.5 Hz), 2.85−2.81 (2H, m), 2.57−2.52 (2H,
m); 13C NMR (75 MHz, CDCl3) δ 202.91, 161.31, 146.89, 140.47,
135.79, 134.39, 127.65, 124.43, 121.22, 120.94, 117.04, 112.65,
109.46, 99.11, 34.32, 30.28; HRESIMS, m/z C21H16ClO4 [M + H]+,
calcd 367.0732, found 367.0735.
Synthesis of 6-Chloropalmarumycin CP17 (Revised palmar-
umycin B6, 4). 2-Chlorophenyl acetate (11). 2-Chlorophenol
(12.861 g, 100 mmol) and Et3N (24.2 mL, 3 equiv) were added
into a 250 mL round-bottom flask containning DCM (100 mL).
Acetyl chloride (8.49 mL, 1.2 equiv) was added dropwise into the
solution at 0 °C, and the mixture was stirred at room temperature for
2 h. The mixture was washed with 1 M HCl (1 × 100 mL) and brine.
The organic phase was dried over anhydrous Na2SO4. The solvent
was evaporated under vacuum, and the crude product was purified by
flash column chromatography on silica gel (petroleum ether/EtOAc,
20:1) to afford compound 11 (16.61 g, 97%) as a colorless liquid: 1H
NMR (300 MHz, CDCl3) δ 7.44 (1H, dd, J = 7.8, 1.8 Hz), 7.31−7.25
(1H, m), 7.22−7.16 (1H, m), 7.13 (1H, dd, J = 7.8, 1.8 Hz), 2.36
(3H, s); ESIMS, m/z 171 [M + H]+. These data were identical with
the reported data.35,36
2-Chloro-6-acetylphenol (12). AlCl3 (3.231 g, 24 mmol, 1.2
equiv) was added to a 50 mL round-bottom flask containing 2-
chlorophenyl acetate (3.443 g, 20.2 mmol) and 1,2-dichlorobenzene
(10 mL). The mixture was heated to 100−110 °C in an oil bath for 4
h. The reaction was quenched by crushed ice and extracted with
EtOAc (2 × 50 mL). The organic phase was washed with brine twice,
the solvent was removed under vacuum, and the crude product was
purified by flash column chromatography on silica gel (petroleum
ether/EtOAc, 10:1) to give compound 12 (1.111 g, 32%) as a yellow
solid: mp 54−56 °C; 1H NMR (300 MHz, CDCl3) δ 12.85 (1H, brs),
7.68 (1H, dd, J = 7.8, 1.8 Hz), 7.57 (1H, dd, J = 7.8, 1.8 Hz), 6.87
E
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Journal of Natural Products
Article
(1H, t, J = 7.8 Hz), 3.81 (3H, s), 3.16 (1H, t, J = 7.2 Hz), 2.57 (2H, t,
J = 7.2 Hz); HRESIMS, m/z C11H12ClO4 [M + H]+, calcd 243.0419,
found 243.0415. These data were consistent with the reported data.37
4-(3-Chloro-2-methoxyphenyl)butanoic acid (17). Et3SiH (2.899
g, 24.52 mmol, 3 equiv) was added to a TFA (24 mL) solution of
compound 16 (1.982 g, 8.17 mmol) in a 50 mL round-bottom flask at
room temperature. The mixture was stirred at room temperature for 6
h, the solvent removed under vacuum, and the residue diluted with
EtOAc (50 mL) and washed with water until pH > 4. The organic
phase was treated with 1 M KOH (3 × 20 mL). The aqueous phase
was combined, and the pH adjusted to <1 with 1 M HCl. Then the
aqueous phase was extracted with EtOAc (2 × 30 mL), and the
organic phase was concentrated under reduced pressure to afford 4-
(3-chloro-2-methoxyphenyl)-4-hydroxybutanoic acid (1.695 g, 96%)
as a colorless liquid.
PdCl2 (119 mg, 0.63 mmol, 0.12 equiv) was added to a solution of
the 4-hydroxybutanoic acid (1.329 g, 5.44 mmol) and Et3SiH (1.899
g, 16.4 mmol, 3 equiv) in THF (30 mL) at room temperature under a
N2 atmosphere. The mixture solution was stirred for 2 h, and the
solvent removed under reduced pressure. The residue was diluted
with EtOAc (50 mL), and the solution was washed with water and
brine and dried over anhydrous Na2SO4. The solvent was removed
under vacuum to give compound 17 (1.018 g, 82%) as a yellow solid:
mp 75−77 °C; 1H NMR (300 MHz, CDCl3) δ 7.22 (1H, dd, J = 7.8,
1.8 Hz), 7.08 (1H, dd, J = 7.8, 1.8 Hz), 6.98 (1H, t, J = 7.8 Hz), 3.85
(3H, s), 2.72 (2H, t, J = 7.5 Hz), 2.41 (2H, t, J = 7.5 Hz), 2.00−1.90
(2H, m); HRESIMS, m/z C11H14ClO3 [M + H]+, calcd 229.0626,
found 229.0624.
6-Chloro-5-methoxy-3,4-dihydronaphthalen-1(2H)-one (18). A
mixture of P2O5 (3 g) and phosphoric acid (4.5 mL; 85%) was stirred
in a 25 mL round-bottom flask at 90 °C for 2 h to obtain the
polyphosphoric acid. Compound 17 (321 mg, 1.4 mmol) was added
into the polyphosphoric acid at 90 °C, and the mixture stirred for 2 h,
quenched with crushed ice, and extracted with EtOAc (3 × 20 mL).
The organic phase was combined, washed with a saturated aqueous
NaHCO3 solution, and dried over anhydrous Na2SO4. The crude
product was purified by flash column chromatography on silica gel
(petroleum ether/EtOAc, 10:1) to afford compound 18 (229 mg,
78%) as a white solid: mp 82−84 °C; 1H NMR (300 MHz, CDCl3) δ
7.76 (1H, d, J = 8.4 Hz), 7.32 (1H, d, J = 8.4 Hz), 3.85 (s), 2.99 (2H,
t, J = 6.0 Hz), 2.63 (2H, t, J = 6.0 Hz), 2.16−2.08 (2H, m);
HRESIMS, m/z C11H12ClO2 [M + H]+, calcd 211.0520, found
211.0522.
1.11 mmol) and PDC (1.235 g, 3.34 mmol, 3 equiv) were added to a
100 mL round-bottom flask with Celite (4 g) and benzene (30 mL) in
an ice−water bath. To the stirred solution was added 5−6 M t-
BuOOH (9 mL) via syringe in 15 min. The ice−water bath was
removed, and the mixture stirred at ambient temperature for 24 h.
The solution was diluted with EtOAc, and the solid was removed. The
organic phase was washed with 1 M HCl solution (25 mL) and brine
and dried over anhydrous Na2SO4. The solvent was removed under
reduced pressure, and the residue subjected to flash column
chromatography on silica gel (petroleum ether/EtOAc, 10:1) to
give compound 21 (296 mg, 73%) as a yellow solid: mp 115−117 °C;
1H NMR (300 MHz, CDCl3) δ 7.70 (2H, s), 7.53 (2H, d, J = 8.4 Hz),
7.45 (2H, t, J = 8.1 Hz), 6.96 (2H, d, J = 7.2 Hz), 3.99 (3H, s), 2.78
(2H, t, J = 6.9 Hz), 2.47 (2H, t, J = 6.9 Hz); 13C NMR (75 MHz,
CDCl3) δ 194.39, 155.86, 147.43, 140.99, 135.19, 134.32, 132.09,
127.67, 127.22, 122.63, 121.11, 113.50, 109.59, 98.50, 62.08, 35.44,
29.46; HRESIMS, m/z C21H16ClO4 [M + H]+, calcd 367.0732, found
367.0736.
6-Chloro-5-hydroxy-2,3-dihydro-4H-spiro[naphthalene-1,2′-
naphtho[1,8-de][1,3]dioxin]-4-one (4) (revised palmarumycin B6).
TMSI (1.242 mL, 8.73 mmol, 10 equiv) was added to a CHCl3 (10
mL) solution of compound 21 (320 mg, 0.87 mmol) in a 25 mL flask
in an ice−water bath. The mixture was stirred at 0 °C for 12 h, the
solvent removed under vacuum, and the residue subjected to flash
column chromatography on silica gel and eluted with petroleum/
EtOAc (10:1) to afford compound 4 (249 mg, 81%) as a light yellow
1
solid: mp 190−192 °C; H NMR (300 MHz, CDCl3) δ 13.02 (1H,
s), 7.70 (1H, d, J = 8.1 Hz), 7.54 (2H, dd, J = 8.4, 1.0 Hz), 7.46 (2H,
t, J = 8.1 Hz), 7.42 (1H, d, J = 8.1 Hz), 6.97 (2H, dd, J = 7.2, 1.0 Hz),
2.88 (2H, t, J = 6.6 Hz), 2.49 (2H, t, J = 6.6 Hz); 13C NMR (75 MHz,
CDCl3) δ 203.36, 158.12, 147.32, 139.55, 137.07, 134.34, 127.69,
124.14, 121.16, 117.19, 116.30, 113.38, 109.61, 98.16, 34.31, 29.49;
HRESIMS, m/z C20H14ClO4 [M + H]+, calcd 353.0575, found
353.0578.
X-ray Diffraction Analysis of Palmarumycin B6 (4). Colorless
plate-like crystals of palmarumycin B6 (4) were obtained from a
slowly evaporating CH2Cl2/n-hexane solution. A 0.40 × 0.35 × 0.30
mm3 crystal was selected for analysis. The parameters and structure
information for compound 4 have been deposited at the Cambridge
Crystallographic Data Centre. CCDC ID 1834663 contains the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
6-Chloro-5-methoxy-3,4-dihydro-2H-spiro[naphthalene-1,2′-
naphtho[1,8-de][1,3]dioxine] (20). Compound 18 (209 mg, 1
mmol) and PPTs (51 mg, 0.2 mmol, 0.2 equiv) were added to a
MeOH solution (30 mL) of trimethyl orthoformate (10 mL) in a 100
mL flask at room temperature. The mixture was heated at reflux for 12
h, Et3N (0.5 mL) added, the solvent removed, and the residue
purified by flash column chromatography on neutral Al2O3
(petroleum ether/EtOAc, 20:1) to afford compound 19 as a white
solid.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jnat-
prod.8b00258.
■
S
Additional information (PDF)
A mixture of compound 19, 1,8-dihydroxynaphthalene (194 mg,
1.2 mmol, 1.2 equiv), and p-toluenesulfonic acid (38 mg, 0.2 mmol,
0.2 equiv) in toluene (30 mL) in a 50 mL round-bottom flask was
stirred and heated for 72 h at reflux under a N2 atmosphere. The
solution was diluted with EtOAc (60 mL), washed with brine, and
dried over anhydrous Na2SO4. The solvent was removed, and the
crude product was purified by flash column chromatography on silica
gel (petroleum ether/EtOAc, 30:1) to produce compound 20 (160
AUTHOR INFORMATION
Corresponding Author
*Tel: +86-10-62734093. E-mail: wangma@cau.edu.cn.
ORCID
Xinlei Liu: 0000-0002-2630-7751
Ligang Zhou: 0000-0003-4681-191X
Notes
■
1
mg, 46% for two steps) as a light yellow solid: mp 179−181 °C; H
NMR (300 MHz, CDCl3) δ 7.60 (1H, d, J = 8.4 Hz), 7.52−7.36 (5H,
m), 6.92 (2H, dd, J = 7.2, 1.2 Hz), 3.88 (3H, s), 2.92 (2H, t, J = 6.3
Hz), 2.15−2.11 (2H, m), 1.98−1.91 (2H, m); 13C NMR (75 MHz,
CDCl3) δ 153.32, 148.09, 135.64, 134.31, 134.19, 128.93, 128.51,
127.52, 124.33, 120.58, 113.70, 109.50, 100.26, 60.19, 30.55, 23.94,
19.11; HRESIMS, m/z C21H18ClO3 [M + H]+, calcd 353.0939, found
353.0938.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The National Natural Science Foundation of China (Nos.
21772229, 21172254) and National Key R & D Program of
China (2017YFD0201105) are appreciated for the financial
support.
6-Chloro-5-methoxy-2,3-dihydro-4H-spiro(naphthalene-1,2′-
naphtho[1,8-de][1,3]dioxin)-4-one (21). Compound 20 (393 mg,
F
DOI: 10.1021/acs.jnatprod.8b00258
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Journal of Natural Products
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