Total Synthesis of Violaceoid A and (-)- and (+)-Violaceoid B

Article abstract of DOI:10.1021/acs.jnatprod.8b00215

The first total synthesis of violaceoid A, a cytotoxic agent, and the asymmetric total synthesis of (-)- and (+)-violaceoid B are reported. The precursor was accessed by desymmetrization of a substituted quinol moiety, and the racemic secondary alcohol wa

Full text of DOI:10.1021/acs.jnatprod.8b00215

Article
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Cite This: J. Nat. Prod. XXXX, XXX, XXX−XXX
Total Synthesis of Violaceoid A and (−)- and (+)-Violaceoid B
Takatsugu Murata,† Teppei Kuboki,† Ryo Ishikawa,† Takahiro Saito,† Shotaro Taguchi,†
Kazuma Takeuchi,† Emiko Hatano,† Motoyuki Shimonaka,‡ and Isamu Shiina*,†
†Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601,
Japan
‡Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
S
* Supporting Information
ABSTRACT: The first total synthesis of violaceoid A, a cytotoxic agent, and the asymmetric total synthesis of (−)- and
(+)-violaceoid B are reported. The precursor was accessed by desymmetrization of a substituted quinol moiety, and the racemic
secondary alcohol was kinetically resolved using a chiral nucleophilic catalyst. The asymmetric synthesis of (−)- and
(+)-violaceoid B elucidated the absolute configuration of the naturally occurring violaceoid B. Synthetic violaceoid A inhibited
the growth of human breast cancer cell lines MCF-7 and Hs 578T at concentrations of less than 100 μM, while (S)- and (R)-
violaceoid B were inactive.
In 2014, Sugawara and co-workers reported a series of unique
alkylated hydroquinones, violaceoids A−F (compounds 1−6),
which were isolated from a culture broth of Aspergillus
violaceofuscus Gasperini coexisting with moss.1 Violaceoids B
and D−F are chiral compounds, and the absolute config-
urations of violaceoids B, D, and E have not yet been
determined. Sugawara’s group also reported that violaceoids
exhibit cytotoxicity. Among them, violaceoid A (1) inhibited
the growth of several human cancer cell lines. In addition,
neither the total synthesis nor the synthetic approach for
violaceoids A−E has ever been reported to the best of our
knowledge.2 Considering this, we initiated a program to
synthesize 1 and 2, and the details are presented in this paper.
Following this strategy, we first tried to protect two hydroxy
groups of 3,6-dihydroxyphthalonitrile as tert-butyldiphenylsilyl
(TBDPS) ether depicted in Scheme 2. However, we could not
RESULTS AND DISCUSSION
■
obtain the desired phthalonitrile 10. Then we conducted the
protection transforming it into methoxymethyl (MOM) ether
groups, but we could get the desired compound 12 in low
yield. On the basis of the above results, we decided to change
the synthetic plan for providing violaceoids A and B as shown
in Scheme 3.
The retrosynthetic analysis of 1 and 2 is shown in Scheme 1.
Optically active (S)- or (R)-violaceoid B (2) can be obtained
by the deprotection of the acetonide group from (S)- or (R)-7,
respectively. Diols (S)- and (R)-7 can be separated by using a
kinetic resolution method of a racemic compound (rac-7) that
our group has already reported.3 The substrate for kinetic
resolution can be obtained from the racemic secondary alcohol
8, which can be prepared from 3,6-dihydroxyphthalonitrile
(11) via direct protection of 11.
Received: March 14, 2018
© XXXX American Chemical Society and
American Society of Pharmacognosy
A
DOI: 10.1021/acs.jnatprod.8b00215
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Scheme 1. Retrosynthetic Analysis of 1 and 2
In the revised synthetic plan, rac-7, the substrate for kinetic
resolution, can be obtained from the racemic secondary
alcohol 14 by several functional transformations. Violaceoid A
(1) can also be derived from 14. The key intermediate 14 can
be prepared from 11 via reduction of a derivative of 11.
The preparation of 14 was carried out as depicted in Scheme
4. The hydrolysis of 11 followed by esterification gave the
dimethyl ester 17.4 The two hydroxy groups of 17 were
protected with TBDPS groups to afford 18. The symmetric
diol 16 was obtained by the reduction of diester 18.
Monotetrahydropyranylation gave the unilateral protected
alcohol 19. After the oxidation of alcohol 19 to aldehyde 15
followed by alkylation with an alkyl lithium reagent, we
obtained the key intermediate 14.
Scheme 2. Protection of 3,6-Dihydroxyphtalonitrile (11)
Scheme 3. Revised Retrosynthetic Analysis of 1 and 2
B
DOI: 10.1021/acs.jnatprod.8b00215
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Scheme 4. Synthesis of Key Intermediate 14
Scheme 5. Synthesis of 1 and rac-2
1
We next attempted to synthesize 1 and rac-2 (Scheme 5).
The double bond in 20 was generated by mesylation of 14.
Then, the THP group was removed, and we obtained the
alcohol 21. Finally, by deprotection of the two TBDPS groups,
we accomplished the total synthesis of violaceoid A (1). In
addition, by sequential deprotection of the THP and TBDPS
groups of 14, we achieved the synthesis of rac-violaceoid B
Table 1. Comparison of the H and 13C NMR Data of
Naturally Occurring Violaceoid A with Those of Synthetic 1
in CD3OD
1
(rac-2). We compared the H and 13C NMR data of the
synthetic 1 and 2 with those of naturally occurring violaceoids
A and B reported in the literature to determine the true
structure. The results are shown in Tables 1 and 2. As a result,
the 1H and 13C NMR data of synthetic 1 and 2 were shown to
be in accordance with those reported for the natural
compounds.
Subsequently, we attempted to determine the absolute
configuration of naturally occurring violaceoid B (2), as
depicted in Scheme 6.
We obtained the alcohol 24 by acetylation of 14 followed by
deprotection of the THP group. Next, deprotection of the
TBDPS group was conducted to afford the 1,3-diol 13. The
acetyl and TBDPS groups were reductively removed together
to yield rac-7, which is the substrate for kinetic resolution.3
The kinetic resolution was conducted to afford the enantiorich
ester (R)-265 and enantiorich alcohol (S)-7. Further, the same
reaction was repeated five times to obtain the enantiorich
alcohol in 95% ee. Finally, by deprotection of the acetonide
group, we accomplished the asymmetric total synthesis of (S)-
violaceoid B ((S)-2). However, the optical rotation of (S)-2
was not consistent with the natural product, which motivated
us to synthesize the enantiomer, (R)-violaceoid B ((R)-2)
(Scheme 7).
a
natural violaceoid A
δC
synthetic violaceoid A
b
c
position
δH (J in Hz)
position
δC
δH (J in Hz)
1
2
3
4
5
6
1′
148.8
127.6
125.9
150.6
115.0 6.57, d (8.7)
116.3 6.60, d (8.7)
1
2
3
4
5
6
1′
148.8
127.6
125.9
150.6
114.9 6.54, d (8.7)
116.2 6.59, d (8.7)
124.6 6.50−6.42, m
124.6 6.45, dt (16.1,
6.9)
137.9 6.06, dt (16.1,
6.9)
2′
3′
2′
3′
137.8 6.08, dt (15.6,
6.9)
34.9
2.24, tdd (6.9, 6.9,
1.5)
34.9
2.32−2.19, m
4′
5′
6′
7′
1″
30.3
32.7
23.6
14.4
58.5
1.51, m
1.37, m
1.37, m
0.93, t (7.1)
4.72, s
4′
5′
6′
7′
1″
30.4
32.7
23.7
14.5
58.4
1.61−1.43, m
1.43−1.30, m
1.43−1.30, m
0.94, t (7.2)
4.71, s
a
b
300 MHz for H NMR and 75 MHz for 13C NMR. See ref 1. 125
MHz using JNM-LA500. 300 MHz using JNM-AL300.
1
c
As for enantiopure (S)-7, we conducted the kinetic
resolution procedure. Using the recovered enantiorich alcohol,
the same reaction was then repeated five times to obtain the
enantiorich alcohol in 93% ee. Finally, deprotection of the
C
DOI: 10.1021/acs.jnatprod.8b00215
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1
Table 2. Comparison of the H and 13C NMR Data of
violaceoid B, and cell numbers were estimated using WST-8
reagent. Violaceoid A (1) inhibited the growth of MCF-7 cells
and Hs 578T cells at concentrations of 10−100 μM in a dose-
dependent manner. The GI50 values of 1 in MCF-7 cells and
Naturally Occurring Violaceoid B with Those of Synthetic 2
in CD3OC
Hs 578T cells were 61.5
18.0 and 59.7
10.0 μM,
respectively. On the other hand, (S)-violaceoid B ((S)-2) and
(R)-violaceoid B ((R)-2) did not inhibit the growth of these
cells within the same range. The GI50 values of (S)-2 and (R)-2
in these cells were therefore estimated as >100 μM.
In conclusion, we have accomplished the first total synthesis
of violaceoids A and B and successfully elucidated the absolute
configuration of the naturally occurring violaceoid B. The
cytotoxicity of the synthetic violaceoid A and (S)- or (R)-
violaceoid B against human cancer cells was assessed using
MCF-7 cells or Hs 578T cells, and it was shown that violaceoid
A inhibited the growth of both breast cancer cell lines at
concentrations of less than 100 μM (with GI50 values of 61.5
18.0 μM for MCF-7 and 59.7 10.0 μM for Hs 578T).
a
natural violaceoid B
synthetic violaceoid B
b
c
position
δC
δH (J in Hz)
position
δC
δH (J in Hz)
1
2
3
4
5
6
1′
150.1
130.4
125.1
150.1
1
2
3
4
5
6
1′
150.1
130.5
125.0
150.0
115.6 6.63, d (8.4)
117.5 6.60, d (8.4)
115.8 6.57, d (8.7)
117.5 6.60, d (8.7)
72.0
5.15, dd (9.0,
4.6)
71.9
5.19, dd (9.2,
4.4)
2′
38.2
1.89, m, 1.71, m
2′
38.2
2.00−1.83,
EXPERIMENTAL SECTION
■
1.80−1.67, m
General Experimental Procedures. Optical rotations were
determined using a Jasco P-1020 polarimeter. Infrared (IR) spectra
were obtained using a Jasco FT/IR-4600 Fourier transform infrared
spectrometer. 1H and 13C NMR spectra were recorded with
chloroform (in CDCl3) or methanol (in CD3OD) on the following
instrument: JEOL JNM-AL300 (1H at 300 MHz and 13C at 75 MHz),
JEOL JNM-LA500 (1H at 500 MHz and 13C at 125 MHz), Bruker
Biospin AVANCE 400 M (1H at 400 MHz and 13C at 100 MHz).
Mass spectra were determined by a Bruker Daltonics micrOTOF
focus (ESI-TOF) mass spectrometer. Thin-layer chromatography was
performed on Wakogel B-5F. HPLC was performed with a Hitachi
LaChrom Elite system composed of the organizer, L-2400 UV
detector, and L-2130 pump.
All reactions were carried out under an argon atmosphere in dried
glassware unless otherwise noted. CH2Cl2 was distilled from
diphosphorus pentoxide and then calcium hydride and dried over
MS 4 Å. All reagents were purchased from Tokyo Kasei Kogyo Co.,
Ltd., Kanto Chemical Co., Inc., or Aldrich Chemical Co., Inc., and
used without further purification unless otherwise noted. Carbon
3′
4′
5′
6′
7
27.2
30.3
33.1
23.7
14.4
56.6
1.55, m
1.33, m
1.33, m
1.33, m
0.89, t (6.8)
4.71, d (11.7),
4.68, d (11.7)
3′
4′
5′
6′
7′
1″
27.2
30.4
33.1
23.7
14.5
56.5
1.67−1.43, m
1.43−1.25, m
1.43−1.25, m
1.43−1.25, m
0.93, t (6.8)
4.75, d (11.6),
4.71, d (11.6)
b
1″
a
300 MHz for H NMR and 75 MHz for 13C NMR. See ref 1. 125
MHz using JNM-LA500. 400 MHz using AVANCE 400M.
1
c
acetonide group was conducted, and we accomplished the
asymmetric total synthesis of (R)-violaceoid B ((R)-2). The
optical rotation of synthetic (R)-2 was consistent with that
reported for naturally occurring violaceoid B.
To evaluate the antiproliferative effect of violaceoids on
human breast cancer cells, MCF-7 cells or Hs 578T cells were
incubated with synthetic violaceoid A and (S)- and (R)-
Scheme 6. Synthesis of (S)-Violaceoid B ((S)-2)
D
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Scheme 7. Synthesis of (R)-Violaceoid B ((R)-2)
atoms of all compounds are numbered according to IUPAC
nomenclature.
16 (1.16 g, 1.79 mmol) in CH2Cl2 (44.8 mL, 0.04 M) were added
pyridinium p-toluenesulfonate (90.4 mg, 0.36 mmol) and 3,4-dihydro-
2H-pyran (0.23 mL, 2.69 mmol), and the reaction mixture was stirred
for 4 h at rt. The reaction mixture was quenched by saturated aqueous
NaHCO3, extracted with CH2Cl2, and concentrated. The residue was
purified by silica gel column chromatography (gradient, n-hexane/
EtOAc = 20:1−1:1). Compound 19 (1.22 g, 93%) was obtained as a
white solid.
3,6-Bis((tert-butyldiphenylsilyl)oxy)-2′-(((tetrahydro-2H-
pyran-2-yl)oxy)methyl)benzaldehyde (15). To a solution of 19
(198 mg, 0.271 mmol) in CH2Cl2 (5.42 mL, 0.05 M) were added
tetrapropylammonium perruthenate (28.6 mg, 0.0813 mmol) and 4-
methylmorpholine N-oxide (95.5 mg, 0.813 mmol), and the reaction
mixture was stirred for 2 h at 0 °C. The reaction mixture was filtrated
through a short pad of silica gel with EtOAc, and the filtrate was
concentrated. Compound 15 (197 mg, quant.) was obtained as a
white solid.
1-(3′,6′-Bis((tert-butyldiphenylsilyl)oxy)-2′-(((tetrahydro-
2H-pyran-2″-yl)oxy)methyl)phenyl)heptan-1-ol (14). To a
solution of 15 (457 mg, 0.628 mmol) in THF (12.6 mL, 0.05 M)
was added a 1.15 M hexyllithium solution in n-hexane at −78 °C. The
reaction mixture was stirred for 5 min, quenched with saturated
aqueous NaHCO3, extracted with EtOAc, and concentrated. The
residue was purified by silica gel column chromatography (n-hexane/
EtOAc = 5:1). Compound 15 (382 mg, 75%) was obtained as a white
solid.
(E)-((2-(Hept-1′-en-1′-yl)-3-(((tetrahydro-2H-pyran-2″-yl)-
oxy)methyl)-1,4-phenylene)bis(oxy))bis(tert-butyl-
diphenylsilane) (20). To a solution of 14 (180 mg, 0.220 mmol) in
CH2Cl2 (4.35 mL, 0.05 M) were added methanesulfonyl chloride
(0.047 mL, 0.660 mmol) and triethylamine (0.186 mL, 1.321 mmol)
at 0 °C. The reaction mixture was stirred for 3 h at rt, quenched with
saturated aqueous NaHCO3, extracted with CH2Cl2, and concen-
trated. The residue was purified by silica gel column chromatography
(n-hexane/EtOAc = 5:1). Compound 20 (122 mg, 70%) was
obtained as a white solid.
(E)-(3,6-Bis((tert-butyldiphenylsilyl)oxy)-2-(hept-1′-en-1′-
yl)phenyl)methanol (21). To a solution of 20 (84.0 mg, 0.106
mmol) in MeOH (4.24 mL, 0.025 M) was added p-toluenesulfonic
acid monohydrate (30.0 mg, 0.158 mmol) at 0 °C. The reaction
mixture was stirred for 2 h at rt, diluted with water, extracted with
chloroform, and concentrated. The residue was purified by silica gel
column chromatography (n-hexane/EtOAc = 5:1). Compound 21
(53.8 mg, 72%) was obtained as a colorless oil.
(E)-2-(Hept-1′-en-1′-yl)-3-(hydroxymethyl)benzene-1,4-diol
(Violaceoid A (1)). To a solution of 21 (18.5 mg, 0.026 mmol) in
THF/pyridine (1.7 mL, v/v = 1/1, 0.015 M) was added hydrogen
fluoride pyridine complex (0.60 mL) at 0 °C. The reaction mixture
was stirred for 2 h, quenched with saturated aqueous NaHCO3,
extracted with EtOAc, and washed with saturated aqueous copper
sulfate, water, saturated aqueous NaHCO3, and brine. The organic
layer was dried over Na2SO4 and concentrated. The residue was
3,6-Bis(methoxymethoxy)phthalonitrile (12). To a solution of
3,6-dihydroxyphthalonitrile (11) (400 mg, 2.56 mmol) in CH2Cl2
(25.6 mL) were added diisopropylethylamine (1.76 mL, 10.24 mmol)
and methyloxymethyl chloride (0.77 mL, 10.24 mmol) at 0 °C. The
reaction mixture was stirred for 1 h and warmed up to room
temperature (rt) and stirred for 12 h. The solution was diluted with
water and extracted with CH2Cl2. The organic layer was washed with
H2O and brine, dried over Na2SO4, filtrated, and concentrated. The
residue was purified by silica gel column chromatography (n-hexane/
EtOAc = 4:1). Compound 12 (127 mg, 20%) was obtained as a white
solid.
Dimethyl 3,6-Dihydroxyphthalate (17). To a solution of KOH
(32.0 g, 570 mmol) in water (32.0 mL) was added 3,6-
dihydroxyphthalonitrile (11) (5.00 g, 31.2 mmol) at rt. The reaction
mixture was refluxed for 1 h. Aqueous 20% H2SO4 (100 mL) was
slowly poured into the reaction mixture and extracted with Et2O and
CHCl3 sequentially. The organic layer was dried over Na2SO4,
filtrated, and concentrated in vacuo, giving 3,6-dihydroxyphthalic acid
(6.20 g) as the crude product including starting material 11. The
mixture was used for the next reaction without further purification.
To a solution of the crude 3,6-dihydroxyphthalic acid (6.20 g) in
MeOH (240 mL) was added boron trifluoride diethyl etherate (12.8
mL, 100 mmol) at room temperature, and reaction mixture was
refluxed for 15 h. The solution was cooled to rt, concentrated in
vacuo, quenched with saturated aqueous NaHCO3, and extracted with
EtOAc. The organic layer was dried over Na2SO4, filtrated, and
concentrated in vacuo to yield dimethyl 3,6-dihydroxyphthalate (17)
(3.69 g, 52% in two steps) as a white solid.
Dimethyl 3,6-Bis((tert-butyldiphenylsilyl)oxy)phthalate
(18). 3,6-Dihydroxyphthalate (17) (500 mg, 2.21 mmol) was
dissolved in CH2Cl2 (22 mL) and cooled to 0 °C. Imidazole (1.20
g, 17.68 mmol) and TBDPSCl (2.27 mL, 8.84 mmol) were added to
the solution, and the reaction mixture was stirred for 2 h at rt. The
mixture was quenched by saturated aqueous NaHCO3 at 0 °C,
extracted with EtOAc, washed with brine, and concentrated. The
residue was purified by silica gel column chromatography (gradient, n-
hexane/EtOAc = 20:1−5:1). Compound 18 (1.61 g, 95%) was
obtained as a white solid.
(3,6-Bis((tert-butyldiphenylsilyl)oxy)-1,2-phenylene)-
dimethanol (16). To a solution of 18 in CH2Cl2 was added
diisobutylaluminum hydride (42.7 mL, 42.7 mmol) at −78 °C, and
the reaction mixture was stirred at 0 °C for 2 h. The reaction mixture
was quenched by MeOH and saturated aqueous Rochell’s salt,
extracted with CH2Cl2, washed with brine, and concentrated. The
residue was purified by silica gel column chromatography (n-hexane/
EtOAc = 10:1). Compound 16 (1.60 g, 93%) was obtained as a
colorless solid.
(3,6-Bis((tert-butyldiphenylsilyl)oxy)-2-(((tetrahydro-2H-
pyran-2′-yl)oxy)methyl)phenyl)methanol (19). To a solution of
E
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Journal of Natural Products
Article
purified by silica gel chromatography (n-hexane/EtOAc = 2:1).
Violaceoid A (1) (4.1 mg, 67%) was obtained as a white solid.
1 - ( 3 ′ , 6 ′ - B i s ( ( t e r t - b u t y l d i p h e n y l s i l y l ) o x y ) - 2 ′ -
(hydroxymethyl)phenyl)heptan-1-ol (22). To a solution of 14
(200 mg, 0.246 mmol) in MeOH (5.00 mL, 0.05 M) was added p-
toluenesulfonic acid monohydrate (56.0 mg, 0.295 mmol) at 0 °C.
The reaction mixture was stirred for 2 h at rt, diluted with water,
extracted with chloroform, and concentrated. The residue was purified
by silica gel column chromatography (n-hexane/EtOAc = 5:1).
Compound 22 (119 mg, 66%) was obtained as a white solid.
2-(1′-Hydroxyheptyl)-3-(hydroxymethyl)benzene-1,4-diol
(rac-Violaceoid B (2)). To a solution of 22 (77 mg, 0.105 mmol) in
THF/pyridine (5.25 mL, v/v = 1:1, 0.02 M) was added hydrogen
fluoride pyridine complex (2.00 mL) at 0 °C. The reaction mixture
was stirred for 2 h, quenched with saturated aqueous NaHCO3,
extracted with EtOAc, and washed with saturated aqueous copper
sulfate, water, saturated aqueous NaHCO3, and brine. The organic
layer was dried over Na2SO4 and concentrated. The residue was
purified by silica gel chromatography (n-hexane/EtOAc = 2:1). rac-
Violaceoid B (2) (19.2 mg, 72%) was obtained as a white solid.
1 - ( 3 ′ , 6 ′ - B i s ( ( t e r t - b u t y l d i p h e n y l s i l y l ) o x y ) - 2 ′ -
(hydroxymethyl)phenyl)heptyl Acetate (24). To a solution of 14
(32.4 mg, 0.040 mmol) in CH2Cl2 (2.00 mL, 0.02 M) were added
acetic anhydride (7.5 μL, 0.0796 mmol), triethylamine (22.2 μL,
0.159 mmol), and N,N-dimethylpyridin-4-amine (1.0 mg, 0.00796
mmol) at rt. The reaction mixture was stirred for 12 h, quenched with
saturated aqueous NaHCO3, extracted with CH2Cl2 × 2 and EtOAc ×
2, and dried over Na2SO4. The organic layer was filtrated and
concentrated. The residue was purified by silica gel chromatography
(n-hexane/EtOAc = 5:1). The crude product 23 was used for the
following reaction without further purification.
To a solution of the crude product 23 in MeOH/THF (3.35 mL,
v/v = 1:1, 0.01 M) was added p-toluenesulfonic acid monohydrate
(9.6 mg, 0.0503 mmol) at 0 °C. The reaction mixture was stirred for 2
h at rt, quenched with saturated aqueous NaHCO3, extracted with
CH2Cl2 × 2 and EtOAc × 2, and dried over Na2SO4. The organic
layer was filtrated and concentrated. The residue was purified by silica
gel chromatography (n-hexane/EtOAc = 5:1). Compound 24 (20.4
mg, 66% in 2 steps) was obtained as a white solid.
1-(6′-((tert-Butyldiphenylsilyl)oxy)-3′-hydroxy-2′-
(hydroxymethyl)phenyl)heptyl Acetate (13). To a solution of 24
(1.45 g, 1.70 mmol) in THF/pyridine (34.0 mL, v/v = 2:1, 0.050 M)
was added hydrogen fluoride pyridine complex (1.0 mL) at 0 °C. The
reaction mixture was stirred for 2 h, quenched with saturated aqueous
NaHCO3, extracted with EtOAc, and concentrated. Pyridine was
removed by using azetrope with benzene, and the residue was purified
by silica gel chromatography (n-hexane/EtOAc = 3:1). Compound 13
(851 mg, 94%) was obtained as a white solid.
mmol), (R)-benzotetramisole (3.0 mg, 0.0118 mmol), and diphenyl-
acetic anhydride (57.0 mg, 0.141 mmol). The reaction mixture was
stirred for 12 h, quenched with saturated aqueous NaHCO3, extracted
EtOAc, and dried over Na2SO4. The organic layer was filtrated and
concentrated. The residue was purified by silica gel chromatography
(n-hexane/EtOAc = 4:1, then CH2Cl2). Compound (R)-26 (33.1 mg,
29%, 12% ee) was obtained as a colorless oil, and compound (S)-7
(37.1 mg, 54%, 50% ee) was obtained as a white solid.
(S)-5-(1′-Hydroxyheptyl)-2,2-dimethyl-4H-benzo[d][1,3]-
dioxin-6-ol ((S)-7) (95% ee). To a solution of (S)-7 (37.1 mg, 0.126
mmol, 50% ee) in Et2O (1.3 mL, 0.1 M) were added N,N-
diisopropylethylamine (0.014 mL, 0.0339 mmol), (R)-benzotetrami-
sole (1.6 mg, 0.00628 mmol), and diphenylacetic anhydride (13.8 mg,
0.0339 mmol). The reaction mixture was stirred for 12 h, quenched
with saturated aqueous NaHCO3, extracted with EtOAc, and dried
over Na2SO4. The organic layer was filtrated and concentrated. The
residue was purified by silica gel chromatography (n-hexane/EtOAc =
4:1, then CH2Cl2). Compound (S)-7 (35.9 mg, 97%, 54% ee) was
obtained as a white solid. The chiral enriched (S)-7 (18.8 mg, 95%
ee) was obtained by the same kinetic resolution after four repetitions
from the above (S)-7 (35.9 mg, 0.126 mmol, 54% ee).
(S)-7 (95% ee): [α]2D5 −46.3 (c 1.12, CHCl3). Other spectra are the
same as those of rac-7.
(S)-2-(1′-Hydroxyheptyl)-3-(hydroxymethyl)benzene-1,4-
diol ((S)-Violaceoid B, (S)-2)). To a solution of (S)-7 (69.5 mg,
0.236 mmol, 95% ee) in THF/H2O (2.36 mL, v/v = 1:1, 0.1 M) was
added p-toluenesulfonic acid monohydrate (22.4 mg, 0.118 mmol).
The reaction mixture was stirred for 6 h, quenched with saturated
aqueous NaHCO3, extracted with EtOAc, and concentrated. The
residue was purified by silica gel chromatography (n-hexane/EtOAc =
3:1) to afford (S)-violaceoid B (9.2 mg, 15%) as a white solid, and
(S)-7 (59.6 mg, 86%) was recovered.
Additionally, (S)-violaceioid B (25.7 mg) and (S)-7 (27.1 mg)
were obtained by the same reaction after two repetitions from the
recovered (S)-7 (59.6 mg, 0.202 mmol). Therefore, 34.9 mg (total
amount) of (S)-violaceoid (58%) was prepared from the starting 69.5
mg of (S)-7.
(S)-Violaceoid B ((S)-2) (95% ee): [α]2D3 −17.0 (c 0.093, CHCl3),
[α]2D3 −22.5 (c 0.667, MeOH). Other spectra are the same as those of
rac-2.
(S)-1-(6′-Hydroxy-2′,2′-dimethyl-4H-benzo[d][1′,3′]dioxin-
5′-yl)heptyl 2,2-diphenylacetate ((S)-26) and (R)-5-(1′-Hydrox-
yheptyl)-2,2-dimethyl-4H-benzo[d][1,3]dioxin-6-ol ((R)-7). To
a solution of rac-7 (123.7 mg, 0.420 mmol) in Et2O (4.2 mL, 0.1 M)
were added N,N-diisopropylethylamine (0.050 mL, 0.303 mmol), (S)-
benzotetramisole (5.0 mg, 0.0210 mmol), and diphenylacetic
anhydride (102 mg, 0.252 mmol). The reaction mixture was stirred
for 12 h, quenched with saturated aqueous NaHCO3, extracted with
EtOAc, and dried over Na2SO4. The organic layer was filtrated and
concentrated. The residue was purified by silica gel chromatography
(n-hexane/EtOAc = 4:1, then CH2Cl2). Compound (S)-26 (65.9 mg,
32%, 19% ee) was obtained as a colorless oil, and compound (R)-7
(60.1 mg, 49%, 58% ee) was obtained as a white solid.
1-(6′-((tert-Butyldiphenylsilyl)oxy)-2′,2′-dimethyl-4H-
benzo[d][1′,3′]dioxin-5′-yl)heptyl Acetate (25). To a solution of
13 (851 mg, 1.59 mmol) in CH2Cl2 (53.1 mL, 0.03 M) were added
2,2-dimethoxypropane (0.49 mL, 3.98 mmol) and p-toluenesulfonic
acid monohydrate (1.5 mg, 0.0796 mmol). The reaction mixture was
stirred for 30 min, quenched with saturated aqueous NaHCO3,
extracted with EtOAc, and concentrated. The residue was purified by
silica gel chromatography (n-hexane/EtOAc = 3:1). Compound 25
(838 mg, 92%) was obtained as a colorless oil.
5-(1′-Hydroxyheptyl)-2,2-dimethyl-4H-benzo[d][1,3]dioxin-
6-ol (rac-7). To a solution of 25 (422 mg, 0.734 mmol) in THF
(24.5 mL, 0.03 M) was added 1.0 M lithium aluminum hydride in
THF (1.84 mL) at 0 °C. The reaction mixture was stirred for 5 min,
quenched with MeOH and saturated aqueous Rochell’s salt, extracted
with EtOAc, and dried over Na2SO4. The organic layer was filtrated
and concentrated. The residue was purified by silica gel chromatog-
raphy (n-hexane/EtOAc = 3:1). Compound rac-7 (214 mg, 99%) was
obtained as a white solid.
(S)-26 (19% ee): [α]2D6 −0.20 (c 2.39, CHCl3). Other spectra are
the same as those of (R)-26.
(R)-5-(1′-Hydroxyheptyl)-2,2-dimethyl-4H-benzo[d][1,3]-
dioxin-6-ol ((R)-7) (93% ee). To a solution of (R)-7 (60.1 mg,
0.204 mmol, 58% ee) in Et2O (2.0 mL, 0.10 M) were added N,N-
diisopropylethylamine (9.6 μL, 0.0550 mmol), (S)-benzotetramisole
(2.6 mg, 0.0118 mmol), and diphenylacetic anhydride (22.3 mg,
0.0550 mmol). The reaction mixture was stirred for 12 h, quenched
with saturated aqueous NaHCO3, extracted with EtOAc, and dried
over Na2SO4. The organic layer was filtrated and concentrated. The
residue was purified by silica gel chromatography (n-hexane/EtOAc =
4:1, then CH2Cl2). Compound (R)-7 (56.8 mg, 95%, 66% ee) was
obtained as a white solid. The chiral enriched (R)-7 (31.0 mg, 93%
ee) was obtained by the same kinetic resolution after four repetitions
from the above (R)-7 (56.8 mg, 0.193 mmol, 66% ee).
(R)-1-(6′-Hydroxy-2′,2′-dimethyl-4H-benzo[d][1′,3′]dioxin-
5′-yl)heptyl 2,2-Diphenylacetate ((R)-26) and (S)-5-(1′-Hy-
droxyheptyl)-2,2-dimethyl-4H-benzo[d][1,3]dioxin-6-ol ((S)-
7). To a solution of rac-7 (69.3 mg, 0.235 mmol) in Et2O (2.3 mL,
0.1 M) were added N,N-diisopropylethylamine (0.030 mL, 0.169
(R)-7 (93% ee): [α]2D5 +44.6 (c 1.87, CHCl3). Other spectra are the
same as those of rac-7.
F
DOI: 10.1021/acs.jnatprod.8b00215
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Journal of Natural Products
Article
(R)-2-(1′-Hydroxyheptyl)-3-(hydroxymethyl)benzene-1,4-
diol ((R)-Violaceoid B, (R)-2)). To a solution of (R)-7 (25.1 mg,
0.0853 mmol, 93% ee) in THF/H2O (1.71 mL, v/v = 1:1, 0.05 M)
was added p-toluenesulfonic acid monohydrate (8.1 mg, 0.0426
mmol). The reaction mixture was stirred for 6 h, quenched with
saturated aqueous NaHCO3, extracted with EtOAc, and concentrated.
The residue was purified by silica gel chromatography (n-hexane/
EtOAc = 3:1) to afford (R)-violaceoid B (4.6 mg, 21%) as a white
solid, and (R)-7 (19.6 mg, 78%) was recovered.
(2) Violaceoid F had already been prepared as a synthetic
intermediate in the total synthesis of the enantiomer of eupenoxide
in 2004. See: Mehta, G.; Roy, S. Org. Lett. 2004, 6, 2389−2392.
(3) (a) Shiina, I.; Nakata, K. Tetrahedron Lett. 2007, 48, 8314−8317.
(b) Shiina, I.; Nakata, K.; Sugimoto, M.; Onda, Y.; Iizumi, T.; Ono, K.
Heterocycles 2009, 77, 801−810. (c) Nakata, K.; Shiina, I. Heterocycles
2010, 80, 169−176. (d) Shiina, I.; Ono, K.; Nakata, K. Chem. Lett.
2011, 40, 147−149. (e) Nakata, K.; Ono, K.; Shiina, I. Heterocycles
2011, 82, 1171−1180. (f) Nakata, K.; Shiina, I. Org. Biomol. Chem.
2011, 9, 7092−7096. (g) Shiina, I.; Nakata, K.; Ono, K.; Mukaiyama,
T. Helv. Chim. Acta 2012, 95, 1891−1911.
Additionally, (R)-violaceoid B (4.6 mg) and (R)-7 (12.2 mg) were
obtained by the same reaction after two repetitions from the
recovered (R)-7 (19.6 mg, 0.0618 mmol). Therefore, 9.2 mg (total
amount) of (R)-violaceoid (54%) was prepared from the starting 25.1
mg of (R)-7.
(4) Kelly, T. R.; Bell, S. H.; Ohashi, N.; Armstrong-Chong, R. J. J.
Am. Chem. Soc. 1988, 110, 6471−6480.
(5) The enantiomeric excess (ee) was determined after being
reduced to the corresponding alcohol 7.
(R)-Violaceoid B ((R)-2) (93% ee): [α]2D3 +15.9 (c 0.087, CHCl3),
[α]2D3 +22.9 (c 0.667, MeOH). Other spectra are the same as those of
rac-2.
Biological Assay. Human breast cancer cell lines MCF-7 and Hs
578T were obtained from Japanese Collection of Research
Bioresources (JCRB) Cell Bank (Osaka, Japan) and American Type
Culture Collection (Manassas, VA, USA), respectively. Cells were
cultured with Dulbecco’s modified Eagle’s medium supplemented
with antibiotic−antimycotic (100 U/mL penicillin, 100 μg/mL
streptomycin, 0.25 μg/mL amphotericin B; Thermo Fisher Scientific,
Inc., Waltham, MA, USA), gentamicin (10 μg/mL, Thermo Fisher
Scientific, Inc.), and 10% heat-inactivated fetal bovine serum. MCF-7
cells or Hs 578T cells were seeded in 96-well plates (1000 cells/well)
and incubated for 48 h at 37 °C. Cells were then incubated with
violaceoid A or (S)- and (R)-violaceoid B (10−100 μM) for 48 h, and
cell number was estimated by WST-8 reagent (Cell Counting Kit-8,
Dojindo Laboratories, Japan).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jnat-
prod.8b00215.
■
S
Copies of 1H and 13C NMR spectra for all new
compounds (1, 2, 7, and 12−26), HPLC data of
compounds rac-7, (S)-7, and (R)-7, and additional data
(procedure for the reduction of ester 26 to the
corresponding alcohol 7 and for the determination of
the absolute configuration of chiral alcohols (S)-7 and
(R)-7) (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: shiina@rs.kagu.tus.ac.jp.
■
ORCID
Isamu Shiina: 0000-0002-8749-2540
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors wish to thank Profs. F. Sugawara and S. Kamisuki
(Tokyo University of Science) for providing the spectroscopic
data for violaceoids A and B. This study was partly supported
by a Research Grant from the Center for Chirality and Grants-
in-Aid for Scientific Research from the Ministry of Education,
Science, Sports and Culture, Japan.
REFERENCES
■
(1) Myobatake, Y.; Takemoto, K.; Kamisuki, S.; Inoue, N.; Takasaki,
A.; Takeuchi, T.; Mizushina, Y.; Sugawara, F. J. Nat. Prod. 2014, 77,
1236−1240.
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DOI: 10.1021/acs.jnatprod.8b00215
J. Nat. Prod. XXXX, XXX, XXX−XXX