Structural study on a naturally occurring terphenyl quinone

Article abstract of DOI:10.1271/bbb.130199

Two terphenyl quinones were synthesized for a structural study on a naturally occurring biologically active terphenyl quinone. 3-Methoxy-5,6- diphenylcyclohexa-3,5-dien-1,2-dione, a possible structure proposed by our analysis of the NMR spectra, was synthesized by Suzuki-Miyaura coupling and subsequent oxidation of the resulting substituted phenol, although not being identical to the natural product. Recently isolated 3-methoxy-2,5- diphenylcyclohexa-2,5-dien-1,4-dione was synthesized from a commercially available 2,5-diphenyl-1,4-benzoquinone in three steps in a good overall yield, and its NMR spectra were identical to those of the natural product.

Full text of DOI:10.1271/bbb.130199

Biosci. Biotechnol. Biochem., 77 (7), 1529–1532, 2013
Structural Study on a Naturally Occurring Terphenyl Quinone
Atsuo NAKAZAKI, Wen-Yu HUANG, Kazushi KOGA, Boon-ek YINGYONGNARONGKUL,
Jutatip BOONSOMBAT, Yusuke SAWAYAMA, Takashi TSUJIMOTO, and Toshio NISHIKAWA
y
Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan
Received March 11, 2013; Accepted April 5, 2013; Online Publication, July 7, 2013
[doi:10.1271/bbb.130199]
Two terphenyl quinones were synthesized for a
structural study on a naturally occurring biologically
active terphenyl quinone. 3-Methoxy-5,6-diphenylcyclo-
hexa-3,5-dien-1,2-dione, a possible structure proposed
by our analysis of the NMR spectra, was synthesized by
Suzuki-Miyaura coupling and subsequent oxidation of
the resulting substituted phenol, although not being
identical to the natural product. Recently isolated 3-
methoxy-2,5-diphenylcyclohexa-2,5-dien-1,4-dione was
synthesized from a commercially available 2,5-diphenyl-
1,4-benzoquinone in three steps in a good overall yield,
and its NMR spectra were identical to those of the
natural product.
Fig. 1. Structures of p-Terphenyl o-Quinone 1 and o-Terphenyl o-
Quinone 2.
Key words: terphenyl quinone; structural study; synthesis
Terphenyl is a common structural motif included in
various naturally occurring products, largely isolated
from microorganisms and mushrooms.1) The diverse
structure and wide variety of biological activities have
attracted great attention these natural products.2,3) In
2006, Singh and co-workers at the Merck company
isolated a new p-terphenyl o-quinone as an amorphous
yellow powder from a Phoma sp. as an inhibitor of
parasite cyclic GMP-dependent protein kinase.4) Its
structure was characterized as that of compound 1 based
on an NMR analysis (Fig. 1). The substitution pattern
was elucidated by observing NOEs between methoxy
protons and quinone proton H-5 and between H-5 and
aromatic protons H-200/H-600 in the NOE differential
spectrum. In the same year, we reported a synthesis of
proposed compound 1, although its NMR spectra were
not identical to the reported data.5) We next proposed
that o-terphenyl o-quinone 2 would be another possible
structure which could also explain the NOE data (from
methoxy protons to the quinone proton) reported by
Singh and co-workers. In order to compare their NMR
spectra, we embarked on the synthesis of 2, although an
o-terphenyl compound has not been isolated from
natural sources.2,3)
Scheme 1. Synthesis of o-Terphenyl o-Quinone 2.
Reagents and conditions: (a) K2CO3, MeI, DMF, rt, 42%; (b)
PhB(OH)2, Pd(PPh3)4, Na2CO3, toluene-H2O, 100 ꢀC, 80%; (c)
mCBPO, CH2Cl2, rt; (d) LiAlH4, THF, rt; (e) Ac2O, pyridine, rt, 9%
(3 steps from 5); (f) LiAlH4, THF, rt; (g) Ag2CO3/Celite, benzene,
rt, 97% (2 steps from 6).
position of phenol 5 was next examined. Such typical
oxidation agents of phenol as Fremy’s salt
(K2[NO(SO3)2])7) and molecular oxygen under basic
conditions were ineffective, this being attributable to
steric hindrance around the phenyl group. In addition,
the oxidation of 5 using IBX (2-iodoxybenzoic acid),8)
chromic acid, CAN (ceric ammonium nitrate) and
benzeneselenic acid anhydride9) resulted in the forma-
tion of the undesired 4,5-diphenyl-1,2-benzoquinone as
a major product. Although the oxidation of 5 using
mCBPO (m-chlorobenzoyl peroxide)10) provided a com-
plex mixture of products, corresponding diacetate 6 was
isolated in a 9% yield from the three steps after two-step
transformation (LiAlH4 and then Ac2O in pyridine)
Results and Discussion
The synthesis of o-terphenyl o-quinone 2 is summa-
rized in Scheme 1. Known dibromocatechol 36) was
monomethylated to obtain 4 which was subjected to the
conventional Suzuki-Miyaura cross-coupling conditions
with phenylboronic acid, leading to o-terphenyl com-
pound 5 in an 80% yield. The key oxidation at the ortho-
y
To whom correspondence should be addressed. Fax: +81-52-789-4111; E-mail: nisikawa@agr.nagoya-u.ac.jp

1530
A. NAKAZAKI et al.
Fig. 2. NMR Spectral Data for o-Terphenyl o-Quinone 2.
Scheme 2. Synthesis of p-Terphenyl p-Quinone 7.
Reagents and conditions: (a) AcCl, MeOH, rt to 45 ꢀC, 71%;
(b) CAN, MeCN, rt, 99%; (c) NaOMe, MeOH, rt, 83%.
Fig. 3. Structure of p-Terphenyl p-Quinone 7.
according to the procedure reported by Tada and co-
workers.11) The acetyl groups in 6 were finally removed
with LiAlH4, and subsequent oxidation by a freshly
prepared Fetizon reagent (Ag2CO3 on Celite)12) gave
desired o-quinone 2 in a 97% yield from the two steps.
o-Terphenyl o-quinone 2 was obtained as an amor-
phous purple solid, the structure being confirmed by
analyses of the NMR spectra, IR spectrum, and high-
resolution mass spectrum. Selected 2D NMR spectral
data are shown in Fig. 2. It was found that the NMR
spectra of 2 were not identical to those of the reported
data.4)
Fig. 4. Observed NOE for Synthesized p-Terphenyl p-Quinone 7.
methoxy protons which was observed by Singh and
co-workers, and that the NOESY data showed no
correlation between the methoxy protons and quinone
proton H-6 (Supplemental Fig. 3).
Oberlies and co-workers have recently isolated a
p-terphenyl compound as an inhibitor of phosphodies-
terase 4B from an ascomycete fungus of the order
Chaetothyriales (MSX 47445).13) Its structure was
assigned to be that of 7 by NMR and X-ray crystallo-
Conclusions
Two terphenyl quinones, 3-methoxy-5,6-diphenylcy-
clohexa-3,5-dien-1,2-dione and 3-methoxy-2,5-diphe-
nylcyclohexa-2,5-dien-1,4-dione, were synthesized for
a structural study on a naturally occurring terphenyl
quinone. The NMR spectra of the latter quinone were
identical to those of the natural compound. This result
indicates that the structural misassignment of 1 by Singh
and co-workers was due to the observed signal enhance-
ment in the NOE differential spectra which may be
attributable to noise in the NMR measurements.
*
graphic analyses. Since its NMR data were consistent
with those of the compound reported by Singh and co-
workers, the authors concluded both natural compounds
to be the same (Fig. 3). A chemical synthesis of
p-quinone 7 had not previously been reported, so we
therefore synthesized 7 and ascertained the reason for
Singh’s and our misassignment of the structure.
Commercially available 2,5-diphenyl-1,4-benzoqui-
none 8 was transformed into chlorophenol 9 with HCl
in MeOH in a 71% yield (Scheme 2).15) Phenol 9 was
oxidized by using CAN to provide chloroquinone 10,
this being subjected to sodium methoxide in MeOH to
afford desired p-terphenyl p-quinone 7 in a good overall
yield as an orange solid. The NMR spectra of 7 were
good agreement with the reported data.4,13) The NOE
differential spectra showed NOEs from quinone proton
H-6 to H-20/H-60 and from methoxy protons to H-200/
H-600 (Fig. 4 and Supplemental Figs. 1 and 2; see the
Biosci. Biotechnol. Biochem. Web site). These observa-
tions were also good agreement with those reported by
Oberlies and co-workers.13) It should be noted that no
signal enhancement of quinone proton H-6 in the NOE
differential spectrum was apparent by irradiating the
Experimental
General information. Infrared spectra (IR) were recorded on a Jasco
FT/IR-4100 type A spectrophotometer and are reported in wave
number (cmꢁ1). Ultraviolet spectra (UV) were recorded on a Jasco
V-530 spectrophotometer and are reported in wave length (nm). Proton
nuclear magnetic resonance (1H-NMR) spectra were recorded on a
Varian Gemini-2000 (300 MHz), Bruker ARX-400 (400 MHz), Bruker
AVANCE-400 (400 MHz), Bruker AMX-600 (600 MHz), or Jeol
ECA-600 (600 MHz) instrument. NMR samples were dissolved in
CDCl3, and chemical shifts are reported in ppm relative to tetrame-
thylsilane (ꢀ ¼ 0:00). 1H-NMR data are expressed as chemical shift,
integration, multiplicity (s ¼ singlet, m ¼ multiplet), coupling con-
stant(s), and assignment. Carbon nuclear magnetic resonance (13C-
NMR) spectra were recorded on a Varian Gemini-2000 (75 MHz),
Bruker ARX-400 (100 MHz), or Bruker AVANCE-400 (100 MHz)
instrument. NMR samples were dissolved in CDCl3, and chemical
shifts are reported in ppm relative to the solvent (CDCl3 as ꢀ ¼ 77:0).
Melting point (mp) data were recorded on Yanaco MP-S3 melting
point apparatus and are uncorrected. The low-resolution mass spectrum
*
Oberlies and co-workers named compound 7 betulinan C in
ref. 13, although this name had already been used for a different
compound in ref. 14.

Structural Study on a Naturally Occurring Terphenyl Quinone
Bruker Esquire 3000 ESI mass
1531
(LR-MS) was recorded on
a
The residue was dissolved in pyridine (1.5 mL) and to the resulting
solution was added Ac2O (0.75 mL) at room temperature. After being
stirred at room temperature for 1.5 h, the reaction was quenched with
1 M HCl. The aqueous layer was extracted with EtOAc. The organic
layer was successively washed with 1 M HCl, a saturated aqueous
solution of NaHCO3 and brine, dried over anhydrous MgSO4, and
concentrated. The residue was purified by silica gel column chroma-
tography (hexane/EtOAc ¼ 9:1) and preparative thin-layer chroma-
tography (toluene:Et2O ¼ 3:1) to afford diacetate 6 (6.2 mg, 9% yield
from the 3 steps) as an amorphous white solid. IR (KBr) ꢁmax (cmꢁ1):
1777, 1483, 1369, 1346, 1208, 1105, 1096, 736, 704; 1H-NMR
(CDCl3, 400 MHz) ꢀ (ppm): 7.20–7.13 (6H, m, Ph), 7.10–7.07 (2H, m,
Ph), 7.05–7.01 (2H, m, Ph), 6.94 (1H, s, Ar), 3.90 (3H, s, –OMe), 2.33
(3H, s, Ac), 1.96 (3H, s, Ac); 13C-NMR (CDCl3, 100 MHz) ꢀ (ppm):
168.2, 167.8, 150.8, 141.3, 140.3, 139.7, 135.1, 130.5, 129.5, 127.7,
127.6, 127.5, 126.7, 126.6, 126.3, 111.6, 56.1, 20.2, 19.9; HR-MS
(ESI, positive): calcd. for C23H20O5Na (M þ Na), 399.1203; found,
399.1214.
spectrometer and is expressed in m=z. High-resolution mass spectra
(HR-MS) were recorded on an Applied Biosystems Mariner ESI-TOF
spectrometer and are expressed in m=z.
4,5-Dibromo-2-methoxyphenol (4). To a solution of dibromocate-
chol 36) (8.10 g, 30.2 mmol) in DMF (91 mL) were added K2CO3
(4.13 g, 30.2 mmol) and MeI (1.9 mL, 30.6 mmol) at room temperature
under nitrogen. After being stirred at room temperature for 22 h, the
reaction was quenched with H2O. The aqueous layer was extracted
with EtOAc. The organic layer was washed with brine, dried over
anhydrous MgSO4, and concentrated. The residue was purified by
silica gel column chromatography (hexane/EtOAc ¼ 12:1 to 6:1 to
1:1) to give 4 (3.57 g, 42%) as a white powder. Mp 91–92 ꢀC (lit.16)
92–93 ꢀC); IR (KBr) ꢁmax (cmꢁ1): 3388, 1492, 1433, 1264, 1203, 1027,
865, 797; 1H-NMR (CDCl3, 300 MHz) ꢀ (ppm): 7.18 (1H, s, Ar), 7.06
(1H, s, Ar), 5.57 (1H, s, –OH), 3.88 (3H, s, –OMe); 13C-NMR (CDCl3,
100 MHz) ꢀ (ppm): 146.4, 145.6, 119.1, 115.5, 115.4, 113.8, 56.3;
LR-MS (ESI, negative): calcd. for C7H5Br2O2 (M ꢁ H), 279; found,
279. The NMR data for 4 are identical to the reported data.16)
3-Methoxy-5,6-diphenylcyclohexa-3,5-dien-1,2-dione (2). To a so-
lution of diacetate 6 (4.0 mg, 0.011 mmol) in THF (0.5 mL) was added
LiAlH4 (1.6 mg, 0.044 mmol) at 0 ꢀC under nitrogen. After being
stirred at room temperature for 1 h, the reaction was quenched with
H2O at 0 ꢀC. The aqueous layer was extracted with EtOAc. The
organic layer was successively washed with 1 M HCl, a saturated
aqueous solution of NaHCO3 and brine, dried over anhydrous MgSO4,
and concentrated to give a crude catechol.
2-Methoxy-4,5-diphenylphenol (5). To a solution of 4 (900 mg,
3.19 mmol) in toluene (9 mL) was added a solution of Na2CO3 (2.0 M
in H2O, 2.3 mL, 4.7 mmol) at room temperature. The resulting mixture
under nitrogen was degassed three times by freezing the mixture under
reduced pressure. To the resulting mixture were added PhB(OH)2
(819 mg, 6.73 mmol) and Pd(PPh3)4 (370 mg, 0.306 mmol). After being
stirred at 100 ꢀC overnight, the reaction was quenched with H2O. The
aqueous layer was extracted with EtOAc. The organic layer was
washed with brine, dried over anhydrous MgSO4, and concentrated.
The residue was separated by silica gel column chromatography
(hexane/EtOAc ¼ 4:1) to give 5 (704 mg, 80%) as a white solid. Mp
147–148 ꢀC (lit.17) 146–148 ꢀC); IR (KBr) ꢁmax (cmꢁ1): 3290, 1487,
1156, 1031, 1014, 767, 701; 1H-NMR (CDCl3, 400 MHz) ꢀ (ppm):
7.24–7.08 (10H, m, Ph), 7.04 (1H, s, Ar), 6.92 (1H, s, Ar), 5.64 (1H, s,
–OH), 3.96 (3H, s, –OMe); 13C-NMR (CDCl3, 100 MHz) ꢀ (ppm):
145.8, 144.8, 141.6, 141.1, 133.8, 132.6, 129.94, 129.91, 127.83,
127.76, 126.2, 116.6, 113.0, 56.1; HR-MS (ESI, positive): calcd. for
C19H16O2Na (M þ Na), 299.1043; found, 299.1028. The NMR data for
5 are identical to the reported data.17)
To a solution of this crude catechol in benzene (1 mL) was added a
freshly prepared Fetizon reagent12) (34 mg, 0.060 mmol) at room
temperature under nitrogen. After being stirred at room temperature for
1.5 h, the reaction mixture was filtered and concentrated to give 2
(3.0 mg, 97% for the 2 steps) as an amorphous purple solid. IR (KBr)
ꢁmax (cmꢁ1): 1697, 1665, 1649, 1624, 1337, 1096, 1074, 719, 698; UV
(MeOH) ꢂmax (log "): 206 (4.76), 229 (4.48), 350 (3.66) nm; 1H-NMR
(CDCl3, 600 MHz) ꢀ (ppm): 7.29–7.24 (3H, m, H-30 and H-40), 7.20–
7.18 (3H, m, H-300 and H-400), 7.16–7.14 (2H, m, H-20), 6.97–6.96 (2H,
m, H-200), 6.22 (1H, s, H-4), 3.84 (3H, s, –OMe); 13C-NMR (CDCl3,
100 MHz) ꢀ (ppm): 178.6, 174.8, 151.8, 149.5, 138.9, 133.1, 131.7,
130.9, 129.1, 128.4, 128.3, 127.9, 127.6, 114.0, 56.0; HR-MS (ESI,
positive): calcd. for
313.0824.
C
19H14O3Na (M þ Na), 313.0835; found,
4,5-Diphenyl-1,2-benzoquinone. To a stirred solution of benzene-
seleninic anhydride (16.6 mg, 0.0461 mmol) in THF (1 mL) was added
a solution of 5 (10.2 mg, 0.0369 mmol) in THF (2 mL) at room
temperature under nitrogen. After being stirred at 50 ꢀC for 18 h, the
reaction was quenched with a saturated aqueous solution of NaHCO3
at room temperature. The aqueous layer was extracted with CH2Cl2.
The organic layer was dried over anhydrous MgSO4 and concentrated.
The residue was separated by preparative thin-layer chromatography
(hexane/EtOAc ¼ 9:1) to give 4,5-diphenyl-1,2-benzoquinone (5.9
mg, 61%) as a brown oil. IR (KBr) ꢁmax (cmꢁ1): 1661, 1355, 1283,
862, 769, 700; UV (MeOH) ꢂmax (log "): 206 (4.53), 225 (4.31), 277
(3.92), 349 (3.36) nm; 1H-NMR (CDCl3, 300 MHz) ꢀ (ppm): 7.38–7.29
(2H, m, Ph), 7.29–7.20 (4H, m, Ph), 7.08–7.00 (4H, m, Ph), 6.54 (2H,
s, Ar); 13C-NMR (CDCl3, 75 MHz) ꢀ (ppm): 180.1, 153.9, 137.0,
129.6, 129.3, 128.4, 128.2; HR-MS (ESI, positive): calcd. for
C18H12O2Na (M þ Na), 283.0730; found, 283.0724.
2-Chloro-4-methoxy-3,6-diphenylphenol (9). MeOH (60 mL) and
AcCl (1.3 mL, 18.3 mmol) were mixed at 0 ꢀC for 30 min. To this
solution was added 2,5-diphenyl-1,4-benzoquinone
8 (330 mg,
1.27 mmol) at 0 ꢀC, and then the resulting mixture was stirred at room
temperature for 12 h and at 45 ꢀC for 20 h. After being cooled to room
temperature, the volatile compounds were evaporated. The residue was
separated by silica gel column chromatography (hexane/
CH2Cl2 ¼ 2:1) to give 9 (280.1 mg, 71%) as a white solid. Mp 192–
193 ꢀC; IR (KBr) ꢁmax (cmꢁ1): 1401, 1215, 1192, 1045, 837, 756, 696;
1H-NMR (CDCl3, 400 MHz) ꢀ (ppm): 7.63–7.60 (2H, m, Ph), 7.52–
7.42 (8H, m, Ph), 6.89 (1H, s, Ar), 5.51 (1H, s, –OH), 3.70 (3H, s, –
OMe); 13C-NMR (CDCl3, 100 MHz) ꢀ (ppm): 151.0, 142.7, 137.5,
135.2, 130.2, 129.5, 129.2, 128.5, 128.04, 127.96, 127.8, 127.7, 121.2,
112.5, 56.6; HR-MS (ESI, positive): calcd. for C19H15O2NaCl
(M þ Na), 333.0653; found, 333.0639.
3-Chloro-2,5-diphenylcyclohexa-2,5-dien-1,4-dione (10). To a solu-
tion of 9 (40.5 mg, 0.130 mmol) in MeCN (8 mL) was added CAN
(139 mg, 0.254 mmol) at room temperature. After being stirred at room
temperature for 10 min, the reaction was quenched with H2O. The
aqueous layer was extracted with EtOAc. The organic layer was
washed with brine, dried over anhydrous Na2SO4, and concentrated.
The residue was separated by silica gel column chromatography
(hexane/CH2Cl2 ¼ 3:2 to 1:1) to give 10 (38.1 mg, 99%) as a yellow
solid. Mp 174–178 ꢀC; IR (KBr) ꢁmax (cmꢁ1): 1666, 1644, 1444, 1417,
756, 693; 1H-NMR (CDCl3, 400 MHz) ꢀ (ppm): 7.57–7.45 (8H, m,
Ph), 7.37–7.33 (2H, m, Ph), 7.01 (1H, s, Ar); 13C-NMR (CDCl3,
100 MHz) ꢀ (ppm): 184.4, 179.4, 145.9, 143.4, 140.9, 132.7, 132.4,
130.9, 130.5, 129.7, 129.6, 129.3, 128.6, 128.1; HR-MS (ESI,
1,2-Diacetoxy-3-methoxy-5,6-diphenylbenzene (6). To a solution of
5 (50 mg, 0.18 mmol) in CH2Cl2 (5 mL) was added mCBPO (134 mg,
0.431 mmol) at room temperature under nitrogen. After being stirred at
room temperature for 18 h, the reaction was quenched with a saturated
aqueous solution of Na2S2O3. The aqueous layer was extracted with
EtOAc. The organic layer was washed with brine, dried over
anhydrous MgSO4, and concentrated.
The residue was dissolved in THF (5.5 mL) and to the resulting
solution was added LiAlH4 (23 mg, 0.61 mmol) at 0 ꢀC under nitrogen.
The resulting mixture was warmed to room temperature, and then
stirred at room temperature for 3 h. The reaction was quenched with
H2O at 0 ꢀC. The aqueous layer was extracted with EtOAc. The
organic layer was successively washed with 1 M HCl, a saturated
aqueous solution of NaHCO3 and brine, dried over anhydrous MgSO4,
and concentrated.
positive): calcd. for
317.0326.
C18H11O2NaCl (M þ Na), 317.0340; found,

1532
A. NAKAZAKI et al.
3-Methoxy-2,5-diphenylcyclohexa-2,5-dien-1,4-dione (7). To
a
References
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solution of NaOMe (a 0.87 M solution in MeOH, 1.5 mL, 0.13 mmol) at
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to the reaction mixture was added sodium methoxide (a 0.87 M solution
in MeOH, 0.1 mL, 1 mmol). After being stirred at room temperature for
10 min, the reaction was quenched with a saturated aqueous solution of
NH4Cl. The aqueous layer was extracted with EtOAc. The organic
layer was washed with brine, dried over anhydrous MgSO4, and
concentrated. The residue was separated by silica gel column
chromatography (hexane/CH2Cl2 ¼ 1:1 to 1:2) to give 7 (31.7 mg,
83%) as an orange solid, together with recovered 10 (2.7 mg, 7%). Mp
167–170 ꢀC; IR (KBr) ꢁmax (cmꢁ1): 1660, 1640, 1266, 1092, 766, 707,
691; UV (MeOH/THF, 1:1) ꢂmax (log "): 211 (4.29), 236 (4.29), 331
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128.5, 127.9, 61.4; HR-MS (ESI, positive): calcd. for C19H14O3Na
(M þ Na), 313.0835; found, 313.0835.
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of natural terphenyl quinone. This work was financially
supported by grant-aid for scientific research (B) and
grant-aid for young scientists (B) from JSPS, the
Yamada Science Foundation, the Daiichi-Sankyo Foun-
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Program on Natural Medicine in Pharmaceutical Sci-
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