Practical Synthesis of Polymethylated Flavones: Nobiletin and Its Desmethyl Derivatives

Article abstract of DOI:10.1021/acs.oprd.9b00091

We present a practical synthesis of the polymethoxylated citrus flavone nobiletin that is suitable for use on a hundred gram scale. Ready availability of this compound and its derivatives will aid detailed chemical-biological investigations of their biological activities, including activation of signaling via the cAMP-dependent protein kinase A/extracellular signal-related protein kinase/cAMP response element-binding protein pathway.

Full text of DOI:10.1021/acs.oprd.9b00091

Article
pubs.acs.org/OPRD
Cite This: Org. Process Res. Dev. XXXX, XXX, XXX−XXX
Practical Synthesis of Polymethylated Flavones: Nobiletin and Its
Desmethyl Derivatives
Tomohiro Asakawa,†,‡ Hiroto Sagara,† Masaki Kanakogi,† Aiki Hiza,† Yuta Tsukaguchi,†
Takahiro Ogawa,† Miho Nakayama,† Hitoshi Ouchi,† Makoto Inai,† and Toshiyuki Kan*,†
†School of Pharmaceutical Sciences, University of Shizuoka, 52-1 Yada, Suruga-ku, Shizuoka 422-8526, Japan
‡Tokai University Institute of Innovative Science and Technology, 4-1-1 Kitakaname, Hiratsuka, Kanagawa 259-1292, Japan
S
* Supporting Information
ABSTRACT: We present a practical synthesis of the polymethoxylated citrus flavone nobiletin that is suitable for use on a
hundred gram scale. Ready availability of this compound and its derivatives will aid detailed chemical-biological investigations of
their biological activities, including activation of signaling via the cAMP-dependent protein kinase A/extracellular signal-related
protein kinase/cAMP response element-binding protein pathway.
KEYWORDS: nobiletin, sudatichin, flavonoid, practical synthesis
Scheme 1. Synthetic Strategy for Nobiletin (1)
INTRODUCTION
■
Nobiletin (1) (Figure 1), isolated from citrus fruits,1 has
various biological activities, such as activation of signaling via
Figure 1. Structure of nobiletin (1).
tetramethoxyacetophenone 7. After the Baeyer−Villiger
oxidation reaction of 7 with mCPBA, hydrolysis of the
generated acetate 8 gave phenol 9. Since 9 was expected be
sensitive to oxygen, all handling was carried out under an argon
atmosphere. Next, methylation of labile phenol 9 by treatment
with methyl iodide under basic conditions under an argon
atmosphere to avoid oxidation gave 10. Friedel−Crafts
reaction of 10 and concomitant demethylation gave
acetophenone 3. Although the desired polymethoxyacetophe-
none unit had been obtained, it was difficult to ensure a stable
supply by the first synthetic route. Thus, we modified the
synthetic route of 3.
The modified synthesis of pentamethoxybenzene (10)
started from inexpensive 1,2,3-trimethoxybenzene (11)
(Scheme 3). The introduction of the hydroxyl group was
achieved by formylation followed by Baeyer−Villiger oxidation.
Upon treatment of 11 with dichloromethoxymethane in the
presence of titanium tetrachloride, the formylation reaction
proceeded to afford 12. Previously, silica gel column
chromatography (SiO2 C.C.) had been required to remove
the cAMP-dependent protein kinase A (PKA)/extracellular
signal-related protein kinase (ERK)/cAMP response element-
binding protein (CREB) pathway in cell culture systems,
including cultured rat hippocampal neurons, and may be a lead
compound for novel antidementia agents.2,3 However,
commercially available natural 1 is expensive (47,300 yen/5
mg; ChromaDex, Inc.),4 although synthetic 1 is currently
available at a lower price (55,000 yen/500 mg; Wako
Chemicals).5 During the course of our work on flavone
synthesis, we have developed an efficient synthesis of the
flavone skeleton via a β-diketone intermediate, and we
anticipated that this method might be suitable for the practical
large-scale synthesis of 1 and its demethyl derivatives. which
we report herein.
RESULTS AND DISCUSSION
■
Our synthetic strategy is illustrated in Scheme 1. We
anticipated that the synthesis of tetramethoxyacetophenone
derivative 3 would be the key to this synthesis.
As shown in Scheme 2, the preparation of 3 began with
1,3,4,5-tetramethoxybenzene (5).6 Upon treatment with acetyl
chloride and aluminum trichloride, Friedel−Crafts acylation of
5 and concomitant demethylation proceeded to give
acetophenone 6. Subsequent O-methylation of 6 afforded
Special Issue: Japanese Society for Process Chemistry
Received: February 25, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.oprd.9b00091
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tetramethoxybenzene (15). Different reaction conditions were
required for the second formylation reaction of 15. The
formylation of the stronger electrophile 15, which has four
methoxy groups, would match the mild acidic conditions of the
Vilsmeier−Haack reaction.8 After the Baeyer−Villiger oxida-
tion with selenium dioxide, solvolysis of the generated formyl
group afforded phenol 17. Fortunately, 17 was more stable
than 9 toward oxygen, even though they are regioisomers.
Finally, methylation and acylation of 17 gave 3 without the
sensitive care for the prevention of oxidation. A stable supply
of the important synthetic intermediate 3 was eventually
achieved by the modified synthetic route, which contains no
chromatographic purification.
Scheme 2. Synthesis of the Key Unit 3 from 1,3,4,5-
Tetramethoxybenzene (5)
a
Benzotriazole derivative 4,9 which is readily obtainable from
3,4-dimethoxybenzoic acid (18), is a suitable acyl donor
(Scheme 4). Generally, the reactions of acetophenone with
readily available acyl halides provide the esters generated by
the reaction with the phenolic hydroxyl group. The use of
acylbenzotriazole as a soft nucleophile, however, directly
afforded the C-acyl adduct. Moreover, acylbenzotriazoles
were extremely stable, in contrast to labile acyl halides,
which required preparation at the time of use. It is more
suitable for the practical-scale synthesis that acylbenzotriazole
4 has high long-term storage stability. The desired C-acylation
reaction of 3 with 4 in the presence of LHMDS at 0 °C
afforded β-diketone 2. Upon treatment with trifluoroacetic acid
in methanol, cyclization (2 → 19) and dehydration (19 → 20)
afforded 1. This total synthesis of 1 was accomplished in 35%
total yield in 11 steps starting from 11.10
a
Conditions: (a) AcCl, AlCl3, Et2O, 76%; (b) MeI, K2CO3, DMF, 70
°C, 94%; (c) mCPBA, CH2Cl2 then SiO2 C.C.; (d) K2CO3, MeOH
then SiO2 C.C. under air; (e) SeO2 (cat.), 30% H2O2, t-BuOH, 50 °C;
(f) K2CO3, MeOH then SiO2 C.C. under argon, 51% (two steps); (g)
MeI, K2CO3, acetone (under Ar), 95%; (h) AcCl, AlCl3, CH2Cl2, 45%
(brsm 94%).
Our synthetic route illustrated in Scheme 4 was applied to
the hundred gram scale preparation of nobiletin by
USHIOchemix Ltd., and we and our collaborators are currently
using this material for detailed investigation of its biological
activities11 and toxicological testing.
a
Scheme 3. Improved Synthesis of Acetophenone 3
There have been few pharmacodynamic studies of dietary
flavonoids using labeled synthetic compounds, even though
several flavonoids are known to have beneficial effects both in
vitro and in vivo.16 Thus, we investigated the conversion of
modified acetophenones and acylbenzotriazoles into desmethyl
derivatives of nobiletin (Scheme 5). After the coupling of 4
and 2212 afforded β-diketone 23, acidic cyclization of 23 gave
flavone 24. The demethylation of 24 under Node’s
conditions13 afforded 5,4′-desmethylnobiletin (26) accompa-
nied by debenzylation. The key to the regioselectivity and
enhancement of the reactivity was the chelation of the Lewis
acid to the carbonyl group (intermediate 25). Additionally, 5-
desmethylnobiletin (27) was obtained from 1 under Node’s
conditions by selective demethylation at the 5-position.
In addition, 7-desmethylnobiletin (32) was also obtained
from 3 (Scheme 6). The regioselective demethylation reaction
proceeded at the 4-position of 3 to afford 29 through
intermediate 28 activated by lithium chloride. After the
benzylation of 29, condensation of the resulting compound
30 with 4 in the same manner afforded 31 with concomitant
Bn deprotection. Compound 32 was readily converted to 5,7-
didemethylnobiletin (35). In order to decrease the reactivity of
the electron-rich aromatic A ring of 29, acetate was selected for
the protecting group of phenol 29. Acetylation of 32 by
treatment with acetic anhydride and DMAP afforded 33. After
the demethylation of 33 under Node’s conditions to afford 34,
the following deacetylation provided 35. 5-Desmethylnobiletin
(27) has already been converted into chemical probes labeled
with 11C for positron emission tomography10 or conjugated
a
Conditions: (a) TiCl4, Cl2CHOMe, CH2Cl2; (b) SeO2 (cat.), 30%
H2O2, t-BuOH, 50 °C; (c) Et3N, MeOH; (d) MeI, K2CO3, acetone,
84% (four steps); (e) PhN(Me)CHO, POCl3, 60 °C; (f) SeO2 (cat.),
30% H2O2, t-BuOH, 50 °C; (g) Et3N, MeOH; (h) MeI, K2CO3,
acetone, 94% (four steps); (i) AcCl, AlCl3, CH2Cl2, 45% (brsm 94%).
remaining mCPBA and mCBA in the Baeyer−Villiger
oxidation. Here we used only a catalytic amount of selenium
dioxide7 in the presence of hydrogen peroxide in the oxidation
step, and the product could be purified by solvent extraction.
After the Baeyer−Villiger oxidation with selenium dioxide,
methanolysis and subsequent methylation afforded 1,2,3,4-
B
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a
Scheme 4. Completion of the Synthesis of Nobiletin (1)
a
Conditions: (a) LHMDS, THF, 0 °C, 91%; (b) TFA, MeOH, 50 °C, 98%; (c) SOCl2, DMF, CH2Cl2 then benzotriazole.
a
Scheme 5. Synthesis of Nobiletin Derivatives
a
Conditions: (a) LHMDS, THF; (b) TsOH·H2O, toluene; (c) EtSH, AlCl3, CH2Cl2, 75%; (d) EtSH, AlCl3, CH2Cl2, 75%.
with fluorescein (TokyoGreen) for visualizing the biodistribu-
tion or with biotin for identification of binding proteins.14
By means of a similar strategy for the desmethylation and
flavone synthesis, efficient syntheses of sudatichin (36) and
desmethylnobiletin derivatives 37−3915 were accomplished,16
as shown in Figure 2.
methylated flavone derivatives. The desmethyl analogues
should be applicable as tools for chemical biology studies by
conversion into various probe molecules. Detailed studies of
the biological activities of these compounds and metabolism of
1 are in progress.
EXPERIMENTAL SECTION
■
SUMMARY
1
Materials and Instrumentation. H NMR (270 MHz)
■
We have developed a practical synthesis of nobiletin (1)
suitable for use on a large (100 g) scale. The methodology was
also developed for efficient and flexible synthesis of a variety of
and 13C NMR (68 MHz) spectra were determined on a JEOL
EX-270 instrument, 1H NMR (500 MHz) and 13C NMR (125
MHz) spectra were determined on JEOL ECA-500 and JEOL
C
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a
Scheme 6. Synthesis of 7-Desmethylnobiletin Derivatives
a
Conditions: (a) LiCl, DMF, 120 °C, 59% (brsm 70%); (b) BnBr, K2CO3, DMF, 79%; (c) LHMDS, THF, −78 to 0 °C, 70%; (d) TsOH, toluene,
80 °C, 63%; (e) Ac2O, DMAP, pyridine; (f) EtSH, AlCl3, CH2Cl2, 0 °C to rt, 80% (two steps); (g) K2CO3, MeOH, 99%.
2,3,4-Trimethoxybenzaldehyde (12). To a stirred
solution of 11 (44.7 g, 266 mmol) in CH2Cl2 (500 mL)
were added TiCl4 (58.3 mL, 532 mmol, 2.0 equiv) and
Cl2CHOMe (47 mL, 532 mmol, 2.0 equiv) at 0 °C under an
Ar atmosphere. The reaction mixture was stirred at room
temperature for 3 h, poured into cold water, and extracted with
Figure 2. Structures of desmethyl derivatives 36−39 of nobiletin (1).
CH2Cl2. The organic layer was washed with 6 M HCl followed
by saturated aqueous NaHCO3 and brine, dried over
anhydrous MgSO4, and concentrated under reduced pressure.
The crude residue 12 (52.7 g) was applied to the following
reaction without further purification.
1
α-500 instruments, and H NMR (400 MHz) and 13C NMR
(100 MHz) spectra were determined on a JEOL LA-400
instrument. Chemical shifts (δ) for 1H NMR spectra are
reported in parts per million downfield from tetramethylsilane
as the internal standard, and coupling constants are given in
hertz. The following abbreviations are used for spin multi-
plicity: s = singlet, d = doublet, t = triplet, q = quartet, m =
multiplet, br = broad. Chemical shifts for 13C NMR are
reported in parts per million relative to the center line of the
triplet at 77.0 ppm for deuteriochloroform. High-resolution
mass spectrometry (HRMS) was performed on a JEOL
MStation 700. Fast atom bombardment (FAB) mass spectra
were obtained with 3-nitrobenzyl alcohol as the matrix and a
Bruker Daltonics micrOTOF (ESI). Analytical thin-layer
chromatography (TLC) was performed on Merck precoated
analytical plates (0.25 mm thick, silica gel 60 F254). Preparative
TLC separations were made on 7 cm × 20 cm plates prepared
2,3,4-Trimethoxyphenol (14). To a solution of crude
residue 12 (52.7 g) in t-BuOH (500 mL) were added SeO2
(590 mg, 5.32 mmol, 0.02 equiv) and H2O2 (60 mL, 532
mmol, 2.0 equiv) at 50 °C. The reaction mixture was stirred at
50 °C for 1.5 h and then allowed to cool to room temperature.
The resulting mixture was quenched with saturated aqueous
Na2SO3 at 0 °C, poured into water, and extracted with EtOAc.
The organic layer was washed with brine, dried over anhydrous
MgSO4, and concentrated under reduced pressure. The crude
residue 13 (53.6 g) was applied to the following reaction
without further purification.
To a solution of crude residue 13 (53.6 g) in MeOH (500
mL) was added Et3N (35 mL, 253 mmol, 1.0 equiv) at 0 °C.
The mixture was stirred at room temperature for 0.5 h and
then concentrated under reduced pressure. The crude residue
was dissolved in CH2Cl2, and the solution was acidified with 2
M HCl. The organic layer was extracted with 2 M NaOH. The
water layer was acidified with 6 M HCl and extracted with
CH2Cl2. The organic layer was dried over anhydrous MgSO4
and concentrated under reduced pressure. The crude residue
14 (46.3 g) was applied to the following reaction without
further purification.
with a 0.25 mm or 0.50 mm layer of Merck silica gel 60 F254
.
Compounds were eluted from the adsorbent with 10%
methanol in chloroform. Column chromatography separations
were performed on KANTO CHEMICAL Silica Gel 60
(spherical) 40−50 μm, Silica Gel 60 (spherical) 63−210 μm,
or Silica Gel 60 N (spherical, neutral) 63−210 μm. Reagents
and solvents were commercial grade and were used as supplied
with the following exceptions: dichloromethane, diethyl ether,
n-hexane, tetrahydrofuran, and toluene were dried over 4 Å
molecular sieves, and methanol and acetonitrile were dried
over 3 Å molecular sieves. All reactions sensitive to oxygen or
moisture were conducted under an argon atmosphere.
1,2,3,4-Tetramethoxybenzene (15). To a stirred sol-
ution of crude residue 14 (46.3 g) in acetone (500 mL) were
added K2CO3 (69.5 g, 503 mmol, 1.9 equiv) and MeI (31.3
mL, 503 mmol, 1.9 equiv) at 0 °C under an Ar atmosphere.
D
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The reaction mixture was stirred at room temperature for 40 h.
The resulting mixture was poured into Et2O and filtered
through a pad of Celite. The filtrate was concentrated under
reduced pressure. The crude residue was washed with MeOH
and filtered to afford 15 (44.3 g, 233 mmol, 84%, four steps
from 11) as a white solid (mp 86−89 °C, lit. 85−88 °C).
Spectral data for 15 were in good agreement with the reported
data.17
2,3,4,5-Tetramethoxyphenol (17). POCl3 (0.73 mL,
8.07 mmol, 1.6 equiv) and N-methyl formanilide (0.18 g,
1.35 mmol) were stirred at room temperature for 20 min. To
the reaction mixture was added 15 (1.0 g, 5.04 mmol), and the
resulting mixture was stirred at 60 °C for 6 h. After cooling, the
reaction mixture was quenched with water at 0 °C and
extracted with EtOAc. The organic layer was washed with
brine, dried over anhydrous MgSO4, and concentrated under
reduced pressure. The crude residue (1.5 g) was applied to the
following reaction without further purification.
To a solution of the crude residue (1.5 g) in t-BuOH (10
mL) were added SeO2 (11 mg, 0.151 mmol, 0.03 equiv) and
H2O2 (1.14 mL, 10.1 mmol, 2.0 equiv) at 50 °C. The reaction
mixture was stirred at 50 °C for 2.5 h and allowed to cool to
room temperature. The resulting mixture was quenched with
saturated aqueous Na2SO3 at 0 °C, poured into water, and
extracted with EtOAc. The organic layer was washed with
water followed by brine, dried over anhydrous MgSO4, and
concentrated under reduced pressure. The crude residue 16
(1.7 g) was applied to the following reaction without further
purification.
To a solution of crude residue 16 (1.7 g) in MeOH (16 mL)
was added Et3N (0.7 mL, 5.04 mmol, 1.0 equiv) at 0 °C. The
mixture was stirred at room temperature for 45 min and then
concentrated under reduced pressure. The residue was
dissolved in CH2Cl2 and acidified with 2 M HCl. The organic
layer was extracted with 2 M NaOH. The water layer was
acidified with 6 M HCl and extracted with CH2Cl2. The
organic layer was dried over anhydrous MgSO4 and
concentrated under reduced pressure. The crude residue 17
(1.03 g) was applied to the following reaction without further
purification.
1,2,3,4,5-Pentamethoxybenzene (10). To a mixture of
crude 17 (1.03 g) and K2CO3 (1.33g, 9.62 mmol, 1.9 equiv) in
acetone (16 mL) was added MeI (0.6 mL, 9.62 mmol, 1.9
equiv) under an Ar atmosphere. The reaction mixture was
stirred at room temperature for 36 h. The resulting mixture
was poured into Et2O and filtered through a pad of Celite. The
filtrate was concentrated under reduced pressure to afford 10
(1.08 g, 4.74 mmol, 94%, four steps from 15) as a pale-yellow
solid (mp 60−61 °C, lit. 59−61 °C). Spectral data for 10 were
in good agreement with the reported data.18
1-(2-Hydroxy-3,4,5,6-tetramethoxyphenyl)ethanone
(3). To a solution of 10 (1.0 g, 4.38 mmol) and AcCl (485 μL,
6.14 mmol, 1.4 equiv) in CH2Cl2 (10 mL) was added AlCl3
(560 mg, 4.38 mmol, 1.0 equiv) at 0 °C. The reaction mixture
was stirred at room temperature for 1 h. The resulting mixture
was poured into 2 M NaOH at 0 °C and washed with Et2O.
The organic layer was washed with 2 M NaOH and
concentrated under reduced pressure to recover the starting
material 10 (513 mg, 51%). The water layers were combined,
acidified with 6 M HCl at 0 °C, and extracted with Et2O. The
organic layer was dried over anhydrous MgSO4 and
concentrated under reduced pressure to afford 3 (508 mg,
1.98 mmol, 45%) as a yellow oil. 1H NMR (270 MHz,
CDCl3): δ 13.2 (s, 1H, OH), 4.08 (s, 3H, OCH3), 3.95 (s, 3H,
OCH3), 3.90 (s, 3H, OCH3), 3.85 (s, 3H, OCH3), 3.82 (s, 3H,
OCH3), 2.71(s, 3H, CH3). Spectral data for 3 were in good
agreement with the reported data.19
( 1 H - B e n z o [ d ] [ 1 , 2 , 3 ] t r i a z o l - 1 - y l ) ( 3 , 4 -
dimethoxyphenyl)methanone (4). To a solution of 18
(15.0 g, 82.3 mmol) in CH2Cl2 (200 mL) were added SOCl2
(9.0 mL, 123 mmol, 1.5 equiv) and DMF (2.5 mL, 33 mol, 0.4
equiv) at 0 °C under an Ar atmosphere. The reaction mixture
was stirred at 0 °C for 1.5 h and then concentrated under
reduced pressure to give a crude residue including acid
chloride. To a solution of the crude acid chloride in CH2Cl2
(170 mL) were added Et3N (11.4 mL, 82.3 mmol, 1.0 equiv)
and benzotriazole (9.8 g, 82.3 mmol, 1.0 equiv) at 0 °C under
an Ar atmosphere. The reaction mixture was stirred at 0 °C for
30 min, quenched with 1 M NaOH at room temperature, and
extracted with CH2Cl2. The organic layer was dried over
anhydrous MgSO4 and concentrated under reduced pressure
to give a residue including 4. The crude residue was purified by
recrystallization from n-hexane/CH2Cl2 to afford 4 (19.1 g,
67.5 mmol, 82%, two steps from 18) as a white solid (mp
1
131−132 °C). IR (film): 1691, 1271, 1047 cm−1. H NMR
(270 MHz, CDCl3): δ 8.37 (d, J = 8.5 Hz, 1H, Ar), 8.05 (d, J =
8.5 Hz, 1H, Ar), 7.82 (s, 1H, Ar), 7.70 (t, J = 7.5 Hz, 1H, Ar),
7.55 (t, J = 7.5 Hz, 1H, Ar), 7.04 (d, J = 7.5 Hz, 1H, Ar), 4.01
(s, 3H, OCH3), 4.00 (s, 3H, OCH3). 13C NMR (67.5 MHz,
CDCl3): δ 165.5, 153.9, 148.7, 145.5, 132.5, 130.2, 127.2,
126.1, 123.3, 120.0, 114.8, 113.9, 110.2, 56.1, 56.0. MS (FAB):
m/z 283 (M)+. HRMS (ESI): calcd for C15H13N3O3Na+ (M +
Na)+, 306.0849; found, 306.0855.
Nobiletin (1). To a mixture of 3 (5.2 g, 0.69 mmol) and 4
(6.3 g, 0.76 mmol, 1.1 equiv) in THF (40 mL) was added
LHMDS (81 mL, 81 mmol, 4.0 equiv, 1 M solution in THF)
under an Ar atmosphere at −30 °C. The reaction mixture was
stirred at 0 °C for 2 h, and the resulting mixture was quenched
with saturated aqueous NH4Cl at 0 °C and extracted with
EtOAc. The organic layer was washed with brine, dried over
anhydrous MgSO4, and concentrated under reduced pressure
to give a residue including 2. The residue was purified by
recrystallization from EtOAc/n-hexane to afford 2 (7.8 g, ca.
91%) as a yellow solid. The crude solid was applied to the
following reaction without further purification.
To a solution of crude solid including 2 (3.1 g) in MeOH
(30 mL) was added TFA (3 mL) at room temperature. The
reaction mixture was stirred at 50 °C for 24 h. After stirring,
the reaction mixture was concentrated under reduced pressure
to give a residue including 1. The residue was purified by
recrystallization from hot MeOH to afford 1 (2.9 g, 98%) as a
1
pale-yellow solid (mp 138 °C, lit. 137−138 °C). H NMR
(270 MHz, CDCl3): δ 7.57 (d, J1 = 8.6 Hz, J2 = 2.1 Hz, 1H,
Ar), 7.42 (d, J = 2.1 Hz, 1H, Ar), 7.00 (d, J = 8.6 Hz, 1H, Ar),
6.62 (d, 1H, CH), 4.11 (s, 3H, OCH3), 4.03 (s, 3H, OCH3),
4.00−3.95 (m, 12H, OCH3). 13C NMR (125 MHz, CDCl3): δ
177.1, 160.8, 151.7, 151.2, 149.1, 148.2, 147.5, 143.9, 137.8,
123.8, 119.4, 114.7, 111.1, 108.3, 106.7, 62.1, 61.8, 61.7, 61.5,
55.9, 55.8. Spectral data for 1 were in good agreement with the
reported data.20
4′-Benzyloxyflavone 24. To a stirred solution of 3 (500
mg, 1.95 mmol) and 22 (842 mg, 2.34 mmol, 1.2 equiv) in
THF (6.5 mL) was added LHMDS (7.8 mL, 7.8 mmol, 4.0
equiv, 1 M solution in THF) under an Ar atmosphere at −30
°C, and the reaction mixture was stirred for 1 h at 0 °C,
quenched with saturated aqueous NH4Cl at 0 °C, and
E
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extracted with EtOAc. The organic layer was washed with
brine, dried over anhydrous Na2SO4, and concentrated under
reduced pressure. The residue was purified by silica gel column
chromatography (n-hexane/EtOAc = 9/1 to 5/2) to afford a
mixture of β-diketone 23 and slight amount of impurities (877
mg, ca. 91%) as a yellow solid. The crude solid was applied to
the following reaction without further purification.
the resulting mixture was diluted with saturated aqueous
NH4Cl and extracted with EtOAc. The organic layer was dried
over anhydrous MgSO4 and concentrated under reduced
pressure. The residue was purified by silica gel column
chromatography (n-hexane/EtOAc = 9/1 to 5/2) to afford 29
(112 mg, 59%) as a yellow solid and recovered starting
1
material (22 mg). Mp: 93.5−94.5 °C. H NMR (500 MHz,
To a solution of crude β-diketone 23 (776 mg) in toluene
(5.2 mL) was added TsOH monohydrate (297 mg, 2.34 mmol,
1.5 equiv) at room temperature, and the reaction mixture was
stirred for 3 h at 80 °C, diluted with water, and extracted with
EtOAc. The organic layer was dried over anhydrous Na2SO4
and concentrated under reduced pressure. The residue was
purified by silica gel column chromatography (n-hexane/
EtOAc = 10/3 to 10/7) to afford 24 (685 mg, 92%) as a
CDCl3): δ 6.50 (s, 1H), 3.96 (s, 3H), 3.93 (s, 3H), 3.86 (s,
3H), 2.67 (s, 3H). 13C NMR (125 MHz, CDCl3): δ 204.0,
153.9, 151.3, 149.7, 133.0, 131.0, 108.0, 61.0, 60.9, 60.8, 32.1.
HRMS (FAB): calcd for C11H14O6Na [M + Na]+, 265.0688;
found, 265.0683.
Acetophenone 30. To a stirred solution of 29 (112 mg,
0.46 mmol) and K2CO3 (127 mg, 0.93 mmol, 2.0 equiv) in
DMF (1.5 mL) was added benzyl bromide (0.06 mL, 0.51
mmol, 1.1 equiv) at room temperature, and the reaction
mixture was stirred for 3 h, quenched with saturated aqueous
NH4Cl, and extracted with EtOAc. The organic layer was
washed with brine, dried over anhydrous MgSO4, and
concentrated under reduced pressure. The residue was purified
by silica gel column chromatography (n-hexane/EtOAc = 9/1
1
colorless solid (mp 156−156.5 °C). H NMR (500 MHz,
CDCl3): δ 7.49 (dd, J = 8.5, 2.3 Hz, 1H), 7.45 (t, J = 7.4 Hz,
2H), 7.39 (t, J = 7.4 Hz, 2H), 7.33 (t, J = 7.4 Hz, 1H), 6.99 (d,
J = 8.5 Hz, 1H), 6.61 (s, 1H), 5.25 (s, 2H), 4.10 (s, 3H), 4.02
(s, 3H), 3.99 (s, 3H), 3.95 (s, 6H). 13C NMR (125 MHz,
CDCl3): δ 177.3, 161.0, 151.4, 151.0, 149.7, 148.3, 147.7,
144.0, 138.0, 136.3, 128.7, 128.1, 127.2, 124.2, 119.4, 114.8,
113.4, 108.9, 106.9, 70.8, 62.2, 61.9, 61.8, 61.6, 56.0. HRMS
(FAB): calcd for C27H26O8Na [M + Na]+, 501.1525; found,
501.1520.
1
to 5/2) to afford 30 (121 mg, 79%) as a yellow oil. H NMR
(500 MHz, CDCl3): δ 7.49 (d, J = 7.4 Hz, 2H), 7.38 (t, J = 7.4
Hz, 2H), 7.33 (t, J = 7.4 Hz, 1H), 5.28 (s, 2H), 3.95 (s, 3H),
3.83 (s, 3H), 3.76 (s, 3H), 2.68 (s, 3H). 13C NMR (125 MHz,
CDCl3): δ 204.3, 154.4, 152.6, 151.4, 138.4, 137.4, 137.1,
128.4, 128.2, 128.1, 110.8, 75.4, 61.2, 61.1, 61.0, 32.3. HRMS
(FAB): calcd for C18H20O6Na [M + Na]+, 355.1158; found,
355.1152.
5,4′-Didemethylnobiletin (26). To a stirred solution of
24 (50.0 mg, 0.10 mmol) in CH2Cl2 (0.5 mL) was added
AlCl3 (42.0 mg, 0.31 mmol, 1.5 equiv) in EtSH (1.0 mL) at 0
°C, and the reaction mixture was stirred for 2 h at room
temperature, quenched with 2 M aqueous HCl at 0 °, and
extracted with EtOAc. The organic layer was dried over
anhydrous Na2SO4 and concentrated under reduced pressure.
The residue was purified by silica gel column chromatography
(n-hexane/EtOAc = 4/1 to 5/2) to afford 26 (18.4 mg, 47%)
7-Demethylnobiletin (32). To a stirred solution of 30
(730 mg, 2.20 mmol) and 4 (933 mg, 3.29 mmol, 1.5 equiv) in
THF (7.3 mL) was added LHMDS (8.8 mL, 8.80 mmol, 4.0
equiv, 1 M solution in THF) under an Ar atmosphere at −30
°C, and the reaction mixture was stirred for 2.5 h at 0 °C,
quenched with saturated aqueous NH4Cl at 0 °C, and
extracted with EtOAc. The organic layer was washed with
brine, dried over anhydrous Na2SO4, and concentrated under
reduced pressure. The residue was purified by silica gel column
chromatography (n-hexane/EtOAc = 9/1 to 5/2) to afford a
mixture of β-diketone 31 and a slight amount of impurities
(763 mg, ca. 70%) as a yellow solid. To a solution of crude β-
diketone 31 (294 mg) in toluene (2.0 mL) was added TsOH
monohydrate (168 mg, 0.89 mmol, 1.5 equiv) at room
temperature, and the reaction mixture was stirred for 6 h at 80
°C, diluted with water, and extracted with EtOAc. The organic
layer was dried over anhydrous Na2SO4 and concentrated
under reduced pressure. The residue was purified by silica gel
column chromatography (n-hexane/EtOAc = 10/3 to EtOAc
only) to afford 32 (145 mg, 63%) as a colorless powder (mp
1
as a yellow solid (mp 166−167 °C). H NMR (500 MHz,
CDCl3): δ 12.55 (s, 1H), 7.55 (dd, J = 8.5, 2.3 Hz, 1H), 7.42
(d, J = 2.3 Hz, 1H), 7.06 (d, J = 8.5 Hz, 1H), 6.60 (s, 1H),
6.07 (s, 1H), 4.12 (s, 3H), 4.01 (s, 3H), 3.98 (s, 3H), 3.96 (s,
3H). 13C NMR (125 MHz, CDCl3): δ 183.0, 164.0, 153.0,
149.5, 149.4, 146.9, 145.7, 136.5, 132.9, 123.2, 120.7, 115.1,
108.3, 106.9, 103.8, 62.1, 61.7, 61.1, 56.0. HRMS (FAB): calcd
for C19H18O8Na [M + Na]+, 397.0899; found, 397.0893.
5-Demethylnobiletin (27). To a stirred solution of 1
(50.0 mg, 0.12 mmol) in CH2Cl2 (1.2 mL) was added AlCl3
(25.0 mg, 0.18 mmol, 1.5 equiv) in EtSH (1.2 mL) at 0 °C,
and the reaction mixture was stirred for 2 h at room
temperature, quenched with 2 M aqueous HCl at 0 °C, and
extracted with EtOAc. The organic layer was dried over
anhydrous Na2SO4 and concentrated under reduced pressure.
The residue was purified by silica gel column chromatography
(n-hexane/EtOAc = 4/1 to 5/2) to afford 27 (36.5 mg, 76%)
as yellow solid (mp 146 °C). 1H NMR (500 MHz, CDCl3): δ
12.55 (s, 1H), 7.59 (dd, J = 8.5, 2.0 Hz, 1H), 7.43 (d, J = 2.0
Hz, 1H), 7.01 (d, J = 8.5 Hz, 1H), 6.62 (s, 1H), 4.12 (s, 3H),
3.99 (s, 3H), 3.984 (s, 3H), 3.978 (s, 3H), 3.96 (s, 3H). 13C
NMR (125 MHz, CDCl3): δ 182.9, 163.9, 152.9, 152.4, 149.5,
149.3, 145.7, 136.5, 132.9, 123.6, 120.1, 111.2, 108.6, 106.9,
103.9, 62.0, 61.7, 61.1, 56.1, 55.9. HRMS (FAB): calcd for
C20H20O8Na [M + Na]+, 411.1056; found, 411.1050.
1
203−204 °C). H NMR (500 MHz, CDCl3): δ 7.57 (dd, J =
8.5, 2.3 Hz, 1H), 7.40 (d, J = 2.3 Hz, 1H), 7.00 (d, J = 8.5 Hz,
1H), 6.61 (s, 1H), 6.29 (s, 1H), 4.07 (s, 3H), 4.05 (s, 3H),
3.98 (s, 3H), 3.97 (s, 3H), 3.96 (s, 3H). 13C NMR (125 MHz,
CDCl3): δ 177.4, 160.8, 151.8, 149.2, 147.6, 147.4, 147.3,
138.5, 131.8, 123.9, 119.5, 111.9, 111.2, 108.5, 106.7, 62.1,
61.8, 61.6, 56.0, 55.9. HRMS (FAB): calcd for C20H20O8Na
[M + Na]+, 411.1056; found, 411.1050.
7-Acetoxyflavone 34. To a stirred solution of 32 (100
mg, 0.26 mmol) in pyridine (0.87 mL) were added acetic
anhydride (0.10 mL, 1.29 mmol, 5.0 equiv) and DMAP (14
mg, 0.03 mmol, 0.1 equiv) at room temperature, and the
reaction mixture was stirred for 30 min. The resulting mixture
was evaporated under reduced pressure to afford a crude
Acetophenone 29. To a stirred solution of 3 (200 mg,
0.78 mmol) in DMF (2.6 mL) was added LiCl (331 mg, 7.80
mmol, 10 equiv) at room temperature, and the mixture was
stirred for 18 h at 120 °C. After cooling to room temperature,
F
DOI: 10.1021/acs.oprd.9b00091
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Article
mixture of 33 (123 mg). To a stirred solution of crude 33
(50.0 mg) in CH2Cl2 (1.2 mL) was added AlCl3 (23.0 mg,
0.17 mmol, 1.5 equiv) in EtSH (1.2 mL) at 0 °C, and the
reaction mixture was stirred for 2 h at room temperature,
quenched with 2 M aqueous HCl at 0 °C, and extracted with
EtOAc. The organic layer was dried over anhydrous Na2SO4
and concentrated under reduced pressure. The residue was
purified by silica gel column chromatography (n-hexane/
EtOAc = 4/1 to 2/1) to afford 34 (38.2 mg, 80%, two steps
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.oprd.9b00091.
■
S
1H and 13C NMR spectra for compounds 1, 3, 4, 10, 15,
24, 26, 27, 29, 30, 32, 34, and 35 (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: kant@u-shizuoka-ken.ac.jp.
■
ORCID
́
́
́
Toshiyuki Kan: 0000-0002-9709-6365
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. Kazuo Okamoto, CEO of Ushio ChemiX
Corporation, for the development of the industrial synthesis
for nobiletin. This work was financially supported by MEXT/
JSPS KAKENHI Grants JP17H03973 and JP17K15424, Grant-
in-Aid for Scientific Research on Priority Areas JP16H01160
from the Ministry of Education, Culture, Sports, Science and
Technology (MEXT) of Japan, and the Platform Project for
Supporting in Drug Discovery and Life Science Research
(Platform for Drug Discovery, Informatics and Structural Life
Science) from MEXT.
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Y.; Shibuya, M.; Yamakoshi, H.; Tomioka, Y.; Iwabuchi, Y.; Ohizumi,
G
DOI: 10.1021/acs.oprd.9b00091
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Article
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H
DOI: 10.1021/acs.oprd.9b00091
Org. Process Res. Dev. XXXX, XXX, XXX−XXX