A concise synthetic approach to brazilin via Pd-catalyzed allylic arylation

Article abstract of DOI:10.1039/c5ob00216h

A short synthetic route to the trimethyl ether of brazilin was developed in 6 steps from 7-methoxychromene with 78% overall yield. Regioselective installation of a formyl group onto 7-methoxychromene followed by reduction and acetylation afforded allylic acetate. Palladium-catalyzed allylic coupling of allylic acetate with arylboronic acid provided direct access to 3-benzylchromene which was converted to the target molecule upon ensuing dihydroxylation and acid-catalyzed cyclization in a highly concise manner. This journal is

Full text of DOI:10.1039/c5ob00216h

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A concise synthetic approach to brazilin via
Pd-catalyzed allylic arylation†
Cite this: DOI: 10.1039/c5ob00216h
Youngeun Jung and Ikyon Kim*
A short synthetic route to the trimethyl ether of brazilin was developed in 6 steps from 7-methoxy-
chromene with 78% overall yield. Regioselective installation of a formyl group onto 7-methoxychromene
followed by reduction and acetylation afforded allylic acetate. Palladium-catalyzed allylic coupling of allylic
acetate with arylboronic acid provided direct access to 3-benzylchromene which was converted to the
target molecule upon ensuing dihydroxylation and acid-catalyzed cyclization in a highly concise manner.
Received 2nd February 2015,
Accepted 26th February 2015
DOI: 10.1039/c5ob00216h
www.rsc.org/obc
Recently, we have communicated on the succinct total syn- requisite 5 from the known chromene 6 in gram quantities by
thesis of a homoisoflavanoid natural product, brazilin, by this procedure.
relying on a highly efficient Mitsunobu coupling/alkyne-alde-
When we reacted allylic acetate 5 with 3,4-dimethoxyphenyl-
hyde metathesis sequence to facilitate construction of the core boronic acid in the presence of Pd(PPh3)4 and several bases at
framework.1 The key intermediate 2 was rapidly elaborated to 100 °C, K3PO4 afforded the best isolated yield of 4 (Table 1,
the trimethyl ether analogue (3) of brazilin via a short entries 1–3).8 No regioisomer was observed. Lowering the reac-
sequence of high-yielding and simple synthetic manipulations tion temperature to 80 °C resulted in incomplete conversion
(Scheme 1).
even at a prolonged reaction time (entry 4). The other solvents
Although the efficiency of this route was demonstrated by instead of THF gave inferior results (entries 5–7). Under these
its very high overall yield (∼70%), deoxygenation of the ketone optimized conditions, several arylboronic acids were allowed
moiety of 2 using the Barton–McCombie procedure was to react with 5 to give the corresponding products 8–10 in
required as a means to modulate the oxidation levels. To good yields (entries 8–10). For comparison, allylic t-BOC car-
obviate these steps, compound 4 was deemed as a viable bonate instead of 5 was also subjected to the identical con-
alternative precursor, which would be available via palladium- ditions but only 26% yield of 4 was isolated (not listed).
catalyzed allylic substitution2 with allylic acetate 5 and aryl-
The resulting chromene 4 was then subjected to the Upjohn
boronic acid.3 Compared with ample reports on Pd-catalyzed dihydroxylation process9 to furnish the diol 11 in 100% yield
allylic substitution reactions with various C-, N-, or O-nucleo- (Scheme 3). Cyclization of 11 under acid catalysis gave 3 in
philes via the formation of π-allylpalladium complexes, the use 94% yield.10 Spectral data of 3 matched the literature values in
of boronic acid as a coupling partner is rare. Moreover, to the all respects. In the meantime, asymmetric dihydroxylation11
best of our knowledge, Pd-catalyzed allylic arylation with aryl- was also conducted as it would engender this asymmetric
boronic acid has not been applied to the total synthesis of route.12 When 4 was treated with AD-mix-α at 0 °C for 48 h,
natural products. Here we wish to report another succinct syn- 76 : 24 er was observed with 59% isolated yield (84% based on
thetic route to brazilin and its analogues.4
the recovered starting material).13
As shown in Scheme 2, we commenced our synthesis by
In summary, we have devised another succinct synthetic
preparation of 5. We expected that Vilsmeier–Haack formyla- route to brazilin in connection with our previous synthetic
tion5 of chromene 66 would allow for regioselective introduc- efforts towards this tetracyclic natural product. Regioselective
tion of a formyl group at the C3 position to produce 77 which Vilsmeier–Haack formylation of chromene and Pd-catalyzed
would be transformed to allylic acetate 5 via subsequent regioselective allylic substitution of allylic acetates with an
reduction and acetylation. Indeed, we were able to obtain the arylboronic acid facilitated the construction of the full carbon
skeleton of brazilin from which oxidation and cyclization
enabled ready access to the trimethyl ether 3 of brazilin in
College of Pharmacy and Yonsei Institute of Pharmaceutical Sciences, Yonsei
high overall yield. Asymmetric access to 3 was also investigated
University, 85 Songdogwahak-ro, Yeonsu-gu, Incheon 406-840, Republic of Korea.
by Sharpless asymmetric dihydroxylation of 4 although moder-
E-mail: ikyonkim@yonsei.ac.kr; Fax: +82 32 749 4105; Tel: +82 32 749 4515
†Electronic supplementary information (ESI) available: 1H and 13C NMR spectra
of 3–5, 7–11. See DOI: 10.1039/c5ob00216h
ate er was observed. This flexible and practical approach
should be particularly suitable for the synthesis of diverse
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Scheme 1 Synthetic approaches to brazilin.
Scheme 2 Synthesis of 5.
brazilin analogues as a number of (hetero)arylboronic acids
could be employed in Pd-catalyzed cross-coupling reactions.
Table 1 Pd-catalyzed allylic substitutiona
Experimental section
General methods
Unless specified, all reagents and starting materials were pur-
chased from commercial sources and used as received without
purification. “Concentrated” refers to the removal of volatile
solvents via distillation using a rotary evaporator. “Dried”
refers to pouring onto, or passing through, anhydrous mag-
nesium sulfate followed by filtration. Flash chromatography
was performed using silica gel (230–400 mesh) with hexanes,
ethyl acetate, and dichloromethane as eluents. All reactions
were monitored by thin-layer chromatography on 0.25 mm
silica plates (F-254) visualizing with UV light. 1H and 13C NMR
spectra were recorded using a 400 MHz NMR spectrometer
and were described as chemical shifts, multiplicity (s, singlet;
d, doublet; t, triplet; q, quartet; m, multiplet), coupling con-
stant in hertz (Hz), and number of protons. IR spectra were
recorded by FT-IR using the diamond ATR technique and were
described as wavenumbers (cm−1). HRMS were measured by
electrospray ionization (ESI) and a Q-TOF mass analyzer.
Temp
(°C)
Time
(h)
Yieldb
(%)
Entry
Base
Solvent
Product
1
2
3
4
5
6
7
K2CO3
Et3N
THF
THF
THF
THF
DMF
CH3CN
Toluene
THF
THF
THF
100
100
100
80
100
100
100
100
100
100
20
20
2
26
26
26
26
1
4
4
4
4
4
4
4
8
9
22 (41)
57 (60)
88
28 (57)
60 (100)
37 (51)
13 (26)
78
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
8c
9d
10e
1
3
60
93
10
a A mixture of
5 (0.17 mmol), 3,4-dimethoxyphenylboronic acid
(3 equiv.), base (2 equiv.), and Pd(PPh3)4 (0.05 equiv.) in solvent (2 mL)
was heated unless otherwise stated. b Isolated yield (%). Yields in
parentheses are based on recovered starting materials. c 4-MeOC6H4-
B(OH)2 was used as boronic acid. d 4-ClC6H4B(OH)2 was used as boronic
acid. e 3,4,5-(MeO)3C6H2B(OH)2 was used as boronic acid.
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Scheme 3 Synthesis of 3.
7-Methoxy-2H-chromene-3-carbaldehyde (7). To
a
stirred 0.1 equiv.) and Ac2O (0.38 mL, 1.2 equiv.) were added at 0 °C.
solution of 6 (700 mg, 4.3 mmol in DMF (1 mL), a pre-mixed After being stirred at rt for 1 h, the reaction mixture was
solution of POCl3 (4 mL, 43 mmol, 10 equiv.) and DMF diluted with CH2Cl2 (10 mL) and washed with 10% aq. HCl
(3.5 mL, 43 mmol, 10 equiv.) was added dropwise at 0 °C. After and aq. NaHCO3, successively. The water layer was extracted
being stirred at 0 °C for 17 h, the reaction mixture was with CH2Cl2 (10 mL × 2) two more times. The combined
quenched with H2O (3 mL) at 0 °C. The reaction mixture was organic layers were dried over Na2SO4 and concentrated
basified with an aq. NaOH solution (pH ∼ 8) and extracted in vacuo to afford 5 (785 mg, 100%). White solid, mp:
1
with ethyl acetate (10 mL). The water layer was extracted with 75.8–77.6 °C; H NMR (400 MHz, CDCl3) δ 6.90 (d, J = 8.4 Hz,
ethyl acetate (10 mL × 2) two more times. The combined 1H), 6.44 (dd, J = 2.4, 8.4 Hz, 1H), 6.41 (s, 1H), 6.39 (d, J =
organic layers were dried over Na2SO4 and concentrated 2.4 Hz, 1H), 4.75 (s, 2H), 4.62 (s, 2H), 3.77 (s, 3H), 2.09 (s, 3H);
in vacuo. The residue was purified by silica gel column chromato- 13C NMR (100 MHz, CDCl3) δ 171.0, 161.1, 154.8, 127.7, 125.3,
graphy (hexanes–ethyl acetate–dichloromethane = 10 : 1 : 2) to 123.5, 115.1, 107.4, 101.7, 66.6, 65.0, 55.5, 21.0; IR (ATR) 3443,
1
give 7 (768 mg, 94%). Yellow solid, mp: 81.6–82.9 °C; H NMR 2936, 1721, 1615, 1460, 1260, 1018 cm−1; HRMS (ESI-QTOF)
(400 MHz, CDCl3) δ 9.52 (s, 1H), 7.22 (s, 1H), 7.13 (d, J = m/z [M + Na]+ calcd for C13H15O4 257.0784, found 257.0785.
8.4 Hz, 1H), 6.53 (dd, J = 2.4, 8.4 Hz, 1H), 6.42 (d, J = 2.4 Hz,
3-(3,4-Dimethoxybenzyl)-7-methoxy-2H-chromene (4). To a
1H), 5.03 (s, 2H), 3.82 (s, 3H); 13C NMR (100 MHz, CDCl3) stirred solution of 5 (40 mg, 0.17 mmol) in THF (2 mL), 3,4-
δ 189.6, 164.3, 158.1, 141.7, 130.7, 129.0, 114.0, 109.0, 101.7, dimethoxyphenylboronic acid (93 mg, 3 equiv.), K3PO4 (73 mg,
63.7, 55.7; IR (ATR) 3069, 2815, 1656, 1269, 1557, 1167 cm−1
;
2 equiv.), and Pd(PPh3)4 (9.9 mg, 0.05 equiv.) were added at rt.
HRMS (ESI-QTOF) m/z [M + H]+ calcd for C11H11O3 191.0703, After being stirred at 100 °C for 2 h, the reaction mixture was
found 191.0702. suction-filtered through a pad of Celite and the filtrate was
(7-Methoxy-2H-chromen-3-yl)methyl acetate (5). To a stirred concentrated under reduced pressure to give the residue which
solution of 7 (300 mg, 1.58 mmol) in THF (5 mL) DIBAL (1.0 M was purified by silica gel column chromatography (hexanes–
solution in THF, 4.73 mL, 3 equiv.) was added dropwise at ethyl acetate–dichloromethane = 30 : 1 : 2 to 20 : 1 : 2) to give 4
1
−78 °C. After being stirred at −78 °C for 1 h, the reaction (47 mg, 88%). Yellow gum; H NMR (400 MHz, CDCl3) δ 6.84
mixture was quenched with MeOH at −78 °C and stirred at rt (d, J = 8.4 Hz, 1H), 6.81 (d, J = 8.4 Hz, 1H), 6.76 (dd, J = 1.2,
for 1 h. The mixture was suction-filtered through a pad of Celite 8.4 Hz, 1H), 6.73 (d, J = 1.2 Hz, 1H), 6.41 (dd, J = 2.4, 8.4 Hz,
and the filtrate was concentrated under reduced pressure to give 1H), 6.36 (d, J = 2.4 Hz, 1H), 6.11 (s, 1H), 4.62 (s, 2H), 3.87
allylic alcohol (303 mg, 100%). Colorless liquid; 1H NMR (s, 3H), 3.86 (s, 3H), 3.75 (s, 3H), 3.35 (s, 2H); 13C NMR
(400 MHz, CDCl3) δ 6.89 (d, J = 8.0 Hz, 1H), 6.43 (dd, J = 2.4, (100 MHz, CDCl3) δ 160.2, 154.2, 149.1, 147.8, 131.2, 130.3,
8.0 Hz, 1H), 6.39 (d, J = 2.0 Hz, 1H), 6.34 (s, 1H), 4.79 (s, 2H), 126.8, 121.1, 119.8, 116.1, 112.1, 111.3, 106.9 101.6, 68.2,
4.19 (d, J = 5.6 Hz, 2H), 3.77 (s, 3H), 1.60 (t, J = 6.0 Hz, 1H); 56.01, 55.97, 55.4, 39.5; IR (ATR) 2933, 2831, 1612, 1459, 1258,
13C NMR (100 MHz, CDCl3) δ 160.7, 154.8, 130.5, 127.5, 120.2, 1024 cm−1; HRMS (ESI-QTOF) m/z [M + H]+ calcd for C19H21O4
115.5, 107.3, 101.7, 66.6, 63.7, 55.5; IR (ATR) 3350, 2908, 2834, 313.1434, found 313.1441.
1458, 1271, 1026 cm−1; HRMS (ESI-QTOF) m/z [M + H]+ calcd
for C11H13O3 193.0859, found 193.0854.
7-Methoxy-3-(4-methoxybenzyl)-2H-chromene (8). Prepared
with 5 (40 mg, 0.17 mmol) and 4-methoxyphenylboronic acid
To a solution of allylic alcohol (644 mg, 3.35 mmol) by following the same procedure as that of 4. Yellow gum;
in CH2Cl2 (4 mL), Et3N (0.7 mL, 1.5 equiv.), DMAP (41 mg, 1H NMR (400 MHz, CDCl3) δ 7.13 (d, J = 8.4 Hz, 2H), 6.84
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(d, J = 8.4 Hz, 2H), 6.83 (d, J = 8.0 Hz, 1H), 6.41 (dd, J = 2.0, AD-mix-α (185 mg) were added at 0 °C. After being stirred
8.0 Hz, 1H), 6.35 (d, J = 2.0 Hz, 1H), 6.08 (s, 1H), 4.60 (s, 2H), at 0 °C for 42 h, the reaction mixture was quenched with
3.78 (s, 3H), 3.74 (s, 3H), 3.33 (s, 2H); 13C NMR (100 MHz, aq. Na2SO3 at 0 °C. The mixture was concentrated under reduced
CDCl3) δ 160.2, 158.4, 154.1, 131.3, 130.0, 129.8, 126.8, 119.8, pressure and extracted with ethyl acetate (3 mL). The water
116.1, 114.1, 106.9 101.5, 68.3, 55.43, 55.37, 39.0; IR (ATR) layer was extracted with ethyl acetate (3 mL × 2) two more
2929, 2830, 1608, 1239, 1030, 1154 cm−1; HRMS (ESI-QTOF) times. The combined organic layers were dried over Na2SO4
m/z [M + H]+ calcd C18H19O3 283.1329, found 283.1322.
3-(4-Chlorobenzyl)-7-methoxy-2H-chromene (9). Prepared gel column chromatography (hexanes–ethyl acetate–dichloro-
and concentrated in vacuo. The residue was purified by silica
with 5 (25 mg, 0.11 mmol) and 4-chlorophenylboronic acid by methane = 10 : 1 : 2 to 1 : 1 : 1) to give 11 (15.1 mg, 59%, 84%
following the same procedure as that of 4. Yellow gum; based on the recovered starting material).
1H NMR (400 MHz, CDCl3) δ 7.28 (d, J = 8.0 Hz, 2H), 7.16 (d,
(6aS*,11bR*)-3,9,10-Trimethoxy-6,6a,7,11b-tetrahydroindeno-
J = 8.0 Hz, 2H), 6.84 (d, J = 8.4 Hz, 1H), 6.41 (dd, J = 2.4, [2,1-c]chromen-6a-ol (3). A solution of 11 (58 mg, 0.167 mmol)
8.4 Hz, 1H), 6.36 (d, J = 2.0 Hz, 1H), 6.09 (s, 1H), 4.59 (s, 2H), in 17% HCl–MeOH (1 : 1, 3 mL) was heated at 90 °C for 1 h.
3.75 (s, 3H), 3.36 (s, 2H); 13C NMR (100 MHz, CDCl3) δ 160.4, After being cooled to rt, the reaction mixture was concentrated
154.1, 136.3, 132.5, 130.4, 130.3, 128.8, 126.9, 120.5, 115.9, under reduced pressure to give the crude residue, diluted with
107.1, 101.6, 68.1, 55.5, 39.2; IR (ATR) 2931, 2832, 1737, 1610, CH2Cl2 (3 mL) and H2O (5 mL), and the layers were separated.
1487, 1270, 1154, 1030, 804 cm−1; HRMS (ESI-QTOF) m/z The water layer was extracted with CH2Cl2 (3 mL × 2) two more
[M + H]+ calcd C17H16ClO2 287.0833, found 287.0833.
times. The combined organic layers were dried over Na2SO4
7-Methoxy-3-(3,4,5-trimethoxybenzyl)-2H-chromene (10). Pre- and concentrated in vacuo to give the residue which was puri-
pared with 5 (40 mg, 0.17 mmol) and 3,4,5-trimethoxyphenyl- fied by silica gel column chromatography (hexanes–ethyl
boronic acid by following the same procedure as that of 4. Pale acetate–dichloromethane = 5 : 1 : 2) to give 3 (52.2 mg, 94%).
1
orange solid, mp: 112.8–114.3 °C; H NMR (400 MHz, CDCl3) White solid, mp: 131.8–133.4 °C; 1H NMR (400 MHz, CDCl3)
δ 6.87 (d, J = 8.4 Hz, 1H), 6.43 (s, 2H), 6.42 (dd, J = 2.4, 8.4 Hz, δ 7.31 (d, J = 8.8 Hz, 1H), 6.79 (s, 1H), 6.74 (s, 1H), 6.66 (dd, J =
1H), 6.37 (d, J = 2.0 Hz, 1H), 6.14 (s, 1H), 4.63 (s, 2H), 3.84 2.4, 8.4 Hz, 1H), 6.49 (d, J = 2.4 Hz, 1H), 4.13 (s, 1H), 4.04 (d,
(s, 9H), 3.76 (s, 3H), 3.35 (s, 2H); 13C NMR (100 MHz, CDCl3) J = 11.2 Hz, 1H), 3.84 (s, 3H), 3.83 (s, 3H), 3.82 (d, J = 11.2 Hz,
δ 160.3, 154.2, 153.4, 136.7, 133.6, 130.7, 126.9, 120.2, 116.0, 1H), 3.78 (s, 3H), 3.26 (d, J = 16.0 Hz, 1H), 2.88 (d, J = 15.6 Hz,
107.0, 105.9, 101.6, 68.2, 61.0, 56.2, 55.5, 40.3; IR (ATR) 2934, 1H), 2.46 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 159.6, 154.5,
2833, 1588, 1456, 1235, 1120, 1019 cm−1; HRMS (ESI-QTOF) 148.8, 148.6, 136.2, 131.2, 130.7, 114.5, 109.0, 108.6, 107.8,
m/z [M + H]+ calcd C20H23O5 343.1540, found 343.1546.
102.1, 77.7, 70.4, 56.24, 56.21, 55.5, 50.7, 41.5; IR (ATR) 3449,
(3R*,4S*)-3-(3,4-Dimethoxybenzyl)-7-methoxychroman-3,4- 2998, 2920, 2833, 1617, 1500, 1280, 1157 cm−1; HRMS (ESI-TOF)
diol (11). To a stirred solution of 4 (93 mg, 0.298 mmol) in m/z [M + H]+ calcd for C19H21O5 329.1384, found 329.1382.
acetone–H2O (3 : 1, 8 mL), NMO (105 mg, 3 equiv.) and OsO4
(2.5 wt% in t-BuOH, 0.15 mL, 0.04 equiv.) were added at 0 °C.
After being stirred at 0 °C for 3 h, the reaction mixture was
concentrated under reduced pressure and extracted with
Acknowledgements
CH2Cl2 (5 mL). The water layer was extracted with CH2Cl2 This research was supported in part by the Nano·Material
(5 mL × 2) two more times. The combined organic layers were Technology Development Program through the National
dried over Na2SO4 and concentrated in vacuo. The residue was Research Foundation of Korea (NRF) funded by the Ministry
purified by silica gel column chromatography (hexanes–ethyl of Education, Science and Technology. This work was
acetate–dichloromethane = 3 : 1 : 2) to give 11 (103 mg, 100%). also supported by the National Research Foundation of
White solid, mp: 116.9–118.0 °C; 1H NMR (400 MHz, CDCl3) Korea (NRF) grant funded by the Korea government (MSIP)
δ 7.25 (d, J = 8.4 Hz, 1H), 6.82 (d, J = 8.0 Hz, 1H), 6.78 (d, J = (2014R1A2A1A11050491).
1.6 Hz, 1H), 6.75 (dd, J = 8.0, 1.6 Hz, 1H), 6.58 (dd, J = 8.4,
2.4 Hz, 1H), 6.43 (d, J = 2.4 Hz, 1H), 4.37 (d, J = 6.0 Hz, 1H), 3.95
(d, J = 10.8 Hz, 1H), 3.87 (s, 6H), 3.80 (d, J = 10.8 Hz, 1H), 3.79
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111.1, 108.7, 101.3, 69.7, 69.1, 67.7, 56.0, 55.5, 39.9; IR (ATR)
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proceeded well to produce brazilin as reported previously.
See ref. 1.
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