808
T. S. Navale, R. Rathore
PAPER
The resulting pale-yellow solid was dissolved in CH2Cl2 (100 mL)
and the solution was passed through a short pad of silica gel. The
filtrate was evaporated in vacuo to afford 4.
Yield: 7.0 g (88%); pale-yellow solid; mp 198–200 °C (Lit.11 198–
200 °C).
MS (70 eV): m/z = 298 [M+], 283 [M+ – CH3], 268 [M+ – 2CH3],
253 [M+ – 3CH3], 237, 225, 149, 134.
Synthesis of 1,8-Dimethoxy-9,10-dihydroanthracene (3)
Contaminated with 4
To a suspension of 2 (12.0 g, 45 mmol) in glacial acetic acid (200
mL) was added zinc dust (44.0 g, 675 mmol) in one portion. The re-
action mixture was heated at reflux for 3–4 h. When the reaction
was complete, the mixture was cooled to r.t. and vacuum filtered to
remove the zinc dust. The solid cake was washed with CH2Cl2
(5 × 80 mL) and the collected filtrate was washed with H2O (200
mL), NaHCO3 (2 × 100 mL) and again with H2O (100 mL). The or-
ganic layer was dried over MgSO4 and evaporated under reduced
pressure to afford a mixture containing 1,8-dimethoxy-9,10-dihy-
droanthracene (3) and 1,8-dimethoxyanthracene (4) in ca. 9:1 ratio.
Note: The dihydroanthracene 3, contaminated with ~10% an-
thracene 4, was used for the next step without further separation or
purification.
IR (KBr): 1625, 1567, 1464, 1440, 1321, 1265, 1126, 1058, 1070,
789, 737 cm–1.
lH NMR (CDCl3): d = 4.08 (s, 6 H), 6.73 (d, J = 7.3 Hz, 2 H), 7.37
(t, J = 8.0 Hz, 2 H), 7.56 (d, J = 8.6 Hz, 2 H), 8.30 (s, 1 H), 9.23 (s,
1 H).
13C NMR (CDCl3): d = 55.68, 101.72, 115.86, 120.37, 124.61,
125.32, 125.82, 133.06, 156.09.
MS (70 eV): m/z = 238 [M+], 223 [M+ – CH3], 209 [M+ – CH3 –
CH2], 195, 180, 163, 152, 119.
1,8-Dibromo-4,5-dimethoxyanthracene (5)
To a pre-chilled solution of 4 (4.0 g, 16.8 mmol) in CH2Cl2 (200
mL) in an ice-salt bath (~0 °C) was added NBS (5.98 g, 33.6 mmol)
in small portions over a 15 min period. After the addition, the tem-
perature was slowly allowed to rise to r.t. and the mixture was
stirred for an additional 3 h. The resulting reaction mixture was di-
luted with CH2Cl2 (150 mL) and washed with 5% HCl (50 mL) fol-
lowed by H2O (3 × 150 mL). The organic layer was separated, dried
over anhydrous MgSO4, and evaporated under reduced pressure to
afford 5.
Yield: 10.0 g (94%; 3 + 4); yellow solid.
1H NMR (CDCl3): d = 3.90 (s, 6 H), 3.94 (t, J = 3.0 Hz, 2 H), 4.03
(t, J = 3.0 Hz, 2 H), 6.77 (d, J = 8.30 Hz, 2 H), 6.90 (d, J = 7.5 Hz,
2 H), 7.19 (t, J = 8.1 Hz, 2 H); remaining small signals were as-
signed to 3.
13C NMR (CDCl3): d = 22.14, 35.27, 55.59, 107.63, 120.07, 124.29,
126.67, 137.05, 157.07; remaining small signals were assigned to 3.
MS (70 eV): m/z = 240 [M+], 225 [M+ – CH3], 209, 194, 178, 165,
152, 139, 112.
Yield: 6.1 g (92%); pale-green solid; mp 272–274 °C.
IR (KBr): 1613, 1461, 1386, 1318, 1263, 1221, 1148, 1091, 1073,
806, 610, 600 cm–1.
1H NMR (CDCl3): d = 4.06 (s, 6 H), 6.62 (d, J = 8.1 Hz, 2 H), 7.72
(d, J = 8.1 Hz, 2 H), 9.0 (s, 1 H), 9.25 (s, 1 H).
13C NMR (CDCl3): d = 55.97, 102.97, 113.62, 117.77, 125.53,
125.89, 130.07, 131.54, 155.82.
4,5-Dibromo-1,8-dimethoxy-9,10-dihydroanthracene (7)
To a solution containing a 9:1 mixture of 1,8-dimethoxy-9,10-dihy-
droanthracene and 1,8-dimethoxy-anthracene (9.4 g, 39.0 mmol) in
anhydrous CH2Cl2 (200 mL) was added NBS (13.90 g, 78.0 mmol)
in small portions over 15 min at 0 °C under an argon atmosphere.
After the addition, the temperature was slowly allowed to rise to r.t.
and the mixture was stirred for an additional 3 h. The reaction mix-
ture was diluted with CH2Cl2 (200 mL) and washed with 5% HCl
(100 mL) followed by H2O (3 × 150 mL). The organic layer was
separated, dried over MgSO4 and evaporated under reduced pres-
sure to afford a mixture containing 4,5-dibromo-1,8-dimethoxy-
9,10-dihydroanthracene and 4,5-dibromo-1,8-dimethoxyanthra-
cene in a 9:1 ratio. Note: The dihydroanthracene 7, contaminated
with ~10% anthracene 5, was used for the next step without further
separation or purification.
MS (70 eV): m/z = 396 [M+], 381 [M+ – CH3], 353, 338, 310, 198,
150, 75.
1,4,5,8-Tetramethoxyanthracene (6)
Freshly cut sodium metal (11.0 g, 500 mmol) was added portion-
wise to a 500-mL Schlenk flask containing anhydrous MeOH (100
mL) under an argon atmosphere at 22 °C and the resulting mixture
was heated at reflux until complete dissolution of the sodium metal
was observed. To this solution was added 4,5-dibromo-1,8-
dimethoxyanthracene (2.0 g, 5.0 mmol), toluene (100 mL), MeOAc
(5.0 mL), and copper(I) bromide (0.70 g, 5.0 mmol), successively.
The reaction mixture was then heated at reflux with rapid stirring
for 24 h, after which time, solvent was evaporated under reduced
pressure and the residue was dissolved in CH2Cl2 (ca. 300 mL) and
washed with 10% HCl (100 mL) followed by H2O (2 × 100 mL).
The organic layer was dried over anhydrous MgSO4 and evaporated
under reduced pressure to afford a yellow-green solid, which was
triturated with cold CH2Cl2 to afford 1,4,5,8-tetramethoxyan-
thracene as a yellow solid. An analytically pure sample could be ob-
tained by recrystallization using a 1:1 mixture of chloroform/
toluene.
Yield: 14.0 g (90%; 7 + 5); greenish-yellow solid.
1H NMR (CDCl3): d = 3.86 (s, 6 H), 3.92 (t, J = 3.04 Hz, 2 H), 4.05
(t, J = 3.04 Hz, 2 H), 6.65 (d, J = 8.7 Hz, 2 H), 7.43 (d, J = 8.7 Hz,
2 H); remaining small signals were assigned to 5.
13C NMR (CDCl3): d = 24.01, 26.63, 55.77, 109.30, 115.11, 125.23,
130.34, 134.28, 156.21; remaining small signals were assigned to 5.
MS (70 eV): m/z = 398 [M+], 383 [M+ – CH3], 367, 302, 286, 238,
223, 195, 152, 119.
Alternate Synthesis of 4,5-Dibromo-1,8-dimethoxyanthracene
(5)
A mixture of 4,5-dibromo-1,8-dimethoxy-9,10-dihydroanthracene
and 4,5-dibromo-1,8-dimethoxyanthracene (9.0 g, 22.6 mmol) and
p-chloranil (5.9 g, 24.0 mmol) in toluene (200 mL) was heated at re-
flux for 18 h under an argon atmosphere. The solvent was evaporat-
ed to obtain a green residue, which was diluted with CH2Cl2 (ca. 400
mL) and washed with 10% NaOH (3 × 100 mL) followed by H2O
(2 × 100 mL). The separated organic layer was dried over MgSO4
and the solvent was removed under reduced pressure to afford a
green solid that was further purified by tituration with CH2Cl2 to
Note that in above reaction a catalytic amount of CuBr (1 mmol, 20
mol%) can be employed, however, the reaction required ~72 h to
reach completion.
Yield: 1.40 g (93%); yellow solid; mp 296–298 °C (Lit.9 298–
300 °C).
IR (KBr): 1631, 1471, 1327, 1258, 1129, 1083, 806 cm–1.
1H NMR (CDCl3): d = 4.03 (s, 12 H), 6.63 (s, 4 H), 9.10 (s, 2 H).
13C NMR (CDCl3): d = 55.83, 101.47, 115.37, 125.39, 150.00.
Synthesis 2012, 44, 805–809
© Thieme Stuttgart · New York