Photohydrodimerization of Flavones
J. Chin. Chem. Soc., Vol. 51, No. 6, 2004 1393
silica gel and eluted with n-hexane-ethyl acetate (v/v = 8/1) to
give a white crystal of 3.40 g (12.58 mmol) of 2-benzoyloxy-
5-methoxyacetophenone (5) with a yield of 70.5%, m.p.
38-40 °C. 1HNMR (CDCl3, 500 MHz): d 2.53 (s, 3H), 3.87 (s,
3H), 7.11-7.14 (m, 2H), 7.36 (d, 1H, J = 4.5 Hz), 7.50-7.53
(m, 3H), 8.20-8.22 (dd, 2H, J = 7.0 Hz, J = 4.5 Hz); 13CNMR
(CDCl3, 125 MHz): 55.84, 100.42, 107.57, 114.41, 117.86,
126.16, 127.08, 129.00, 131.40, 131.89, 158.01, 163.02,
164.19, 177.87; MS (EI, m/z, %): 105 (100), 270 (M, 3.7%),
271 (M+1); MS (FAB, m/z, %): 105 (100), 270 (M, 9.9), 271
(13.6); Anal.: Calcd. for C16H14O4: C: 71.10%, H: 5.22%;
Found: C: 70.69%, H: 5.33%.
(CDCl3, 125 MHz): 55.95, 104.79, 106.85, 119.53, 123.85,
124.55, 126.24, 129.03, 131.51, 131.88, 151.10, 157.01,
163.19, 178.36; MS (EI, m/z, %): 107 (48.4), 135 (23.6), 150
(74.5), 222 (29.1), 251 (85.0), 252 (M, 100.0), 253 (M+1,
15.5); MS (FAB, m/z, %): 136 (91.9), 154 (100), 252 (M,
3.1), 253 (M+1, 19.8); Anal.: Calcd. for C16H12O3: C:
76.18%, H: 4.79%; Found: C: 75.97%, H: 4.84%.
Typical Photolysis of 6-Methoxylavone (7)
Photolysis of 6-methoxyflavone (7) was carried out by
using a quartz flask equipped with a Sankyo 306 nm UV-B
Lamp. A solution of 6-methoxyflavone (7) (101.0 mg, 0.40
mmol) and triethyl amine (404.8 mg, 0.40 mmol) in aceto-
nitrile (40 mL) was irradiated for twenty-four hours under a
dry nitrogen atmosphere. Then the reaction solution was
evaporated to remove the solvent under reduced pressure.
The residue was chromatographed on silica gel and was
eluted with n-hexane-ethyl acetate (v/v = 5/1) to give three
white crystals: rac-6,6²-dimethoxy-2,2²-biflavanone (8a)
(38.7 mg, 38.7%, m.p. 225-226 °C), meso-6,6²-dimethoxy-
2,2²-biflavanone (8b) (4.5 mg, 4.5%, m.p. > 300 °C) and 6-
methoxyflavanone (9) (12.9 mg, 12.9%, m.p. 141-142 °C);
rac-6,6²-dimethoxy-2,2²-biflavanone (8a): 1HNMR (CDCl3,
500 MHz): d 3.33 (d, 2H, J = 16.5 Hz), 3.76 (s, 6H), 3.94 (d,
2H, J = 16.5 Hz), 7.08-7.09 (m, 10H), 7.17-7.18 (m, 4H), 7.26
(s, 2H); 13CNMR (CDCl3, 125 MHz): 40.86, 55.63, 87.92,
107.10, 119.53, 121.22, 125.15, 127.67, 128.50, 128.87,
135.15, 153.81, 153.98, 191.38; MS (EI, m/z, %): 103 (41.2),
151 (100.0), 253 (M/2, 73.1), 254 (M/2+1, 12.5); Anal.:
Calcd. for C32H26O6: C: 75.88%, H: 5.17%; Found: C:
75.79%, H: 5.23%. meso-6,6²-dimethoxy-2,2²-biflavanone
(8b): 1HNMR (CDCl3, 500 MHz): d 3.07 (d, 2H, J = 16.0 Hz),
3.61 (d, 2H, J = 16.0 Hz), 3.67 (s, 6H), 7.01-7.07 (m, 4H),
7.23-7.26 (m, 10H), 7.37 (s, 2H,); 13CNMR (CDCl3, 125
MHz): 42.30, 55.64, 87.12, 107.12, 119.22, 121.12, 125.21,
128.03, 128.39, 129.18, 136.06, 153.30, 153.99, 190.66; MS
(EI, m/z, %): 103 (70.3), 151 (100.0), 253 (1/2M+); Anal.:
Calcd. for C32H26O6: C: 75.88%, H: 5.17%; Found: C:
75.75%, H: 5.22%. 6-methoxyflavanone (9): 1HNMR (CDCl3,
500 MHz): d 2.88 (dd, J = 3.0 Hz, J = 17.0 Hz, 1H), 3.07 (dd,
J = 13.5 Hz, J = 17.0 Hz, 1H), 3.62 (s, 3H), 5.45 (dd, J = 3.0
Hz, J = 13.5 Hz, 1H), 7.00 (d, J = 9.0 Hz, 1H), 7.13 (dd, J =
3.5 Hz, J = 9.0 Hz, 1H), 7.26-7.49 (m, 6H); 13CNMR (CDCl3,
125 MHz): 44.56, 55.81, 79.70, 107.34, 119.43, 120.74,
125.40, 126.13, 128.79, 128.82, 138.82, 154.23, 156.28,
192.09; MS (EI, m/z, %): 103 (83.7), 104 (100.0), 150 (57.5),
254 (M+, 15.0), 255 (M+1, 2.5); Anal.: Calcd. for C16H14O3:
2-Hydroxy-5-methoxydibenzoylmethane (6)
2-Benzoyloxy-5-methoxy-acetophenone (5) (3.00 g,
11.10 mmol) was dissolved in 10 mL of dry pyridine. The so-
lution was heated to 50 °C in a water bath and 0.94 g of dry
potassium hydroxide (16.79 mmol) was added. The reaction
solution was stirred for 15 minutes at 50 °C. The solution was
acidified by adding 10% acetic acid (20 mL) and then ex-
tracted with ethyl ether. The organic layer was evaporated to
remove the solvent under reduced pressure. The residue was
chromatographed on silica gel and eluted with n-hexane-
ethyl acetate (v/v = 8/1) to give an orange-yellow crystal of
2.58 g of 2-hydroxy-5-methoxydibenzoylmethane (6) with a
yield of 86.0%, m.p. 42-44 °C. 1HNMR (CDCl3, 500 MHz): d
3.81 (s, 3H), 6.74 (s, 1H), 6.91-6.94 (m, 2H), 7.19 (d, 1H, J =
4.5 Hz), 7.46-7.49 (m, 3H), 7.92 (d, 2H, J = 13.5 Hz), 11.65
(s, 1H), 15.62 (s, 1H); 13CNMR (CDCl3, 125 MHz): 26.58,
31.72, 55.80, 92.15, 111.76, 119.34, 122.98, 123.98, 128.63,
132.32, 151.81, 156.58, 177.58, 194.98; MS (EI, m/z, %):
105 (100.0), 150 (14.2), 270 (M, 14.2), 271 (M+1, 2.4);
HRMS: 270.0892 (M), calcd. 270.0892; Anal.: Calcd. for
C16H14O4: C: 71.10%, H: 5.02%; Found: C: 71.06%, H:
5.08%.
6-Methoxyflavone (7)
A solution of 2-hydroxy-5-methoxy-dibenzoylmethane
(6) (2.00 g, 7.40 mmol) dissolved in 10 mL of glacial acetic
acid was heated in a boiling water bath and 1 mL of concen-
trated sulfuric acid was added. The reaction solution was
stirred for 1 hour in a boiling water bath and then poured into
50 mL of water. The precipitate was filtered and then recrys-
tallized from methanol to produce a red crystal of 1.16 g of
6-methoxyflavone (7) with a yield of 62.1%, m.p. 165-166
°C. 1HNMR (CDCl3, 500 MHz): d 3.92 (s, 3H), 6.83 (s, 1H),
7.26 (s, 1H), 7.50-7.55 (m, 5H), 7.91-7.95 (m, 2H); 13CNMR