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X.-P. Ouyang et al. / Chinese Chemical Letters 24 (2013) 1091–1094
Scheme 1. Synthesis of lignin model compound.
2.2. Synthesis of lignin model compound
small amount of petroleum ether and dried at 25 8C under
vacuum to give compound 4 in 90.6% yield. 1H NMR (400 MHz,
CDCl3):
1.83 (s, 3H, CH3), 3.86–3.92 (2 ꢀ s, 6H, OCH3), 5.21–5.24
(2 ꢀ s, 4H, CH2), 6.06–6.13 (m, 1H, H ), 6.28–6.33 (m, 1 H, H ),
The synthetic route was demonstrated in Scheme 1.
2.3. 3-Methoxy-4-benzyloxyacetophenone (2)
d
00
00
a
b
6.72–6.89 (m, 4 H, aromatics), 7.29–7.60 (m, 7 H, aromatics); 13
NMR (100 MHz, CDCl3):
18.5(C ), 56.1 (2 ꢀ OCH3), 70.8 (C ),
C
d
g
a
0 00
72.2 (C ), 122.9 (C ), 128.5 (C ), 109.4.7–152.9 (C, aromatics),
b b
00
0.830 g (5 mmol) of acetovanillone (1), 1.026 g (6 mmol) of
benzyl bromide, 0.828 g (6 mmol) of powdered potassium
carbonate, and 10 mL of dimethyl formamide were added to a
microwave accelerated reaction system (Milstone Ethos A, Italy).
After reacting for 15 min at 80 8C, the reaction mixture was poured
into 30 mL of water and white precipitates appeared. The
precipitate was collected and washed with distilled water several
times and then dried at room temperature under vacuum to give
a
0
193.3 (C ).
a
3. Results and discussion
3.1. Effect of microwave irradiation on the synthesis
The effects of microwave irradiation on the reaction time and
temperature for the synthesis of 3-methoxy-4-benzyloxyaceto-
phenone (2) are shown in Table 1. The results indicate that when
the reaction time is only 15 min, the yield reaches more than
85%. Conversely, the similar reaction heated with a conventional
water bath requires 3–4 h to give only a yield of 76% [11,12].
Table 1 also shows that with longer reaction time, the yield is
not noticeably improved. With five times longer reaction time,
the yield only increased by 3%. Compared to the conventional
heating method, microwave irradiation results in an almost 60%
increase in the yield under the same condition.
Microwave irradiation not only shortens the reaction time, but
also significantly improves the yield of the desired products. This
may be due to both thermal and specific non-thermal effects of
microwave irradiation in chemical reactions. Basically, the rate
acceleration is ascribed to the increase of the pre-exponential
factor in the Arrhenius equation [13] and the decrease of the
activation energy due to polarization of the solvent molecule,
resulting in an energy transfer between microwave energy and
molecular energy [14].
compound 2 in 95.31% yield. 1H NMR (400 MHz, CDCl3):
3H, CH3), 3.95 (s, 3H, OCH3), 5.25 (s, 2H, CH2), 6.90–7.57 (m, 8H,
aromatics); 13C NMR (100 MHz, CDCl3):
26.3 (CH3), 56.1 (OCH3),
70.8 (CH2), 112.2–152.3 (C, aromatics), 197.0 (C55O).
d 2.57 (s,
d
2.4. 3-Methoxy-4-benzyloxy-a-bromoacetophenone (3)
1.565 g (7 mmol) of cooper bromide in 12 mL ethanol was
added dropwise to a reactor containing 0.768 g (3 mmol) of
compound 2 in chloroform (3 mL). The reaction was carried out
at 70 8C for 15 min under the microwave irradiation. The reaction
mixture was poured into a sodium chloride aqueous solution
after solvent was removed by vacuum distillation, and then the
crude product was extracted with ethyl acetate twice and dried
over MgSO4 and evaporated under vacuum. The resulting
product was further purified by column chromatography with
silica gel using dichlomethane/n-hexane (10/3, v/v) as the mobile
phase. Thin layer chromatography was used to identify the
product, and the purified compound 3 was obtained after
evaporation under vacuum in 87.3% yield. 1H NMR (400 MHz,
CDCl3):
d
3.95 (s, 3H, OCH3), 4.47 (s, 2H, CH2Br), 5.24 (s, 2H, CH2),
30.6
6.50–7.80 (m, 8H, aromatics); 13C NMR (100 MHz, CDCl3):
d
Table 1
(CH2Br), 56.6 (OCH3), 71.1 (CH2), 112.2–153.5 (C, aromatics),
The influence of time and temperature on the yield of 3-methoxy-4-benzylox-
yacetophenone.
190.3 (C55O).
Reaction system
Time (min)
Temperature (8C)
Yield (%)
2.5. 3-Methoxy-4-benzyloxy-a-(3-methoxy-4-(1-propenyl)phenol)-
acetophenone (4)
A
A
A
A
A
A
A
B
15
30
45
60
75
15
15
15
60
60
60
60
60
70
80
80
85.33
85.42
85.51
86.60
88.34
87.12
95.31
35.87
0.720 g (2.15 mmol) compound 3, 0.492 g (3 mmol) isoeu-
genol, and 0.414 g (3 mmol) powdered potassium carbonate in
10 mL dimethyl formamide were added to
accelerated reaction system. After reacting at 80 8C for 30 min,
the reaction mixture was purified using the same processes of
compound 3. The resulting yellow crystals were washed with a
a microwave
A, microwave irradiation; B, traditional heating method.