A new solvent-free synthesis of α,β-unsaturated ketones from acetals with aryl ketones under microwave irradiation

Article abstract of DOI:10.1081/SCC-120003144

A new, rapid and efficient method for the synthesis of α,βunsaturated ketones under microwave irradiation conditions is described. The process involves the reaction of acetals with aryl ketones in the absence of solvent using Lewis acids as catalysts under microwave irradiation to afford the α,βunsaturated ketones in good to excellent isolated yields. The reaction mechanism is briefly discussed.

Full text of DOI:10.1081/SCC-120003144

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A NEW SOLVENT-FREE SYNTHESIS OF α,β-UNSATURATED
KETONES FROM ACETALS WITH ARYL KETONES UNDER
MICROWAVE IRRADIATION
Danfeng Huang a , Jin-Xian Wang a , Yulai Hu a , Yumei Zhang a & Jing Tang a
a Department of Chemistry, Institute of Chemistry, Northwest Normal University, 95 An Ning
Road (E), Lanzhou, 730070, P.R. China
Available online: 21 Aug 2006
To cite this article: Danfeng Huang, Jin-Xian Wang, Yulai Hu, Yumei Zhang & Jing Tang (2002): A NEW SOLVENT-FREE
SYNTHESIS OF α,β-UNSATURATED KETONES FROM ACETALS WITH ARYL KETONES UNDER MICROWAVE IRRADIATION, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 32:7, 971-979
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SYNTHETIC COMMUNICATIONS, 32(7), 971–979 (2002)
A NEW SOLVENT-FREE SYNTHESIS OF
a,b-UNSATURATED KETONES FROM
ACETALS WITH ARYL KETONES UNDER
MICROWAVE IRRADIATION
*
Danfeng Huang, Jin-Xian Wang, Yulai Hu,
Yumei Zhang, and Jing Tang
Institute of Chemistry, Department of Chemistry,
Northwest Normal University, 95 An Ning Road (E),
Lanzhou, 730070, P.R. China
ABSTRACT
A new, rapid and efficient method for the synthesis of a,b-
unsaturated ketones under microwave irradiation conditions
is described. The process involves the reaction of acetals with
aryl ketones in the absence of solvent using Lewis acids
as catalysts under microwave irradiation to afford the a,b-
unsaturated ketones in good to excellent isolated yields. The
reaction mechanism is briefly discussed.
Substituted or unsubstituted chalcones and their derivatives, and more
generally, a,b-unsaturated ketones, are important intermediates in organic
synthesis.1 Several a,b-unsaturated ketones have been found to exhibit
important pharmacological and biological activities,2 and they seem to be
*Corresponding author. E-mail: wangjx99@sohu.com.cn
971
Copyright & 2002 by Marcel Dekker, Inc.
www.dekker.com

972
HUANG ET AL.
involved in the biosynthesis of flavonoids.3 Some a,b-unsaturated ketones
are used as sweeteners, sunscreen agents,4 photoresits, photographic emul-
sions,5 U.V. filters in solar creams6, and in the preparation of liquid crystal
material.7,8
Usually, the preparation of chalcones is achieved with NaOH, KOH
or Ba(OH)2 in hydroalcoholic medium from bezaldehyde and ketones.9–11
However, this method has some disadvantages, such as reaction
reciprocity,12 self-condensation of ketones and aldehydes,13 and long
reaction times.10c Several methods using a catalyst have been developed,
for example, using piperidine,14a cadmium iodide,14b anhydrous ZnCl2,14c
AlPO4-Al2O3,14d bis( p-ethoxyphenyl)telluroxide (BEPTO),15 BMPTO,16
NaOH(S),10 NH4OAc,17 SnCl2/Na2SO3,18 SnCl2,18 Ti(OR)4,19 and p-
toluene sulfonic acid (PTSA).20 Recently, Powers et al., reported respec-
tively the automated parallel synthesis of chalcone-based screening
libraries.21
Acetals have been successfully applied in several organic reactions as a
protective group in organic synthesis.22 There have been many examples of
utilization of this methodology and various nucleophilic species were used
for the reaction with acetals.23 However, there is no literature report on the
synthesis of a,b-unsaturated ketones from acetals with aryl ketones.
The use of microwave energy to activate organic reactions has
recently taken a new dimension.24 It has been used for a great variety
of organic reactions such as esterification, etherification, oxidation, hydro-
lysis, Diels–Alder(4 þ 2), Reformatsky, Knoevenagel, Bischler–Napieralski,
and solid-phase peptide synthesis. In recent years, microwave-induced rate
acceleration technology is becoming
a powerful tool in organic
synthesis, because of milder reaction conditions, reduction of reaction
times, enhanced selectivity, and associated ease of manipulation. A
particularly attractive feature of the microwave technique is the pos-
sibility of carrying out reactions in the absence of solvent.25 In a previous
paper,26 we reported the synthesis of substituted glycerol selenide ethers
and chiral glycerol sulfide ethers under microwave irradiation conditions.
RESULTS AND DISCUSSION
We first studied the reaction of acetals with acetophenone, without
solvent, and under microwave irradiation in the presence of catalytic
amounts of Lewis acids, a new, rapid and efficient method for the synthesis
of a,b-unsaturated ketones from acetals with aryl ketones in solvent free
conditions. The reaction is shown in Scheme 1 and the results are summar-
ized in Table 1.

a,b-UNSATURATED KETONES
973
Scheme 1.
We investigated the effect of the Lewis acid on the reaction of acetals
with acetophenone under microwave irradiation. It was found that the
activities of the catalysts are in the following sequence: AlCl3>TiCl4>
BF3ꢀOEt2>SbCl3. The results are summarized in Table 2 (Entries 1–4).
When KF-AlO3, NaOH(S) and KOH(S) were used as catalysts, the
starting materials were recovered in almost quantitative yields (Entries
9–11). The impact of microwave irradiation and conventional heating for
the synthesis of compound 3a has been compared. Under microwave irra-
diation conditions, the yield of 3a is high (84%), whereas the yield is only 52
and 60%, respectively, when the reaction is carried out at room temperature
for 66 h or 100ꢁC (oil bath) for 10 h (Entry 5). We have also found that this
reaction is sensitive to molecular structure. For example, under the some
conditions, the reaction can not be carried out if aliphatic acetals, cyclic
ketal, and substitute phenyl ethyl ketones are used (Entries 6–8). The effects
of irradiation power and time on the reaction were also studied and the
results are summarized in Tables 3 and 4. It was found the high yield
compounds 3a–k can be obtained in 375 W for 15 min under microwave
irradiation conditions.
In conclusion, this new method for the synthesis of chalcones using
Lewis acid as a catalyst for the reaction of acetal with aryl ketone, without
any solvent under microwave irradiation, offers significant improvements
over existing procedures and thus helps facile entry into a variety of chal-
cones of potentially high synthetic utility. Also, this simple and reproducible
technique affords various chalcones with short reaction times, excellent
yields, and without the formation of undesirable side products.
A possible mechanism for the reaction of aryl acetals with ketone is
outlined in Scheme 2. The first step involves generation of the brown red
complex (A) from arylacetals with Lewis acid via the pathway that was
previously proposed.27 Loss of the EtOMXn from (A) may result in the

974
HUANG ET AL.
Table 1. Synthesis of a,b-Unsaturated Ketones 3a–k Under Microwave
Irradiation Conditions
Irradiation Condition Yielda,b
Entry
Product
Power (W)/Time (min)
(%)
375/8
84
3a
375/15
375/8
85
83
81
85
82
84
79
74
84
68
3b
3c
3d
3e
3f
375/15
375/15
375/8
375/15
375/15
375/15
375/15
375/15
3g
3h
3i
3j
3k
b
aIsolated yield; Using AlCl3 as a catalyst.
formation of the carbocation (B). The addition of the formative
carbocation (B) to enolic arylketones would generate the complex (C)
which then dealcoholize to give complex (D). The deprotonation in the
presence of EtOMXn to give desired a,b-unsaturated ketones and the
Lewis acid is regenerated. The catalyst is thus recycled in the reaction.

a,b-UNSATURATED KETONES
975
Table 2. The Effect of Lewis Acid on the Formation a,b-Unsaturated Ketonesa
Yield
(%)
Entry
1
2
Catalysts
1
2
3
4
5
6
7
8
C6H5COCH3
C6H5COCH3
C6H5COCH3
C6H5COCH3
C6H5COCH3
C6H5COCH3
C6H5COCH3
p-CH3O-C6H4
COCH2CH3
C6H5COCH3
C6H5COCH3
C6H5COCH3
C6H5CH(OEt)2
C6H5CH(OEt)2
C6H5CH(OEt)2
C6H5CH(OEt)2
C6H5CH(OEt)2
CH3(CH2)2CH(OEt)2
Cyclohexanone diacetal
C6H5CH(OEt)2
AlCl3
TiCl4
84b
81b
BF3ꢀOEt2
SbCl3
AlCl3
AlCl3
AlCl3
AlCl3
78b
74b
52 (60)b,c
No reactiond
No reactiond
No reactiond
9
10
11
C6H5CH(OEt)2
C6H5CH(OEt)2
C6H5CH(OEt)2
KF–AlO3
NaOH(S)
KOH(S)
No reactiond
No reactiond
No reactiond
aMol ratio ¼ aryl acetal : aryl ketone : Lewis acid ¼ 1 : 1 : 0.05; Microwave irradiation
conditions: 375 W/15 min.
bIsolated yield.
cConventional heating: r.t./66 h, yield, 52%; 100ꢀC (oil bath)/10 h, yield, 60%.
dThe starting materials were recovered in almost quantitative yields.
Table 3. The Effect of Microwave Irradiation Powera,b
Irradiation Power (W)
Yield (%)
675
64
600
69
525
81
375
84
b
aIrradiation time is 15 min; Using AlCl3 as a catalyst.
Table 4. The Effect of Microwave Irradiation Timea,b
Irradiation Time (min)
Yield (%)
5
8
12
72
15
84
48
61
b
aIrradiation power is 375 W; Using AlCl3 as a catalyst.
EXPERIMENTAL
IR spectra were measured as KBr using an Alpha Centauri FT-IR
spectrophotometer; 1H NMR spectra (80 MHz) were recorded in CDCl3
using an FT-80 spectrometer. Mass spectra were obtained on a Nippon

976
HUANG ET AL.
Scheme 2.
Shimadzu QP-1000 GS-MS spectrometer. Microwave irradiations are car-
ried out in a modified Galanz WP 750B commercial microwave oven at
2450 MHz.
General procedure: Lewis acid (0.4 mmol) was added to a mixture of
aryl acetal (8 mmol) and aryl ketone (8 mmol). The reaction mixture was
stirred at room temperature for 1 min. The mixture, which changed from
colourless to red-brown immediately, was irradiated at 375 W for 8 min.
After completion of the reaction Et2O (or CH2Cl2, 15 ml), H2O (15 ml) was
added. The organic layer was separated, washed with water (3 ꢂ 10 ml) and
then dried (MgSO4). The solvent was evaporated, and the oily reside was
chromatographed via a short silica gel column using petroleum ether :
ethyl acetate (10 : 1, v/v) as an eluent or crystallized from EtOH or ethyl
acetate.
1,3-Diphenyl-2-propen-1-one 3a: Yellowish solid; m.p. 58–59ꢁC (lit.,10a
59ꢁC); IR nmax (KBr)/cmꢃ1 3059, 3026, 1658, 1601, 1574, 1494, 1444, 976,
744, 688; 1H NMR dH (80 MHz, CDCl3), 8.08–7.93 (m. 2H), 7.76–7.24
(m, 14H); EI-MS m/z 208 (Mþ, 99%), 207 (100), 131 (32), 105 (27), 103
(37), 77 (60).
3-(4-Methoxylphenyl)-1-phenyl-2-propen-1-one 3b: Yellowish solid;
m.p. 75–77ꢁC (lit.,28a 77–78ꢁC); IR nmax (KBr)/cmꢃ1 3056, 1658, 1601, 1577,
1512, 1446, 1338, 1018, 985, 825, 779, 688; 1H NMR dH (80 MHz, CDCl3) 8.0
(d, 1H), 7.8 (d, 1H), 7.65–7.86 (m, 8H), 3.84 (s, 3H); EI-MS m/z 208 (Mþ,
100%), 223 (26), 207 (28), 1631 (51), 133 (27), 107 (13), 105 (36), 77 (76).

a,b-UNSATURATED KETONES
977
3-(2-Chlorophenyl)-1-phenyl-2-propen-1-one 3c: Yellowish solid; m.p.
49–51ꢁC (lit.,28b 50–52ꢁC); IR nmax (KBr)/cmꢃ1 3059, 1662, 1606, 1564,
1460, 970, 750, 688; 1H NMR dH (80 MHz, CDCl3), 8.06 (d, 1H), 7.97
(d, 1J), 7.84–7.25 (m, 9H); EI-MS m/z 242 (Mþ, 113%), 207 (100), 137 (7),
105 (17), 77 (26).
1-(4-Methylphenyl)-3-phenyl-2-propen-1-one 3d: Yellowish solid; m.p.
53–54ꢁC (lit.,10c 55–55.5ꢁC); IR nmax (KBr)/cmꢃ1 3047, 3026, 2970, 2914,
1
1657, 1604, 1574, 1493, 1446, 979, 821, 758, 692; H NMR dH (80 MHz,
CDCl3) (m, 11H), 2.44 (s, 3H); EI-MS m/z 222 (Mþ, 81%), 221 (100), 207
(23), 131 (22), 119 (35), 103 (24), 91 (41), 77 (26).
1-(4-Methylphenyl)-3-(4-methoxyphenyl)-2-propen-1-one 3e: Yellowish
solid; m.p. 97–98ꢁC (lit.,10c 98ꢁC); IR nmax (KBr)/cmꢃ1 3030, 2966, 2835,
1
1655, 1604, 1570, 1508, 1458, 1421, 1226, 1033, 817; H NMR dH (80 MHz,
CDCl3), 7.97–6.67 (m, 10H), 3.65 (s, 3H), 2.42 (s, 3H); EI-MS m/z 252 (Mþ,
100%), 237 (62), 221 (16), 161 (43), 133 (25), 119 (30), 91 (54), 77 (22).
3-(2-Chlorophenyl)-1-(4-methylphenyl)-2-propen-1-one 3f: Yellowish
solid; m.p. 53–55ꢁC; IR nmax (KBr)/cmꢃ1 3059, 2974, 1660, 1606, 1566,
1467, 1437, 972, 819, 756, 684, 576; 1H NMR dH (80 MHz, CDCl3),
8.28–7.26 (m, 10H), 2.44 (s, 3H); EI-MS m/z 256 (Mþ, 19%), 241 (5), 221
(100), 165 (6), 137 (5), 119 (17), 91 (25).
1-(2-Chlorophenyl)-3-phenyl-2-propen-1-one 3g: Yellowish solid; m.p.
97–98ꢁC (lit.,28c 98ꢁC); IR nmax (KBr)/cmꢃ1 3053, 1662, 1606, 1587, 1574,
1448, 983, 829, 763, 670; 1H NMR dH (80 MHz, CDCl3), 8.02–7.25 (m, 11H);
EI-MS m/z 242 (Mþ, 100%), 207 (48), 139 (30), 131 (36), 110 (17), 77 (33).
1-(2-Chlorophenyl)-3-(4-methoxyl)-2-propen-1-one 3h: Yellow solid;
m.p. 122–124ꢁC (lit.,28d 121–122ꢁC); IR nmax (KBr)/cmꢃ1 3003, 2937, 1655,
1
1589, 1510, 1460, 1213, 1032, 979, 817; H NMR dH (80 MHz, CDCl3),
8.00–6.88 (m, 10H), 3.85 (s, 3H); EI-MS m/z 272 (Mþ, 92%), 271 (100), 257
(21), 241 (27), 237 (83), 165 (23), 161 (49), 139 (26), 133 (27), 111 (34), 77 (16).
3-(4-Methoxyphenyl)-1-(4-nitrophenyl)-2-propen-1-one 3i: Brown–
yellow solid; m.p. 178–179ꢁC (lit.,28e 176–177ꢁC); IR nmax (KBr)/cmꢃ1
3040, 2974, 1657, 1604, 1572, 1514, 1427, 1338, 1255, 1033, 993, 854, 819;
1H NMR dH (80 MHz, CDCl3), 8.40–6.69 (m, 10H), 3.87 (s, 3H); EI-MS m/z
283 (Mþ, 100%), 268 (22), 252 (54), 237 (31), 161 (51), 150 (8), 133 (42), 77 (11).
1-(4-Biphenyl)-3-phenyl-2-propen-1-one 3j: Yellow solid; m.p. 154–
156ꢁC (lit.,28f 156ꢁC); IR nmax (KBr)/cmꢃ1 3059, 3026, 1658, 1063, 1575,
1
1491, 1448, 999, 839, 754, 686; H NMR dH (80 MHz, CDCl3), 8.16–7.42
(m, 16H); EI-MS m/z 284 (Mþ, 97%), 283 (100), 181 (48), 153 (32), 131 (38),
103 (40), 77 (36).
1-(4-Phenoxyphenyl)-3-phenyl-2-propen-1-one 3k: Yellowish solid; m.p.
87–88ꢁC (lit.,28g 85–86ꢁC); IR nmax (KBr)/cmꢃ1 3055, 1657, 1601, 1493,
1448, 1253, 1168, 995, 837, 750, 694; 1H NMR dH (80 MHz, CDCl3),

978
HUANG ET AL.
8.08–6.99 (m, 16H); EI-MS m/z 300 (Mþ, 83%), 299 (100), 207 (67), 195
(55), 131 (22), 77 (68).
ACKNOWLEDGMENTS
This project was supported by the National Natural Science
Foundation of China and the Northwest Normal University Science and
Technology Development Foundation of China.
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Received in the USA April 5, 2001