Acta Chim. Slov. 2004, 51, 545-550.
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Short Communication
DIELS-ALDER CYCLOADDITIONS OF DIMETHYL
ACETYLENEDICARBOXYLATE AND DIETHYL
ACETYLENEDICARBOXYLATE WITH SOME DIENES UNDER
MICROWAVE IRRADIATION USING A1C13/CH2C12
Javad Safaei-Ghomi,*a Mahmood Tajbakhsh,b and Zohreh Kazemi-Kania
a Department of Chemistry, Faculty of Science, University of Kashan, Zip Code 51167 Kashan,
I. R. Iran E-mail: safaei@kashanu.ac.ir b Department of Chemistry, Faculty of Science, University of Mazandaran, Babolsar, I. R. Iran
Received 23-03-2004
Abstract
Microwave enhanced Diels-Alder reactions of furan, 2,5-dimetylfuran, l,3-cyclohexadiene and anthracene with dimethyl acetylenedicarboxylate (DMAD) and diethyl acetylenedicarboxylate (DEAD) give [4+2]-cycloadducts in high yields in domestic microwave oven. Aluminum(III) chloride and dichloromethane in combination with microwave irradiation increased the reaction rate.
Key words: microwave, Diels-Alder reaction, aluminum(III) chloride, solvent effect
Introduction
Microwave enhancement of organic reactions has interested the chemists recent^.1 Microwave heating is very useful in organic synthesis as the reactions can be performed efficiently with high reaction rates, low by-products, high yields and ease of experimental manipulations.2 Due to ali these factors more focus is being paid nowadays to microwave enhanced chemical reactions.
Diels-Alder cycloaddition is the most useful and widely extended method in organic synthesis and is a crucial step in a wide variety of natural products skeleton constructions. Since this is a thermal reaction, the influence of microwave irradiation should be significant. Solvents that possess high dielectric constants such as dichloromethane superheat rapidly in microwave while those with low dielectric constants heat slowly.3 Other efforts to increase the rate and yield of thermal reactions have been the use of Lewis acid catalysts4 and high pressure.5
Herein we present the results of the reactions of dimethyl acetylenedicarboxylate (DMAD) and diethyl acetylenedicarboxylate (DEAD) with furan, 2,5-dimethyl furan, l,3-cyclohexadiene and anthracene in a teflon container under microwave irradiation.
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Acta Chim. Slov. 2004, 51, 545-550.
These reactions proceed efficiently in the presence of A1C13 as catalyst and in CH2C12 as solvent. To the best of our knowledge there have been no previous studies of these applications of microwave ovens.
Results and discussion
In this article we describe some [4+2]-cycloaddition reactions by using the dienophilics substrates, such as substituted acetylenedicarboxylates with dienic substrates as furan, 2,5-dimethylfuran, l,3-cyclohexadiene and anthracene. The classical method for these Diels-Alder reactions requires long heating times at high temperatures (Table).6"14 In our hands ali reactions proceeded in the presence of polar solvent and catalyst in about one to two minutes (with the power regulation of domestic microwave oven). DMAD and DEAD were allowed to react successively with cyclic dienes in the minimum amount of solvent (CH2C12) and catalytic amount of A1C13. A continuous MW-irradiation with incident 650 W power during 80-120 s gave the adducts 1-7 in excellent yields (90-95%). Reaction conditions for cycloadditions have been optimized to obtain the best yields. Dichloromethane was used because it superheats rapidly due to its high dielectric constant.3 A1C13 was used as catalyst to accelerate the rate of reaction because of the complexation of carbonyl group in dienophile with catalyst. This complexation accentuates both the energy and orbital distortion effects of the carbonyl group and therefore enhance the reactivity of the dienophile, relative to the uncomplexed compound.15
Cycloadducts
o
6
f/
5^-4
COOR
COOR
(1) R=Me
(2) R=Et
O
Me
Me
COOR
-COOR
(3) R=Me
(4) R=Et
//
COOR
COOR
(5) R=Me
(6) R=Et
ROOC
COOR
(7) R=Me
J. Safaei-Ghomi, M. Tajbakhsh, Z. Kazemi-Kani: Diels-Alder Cycloadditions of Dimethyl...
Acta Chim. Slov. 2004, 51, 545-550.
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Othenvise, we observed that adducts, upon more powerful irradiation or longer times with low power were decomposed by a retro Diels-Alder reaction and produced 1,2-substituted benzenes and 3,4-substituted furans.16
Table. Comparison of experimental conditions for Diels-Alder reactions of DMAD and DEAD.
	Microwave irradiation		Conventional	heating	
Adduct	IP: 600	w			
	Conditionsa	Yield (%)*	Conditions		Yield (%)
1	10 s x io	95	Neat, 100 °C, 16 h6 Ether, 20 h7		>95 81
2	10 s x 10	95	Neat, 100 °C, 4 h8, 18 h9		>95
3	10 s x 10	95	Neat, 100 °C, 10 h10		>95
4	10 s x 10	95	Neat, 100 °C, 4 h8		>95
5	10 s x8	95	Neat, room temperature, Neat, reflux, 1 h12	18 h11	>95
6	10 s x8	95	Neat, 200 °C, 1 h13		>95
7	10 s x 12	90	Neat, 180 °C, 15 min14		84
" Reactants used in equimolar amounts or in excess of dienophile for 7 (1.4 : 1). *Isolated product.
As Table shows, microwave irradiation allowed cycloadducts to be obtained in 80-120 s with good yields, avoiding the preparation of 1,2-substituted benzenes and 3,4-substituted furans which occur usually by classical heating.9 Thus in Diels-Alder reactions the control of irradiation tirne and microwave power is very important. Ali prepared compounds conformed to the literature data (see Table).
Conclusions
In summary, the microwave irradiation enhances Diels-Alder reactions of DMAD and DEAD with furan, 2,5-dimethylfuran, l,3-cyclohexadiene and anthracene. The cycloadducts can be synthesized in high yield within minutes. A1C13 as catalyst with CH2C12 as solvent reduced the reaction times and increased the yields
Experimental
The microwave oven used for this study was a domestic microwave oven (National, Model NN-C988W) with six power settings. Ali reactions were carried out in medium high mode (600 W) in PFA teflon containers manufactured by Savillex Corporation. To prevent the danger of explosion of closed vessels under MW irradiation
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Acta Chim. Slov. 2004, 51, 545-550.
the amount of the starting material was adapted by capacity of the vessel and the power of microwave was adjusted for control of the pressure and temperature. In addition, ali reactions were conducted in a safe hood. Ali NMR spectra were measured in CDC13 solution. Chemical shifts are reported in ppm downfield from tetramethylsilane (TMS) as an internal reference at 0 ppm. Proton and carbon spectra were recorded on 300 MHz (Bruker Germany Spectrospin), 500 MHz (Bruker DRX- 500, AVANCE) and 90 MHz (FTNMR Geol, GNM-Ex 90A) spectrometers. Some products were characterized also by GC/MS (Agilent, 5973N mass selectives detector, Agilent 6890 series G.C. system, Hewlett-Packard). Riedel-de Haën plates coated with silica gel 60F254 were used for thin-layer chromatography (TLC). Flash column chromatography was performed on silica gel (spectrum SIL 58, 230-400 mesh, grade 60). The melting points were determined on a Kofler block.
Diels-Alder reaction under microwave irradiation; typical procedure. Dimethyl 7-oxabicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate (1). A mixture of DMAD (0.37 g, 3.0 mmol), furan (0.20 g, 3.0 mmol) and A1C13 (0.026 g, 0.2 mmol) was suspended into a minimum amount of solvent (1 mL CH2C12). The reaction mixture was placed in the MW reactor and was irradiated 100 s with a power 600 W-IP. After cooling, aluminum chloride was separated by filtration through celite and washed with CH2C12. The combined filtrates were concentrated. The crude product was purified by flash column chromatography on silica gel using ethyl acetate/hexane (1:3) as eluent. The yellowish oil adduct (0.60 g, 95%) was identified as (1) by a comparison with the literature data:6'7 lH NMR (300 MHz, CDC13) 5 7.20 (m, 2H, H5 and H6), 5.66 (m, 2H, HI and H4), 3.80 (s, 6H, 2CH3); 13C NMR (90 MHz, CDC13) 5 163.2 (C=0), 152.9, 143.2, 85.0, 53.1. MS m/z (relative intensity) (EI): 210 (3.2%, M+), 184 (10), 153 (56.6), 119 (73.3), 111 (12.3), 68 (100), 59 (17.3), 39 (20).
Diethyl 7-oxabicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate (2). Yellowish oil (0 68 g, 95%); lH NMR (90 MHz; CDC13) 5 7.22 (m, 2H, H5 and H6), 5.66 (m, 2H, HI and H4), 4.26 (q, J 7 Hz, 4H, 2 Ctf2CH3), 1.31 (t, J 7 Hz, 6H, 2 CH2C^); 13C NMR (90 MHz, CDCI3) 5 164.0 (C=0), 152.1, 140.2, 83.6, 60.1, 13.6.
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Dimethyl    1,4-dimethyl-7-oxabicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate    (3).
Yellowish oil (0.68 g, 95%); lH NMR (90 MHz; CDC13) ? 6.95 (m, 2H, H5 and H6), 3.80 (s, 6H, 2 OCH3), 1.81 (s, 6H, 2 CH3); 13C NMR (90 MHz; CDC13) ? 164.4 (C=0), 154.8, 147.2, 92.02, 52.1, 15.4.
Diethyl    1,4-dimethyl-7-oxabicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate    (4).
Yellowish oil (0.76 g, 95%); lH NMR (90 MHz; CDC13) ? 6.95 (s, 2H, H5 and H6), 4.26 (q, J 7 Hz, 4H, 2 Ctf2CH3), 1.80 (s, 6H, 2 CH3), 1.32 (t, J 7 Hz, 6H, 2 CH2CH3); 13C NMR (90 MHz, CDCI3) ? 165.3 (C=0), 151.8, 147.0, 90.1, 60.6, 23.6, 13.6.
Dimethyl bicyclo[2.2.2]octa-2,5-diene-2,3-dicarboxylate (5). Yellowish oil (0 63 g, 95%); lH NMR (500 MHz; CDC13) ? 6.36 (m, 2H, H5 and H6), 4.02 (m, 2H, HI and H4), 3.76 (s, 6H, 2 OCH3), 1.43 (m, 4H, 2 H7 and 2 H8); 13C NMR (500 MHz, CDC13) ? 165.1 (C=0), 140.9, 125.5, 52.3, 47.0, 28.9. MS m/z (relative intensity) (EI): 222 (2%, M+), 195 (6.6), 163 (100), 104 (5), 92 (8.3), 77 (17.3), 50 (6.3), 28 (3.3).
Diethyl bicyclo[2.2.2]octa-2,5-diene-2,3-dicarboxylate (6). Yellowish oil (071 g, 95%); lH NMR (90 MHz; CDC13) ? 6.38 (m, 2H, H5 and H6), 4.27 (q, J 7 Hz, 4H, 2 C//2CH3), 4.02 (m, 2H, HI and H4), 1.43 (m, 4H, 2 H7 and 2 H8), 1.31 (t, JI Hz, 6H, 2 CH2CH3); 13C NMR (90 MHz, CDC13) ? 168.0 (C=0), 138.6, 122.0, 60.9, 47.1, 28.8, 13.6.
Diethyl         tetracyclo[6.6.2.02,7.0914]hexadeca-2,4,6,9(14),10,12,15-heptaene-15,16-
dicarboxylate (7). 0.34 g, 90%; mp 160-161 °C; *H NMR (500 MHz; CDC13) ? 7.31-7.10 (m, 8H, ArH), 4.87 (s, 2H, 2 CH-Ar), 3.77 (s, 6H, 2 CH3); 13C NMR (90 MHz, CDCI3) ? 164.0 (C=0), 155.0, 136.1, 128.4, 126.6, 52.3, 49.9. MS m/z (relative intensitv) (EI): 320 (1.6%, M+), 202 (6.6), 179 (12), 178 (75), 44 (65), 28 (100).
Acknowledgements
Financial supports from the University of Kashan and the University of Mazandaran is gratefully acknowledged. We thank Dr. Hossein Naeimi for his help in submitting this paper.
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Povzetek
Z mikrovalovi pospešene Diels-Alderjeve reakcije med furanom, 2,5-dimetilfuranom, 1,3-cikloheksadienom in antracenom z dimetil acetilendikarboksilatom (DMAD) oz. dietil acetilendikarboksilatom (DEAD) v običajni kuhinjski mikrovalovni pečici dajejo [4+2] cikloadukte z visokimi izkoristki. Aluminijev(III) klorid v diklorometanu ob učinkovanju mikrovalov poveča reakcijske hitrosti.
J. Safaei-Ghomi, M. Tajbakhsh, Z. Kazemi-Kani: Diels-Alder Cycloadditions of Dimethyl...