Microwave-assisted synthesis of substituted imidazoles on a solid support under solvent-free conditions

Article abstract of DOI:10.1016/S0040-4039(00)00771-1

The solvent-free microwave-assisted synthesis of 2,4,5-substituted and 1,2,4,5-substituted imidazoles is reported. Imidazoles are obtained as a result of the condensation of a 1,2-dicarbonyl compound with an aldehyde and an amine using acidic alumina impregnated with ammonium acetate as the solid support. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00771-1

Tetrahedron Letters 41 (2000) 5031±5034
Microwave-assisted synthesis of substituted imidazoles on a
solid support under solvent-free conditions
Alexander Ya. Usyatinsky and Yuri L. Khmelnitsky*
EnzyMed Division, Albany Molecular Research Inc., 2501 Crosspark Rd., Iowa City, IA 52242, USA
Received 3 April 2000; revised 8 May 2000; accepted 9 May 2000
Abstract
The solvent-free microwave-assisted synthesis of 2,4,5-substituted and 1,2,4,5-substituted imidazoles is
reported. Imidazoles are obtained as a result of the condensation of a 1,2-dicarbonyl compound with an
aldehyde and an amine using acidic alumina impregnated with ammonium acetate as the solid support.
# 2000 Elsevier Science Ltd. All rights reserved.
Keywords: microwave chemistry; solvent-free synthesis; substituted imidazoles; combinatorial synthesis.
Microwave-assisted organic synthesis (MAOS) is a new and quickly growing area in synthetic
organic chemistry.1 This synthetic technique is based on the empirical observation that some
organic reactions proceed much faster and with higher yields under microwave irradiation compared
to conventional heating. In many cases reactions that normally require many hours at re¯ux
temperature under classical conditions can be completed within several minutes or even seconds
in a microwave oven, even at comparable reaction temperatures. Recently, we developed a new
highly ecient MICROCOS technology (microwave-assisted combinatorial synthesis) for generating
combinatorial libraries.2 The technology was applied to the high throughput, automated, one-
step, parallel synthesis of diverse substituted pyridines using the Hantzsch synthesis.2 In the present
work we further expand the scope of reactions applicable to MICROCOS technology by intro-
ducing the microwave-assisted synthesis of diverse substituted imidazoles.
The synthetic strategy is based on the condensation of 1,2-diarylethandienones with aldehydes
or aldehydes and amines resulting in 2,4,5-substituted or 1,2,4,5-substituted imidazoles, respectively,
using ammonium acetate as the ammonia source.3 A solid-phase version of this method was
used to generate libraries of substituted imidazoles.4 The multicomponent nature of the synthesis
of 1,2,4,5-substituted imidazoles together with the existence of a wide variety of commercially
available aldehydes and amines makes this reaction an ideal candidate for the MICROCOS
technology. To the best of our knowledge, the microwave-assisted version of this reaction has
* Corresponding author. Tel: (319)-621-5410; fax: (319)-612-5400; e-mail: ykhmelnitsky@enzymed.com
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00771-1

5032
not been reported, although several papers describing other microwave-assisted syntheses of
benzimidazoles and N-substituted imidazoles have been published.5
Generally, the synthetic procedure involved impregnating the mixture of solid support and
ammonium acetate (ammonia source) with ether solution of starting reagents, evaporating the
solvent, and heating the solid residue in a microwave oven (Scheme 1). Preliminary optimization
of reaction conditions was done using the pair benzyl/benzaldehyde (R1=R2=phenyl). Among
di€erent supports tested the expected product 1a was formed on acidic, basic and neutral alumina,
bentonite, montmorillonite K10, montmorillonite KSF, silica gel, and ¯orisil. It was found that
the addition of a small amount (several drops) of acetic acid was required for the reaction to
occur on neutral and basic supports, such as basic and neutral alumina, silica gel, or ¯orisil.6 On
the other hand, when acidic supports were used (bentonite, montmorillonite K10, montmorillonite
KSF, or acidic alumina) the reaction was successful without additional acid. Comparison of
di€erent supports showed that acidic alumina was the most suitable support for the synthesis of
1a, and therefore it was used in all subsequent experiments. Optimal conditions for the synthesis
were found to be 20 min reaction time with a microwave irradiation power of 130 W. No reaction
was observed without heating or when the support was omitted from the reaction mixture.
Scheme 1. Microwave-assisted synthesis of substituted imidazoles
The optimal conditions were then applied for the preparation of the series of 2,4,5-substituted
imidazoles 1a±d and 1,2,4,5-substituted imidazoles 2a±d. In a typical reaction, a mixture of acidic
alumina (9.3 g) and ammonium acetate (4.4 g) was ground in a mortar until a homogeneous
powder was formed. A solution of 0.5 mmol of 1,2-dicarbonyl compound and 0.5 mmol of
aldehyde in 2 ml of diethyl ether or methylene chloride (synthesis of 1a±c), or a solution of 0.5
mmol of 1,2-dicarbonyl compound, 0.5 mmol of aldehyde and 0.5 mmol of amine in 2 ml of ethyl
ether or methylene chloride (synthesis of 2a±c) was added to 2.5 g of the alumina/ammonium
acetate mixture in a 20 ml glass vial. The solvent was allowed to evaporate and the dry residue
was irradiated in a domestic Kenmore microwave oven at 130 W (10% power) for 20 min in the
open vial. The mixture was then allowed to cool to room temperature and washed with a mixture
of acetone/triethylamine (7:3 v:v), 3Â10 ml to extract the product. The combined washes were
®ltered and the solvent was evaporated under reduced pressure. The resulting solid residue was
TM
puri®ed by ¯ash chromatography using a FlashElute system from Elution Solutions on a 90 g
silica cartridge. A mixture of hexane:ethyl acetate (4:1 v:v) was used as eluent for the puri®cation

5033
of 1a±d, 2a, and the solvent system hexane:ethyl acetate (7:1 v:v) was used for the isolation of 2b±
d. The identity of imidazoles 1a,c,d was con®rmed by analytical comparison (HPLC and mass
spectrometry) with authentic standards either obtained from commercial source (Aldrich, for
1a,c), or prepared according to the established procedure3 (for 1d). The structures of 1b and 2a±d
were con®rmed by NMR and mass spectral analysis.7
The structures of products and product yields are shown in Fig. 1. The results show that the
microwave-assisted synthesis provides an ecient way to access diverse, highly functionalized
imidazoles under solvent-free conditions. In contrast to the traditional solid-phase synthesis,4 the
microwave technology does not require the development of solid phase linking and cleaving
chemistries. Similar to solution-phase combinatorial synthesis, the MICROCOS procedure pro-
duces compounds in a soluble form immediately available for biological testing. Indeed, even
without any additional puri®cation the HPLC purity of substituted imidazoles prepared by
microwave-assisted method is typically 75 to 85%, which is usually sucient for primary biological
screening. Furthermore, the MICROCOS technique eliminates the necessity to re¯ux the reaction
mixture in the aggressive solvent (acetic acid) which is required under the standard procedure,4
and the reaction time is reduced from approximately 4 h4 to several minutes. The MICROCOS
procedure is expected to be applicable to a wide range of di€erent building blocks (R1, R2 and R3
in Scheme 1) to provide access to structurally diverse libraries of imidazole derivatives.
Figure 1. Structures and isolated yields of imidazoles prepared by solvent-free microwave-assisted method

5034
Acknowledgements
We thank Dr. Ian C. Cotterill for valuable comments during the preparation of this manuscript.
References
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1665±1692.
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L. Tetrahedron Lett. 1998, 39, 1117±1120.
3. Krieg, B.; Manecke, G. Z. Naturforsch. 1967, 22b, 132±141.
4. Sarshar, S.; Siev, D.; Mjalli, M. M. Tetrahedron Lett. 1996, 37, 835±838.
5. (a) Reddy, A. C. S.; Rao, P. S.; Venkataratham, R. V. Tetrahedron 1997, 53, 5847±5854. (b) Bougrin, K.;
Soufaoui, M. Tetrahedron Lett. 1995, 36, 3683±3686. (c) Ben-Alloum, A.; Bakkas, S.; Soufaoui, M. Tetrahedron
Lett. 1998, 39, 4481±4484. (d) Glas, H.; Thiel, W. R. Tetrahedron Lett. 1998, 39, 5509±5510. (e) Martin-Aranda,
R. M.; Vicente-Rodriguez, M. A.; Lopez-Pestana, J. M.; Lopez-Peinado, A. J.; Jerez, A.; Lopez-Gonzales, J. D.;
Banares-Munoz, M. A. J. Mol. Catal. A 1997, 124, 115±121. (f) Bogdal, D.; Pielichowski, J.; Jaskot, K.
Heterocycles 1997, 45, 715±722.
6. In the original procedure the reaction is performed in re¯uxing acetic acid.3
1
7. Compound 1b: H NMR (DMSO-d6, 360 MHz) ꢀ (7.15, m, 2H), (7.29±7.51, m, 10H), (7.68, dd, 1H), (12.83, s,
1
1H). HRMS [M+H]+=303; compound 1d: H NMR (CD3OD, 360 MHz) ꢀ (2.34, s, 6H), (7.13, m, 5H), (7.33, d,
4H), (7.45, d, 1H), (7.65, d, 1H). HRMS [M+H]+=331; compound 2a: 1H NMR (DMSO-d6, 360 MHz) ꢀ (2.53, t,
2H), (4.08, t, 2H), (6.66, m, 2H), (7.14±7.53, m, 15H), (7.66, d, 2H), (8.09, d, 1H). 13C NMR (DMSO-d6, 90 MHz)
ꢀ 36.6, 45.4, 125.2, 126.0, 128.0, 128.2, 128.4, 128.6, 128.7, 129.1, 130.2, 134.6, 136.5, 145.3, 146.7. HRMS
[M+H]+=401; compound 2b: 1H NMR (DMSO-d6, 360 MHz) ꢀ (2.74, t, 2H), (4.17, t, 2H), (6.80, d, 2H), (7.20, m,
6H), (7.40, m, 5H), (7.55, m, 4H), (7.74, d, 1H). 13C NMR (DMSO-d6, 90 MHz) ꢀ 35.5, 45.7, 125.9, 126.3, 126.6,
127.4, 128.1, 128.3, 128.5, 129.1, 130.2, 130.4, 131.0, 132.8, 134.2, 136.6, 140.6. HRMS [M+H]+=407; compound
2c: 1H NMR (DMSO-d6, 360 MHz) ꢀ (2.23, s, 3H), (2.42, s, 3H), (2.51, t, 2H), (4.08, t, 2H), (6.67, m, 2H), (7.12, d,
2H), (7.14±7.52, m, 16H), (7.63, d, 1H), (8.07, d, 1H). 13C NMR (DMSO-d6, 90 MHz) ꢀ 20.6, 20.9, 35.6, 45.7,
125.1, 126.0, 126.6, 128.0, 128.2, 128.4, 128.5, 128.6, 129.2, 130.7, 131.3, 131.9, 135.1, 136.5, 137.3, 138.1, 146.5.
1
HRMS [M+H]+=429; compound 2d: H NMR (CD3OD, 360 MHz) ꢀ (1.25, t, 3H), (2.70, q, 2H), (5.14, s, 2H)
(6.64, s, 1H), (7.12±7.55, m, 15H), (7.90, d, 1H). 13C NMR (DMSO-d6, 90 MHz) ꢀ 15.3, 27.9, 47.2, 124.3, 125.2,
126.0, 127.9, 128.4, 129.0, 130.4, 130.6, 133.0, 134.4, 139.7, 143.8, 144.7, 145.6, 147.2. HRMS [M+H]+=449.