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Pyridines Using Catalytic Zinc Chloride Amanda L Tetrahedron Letters 48 (2007) 4079–4082 Multicomponent synthesis of imidazo[1,2-a]pyridines using catalytic zinc chloride Amanda L. Rousseau,* Pulane Matlaba and Christopher J. Parkinson Discovery Chemistry, Biosciences, CSIR, Private Bag X2, Modderfontein 1645, South Africa Received 1 March 2007; revised 26 March 2007; accepted 4 April 2007 Available online 11 April 2007 Abstract—The novel use of zinc chloride to catalyze the one-pot, three component synthesis of imidazo[1,2-a]pyridines from a range of substrates using either conventional heating or microwave irradiation is described. This methodology affords a number of imidazo[1,2-a]pyridines in reasonable yields and short reaction times without any significant optimization of the reaction conditions. Ó 2007 Elsevier Ltd. All rights reserved. Imidazo[1,2-a]pyridines 1 and the related imidazo[1,2-a]- approach involves the coupling of 2-aminopyridines pyrimidines 2 (Fig. 1) have received significant atten- with a-halocarbonyl compounds.1 However, this tion from the pharmaceutical industry owing to their approach does not readily lend itself to a diversity interesting biological activities displayed over a broad oriented synthesis. range of therapeutic classes,1 exhibiting antibacterial,2 antifungal,3 antiviral,4 and anti-inflammatory5 proper- A more versatile approach uses a three component ties. They have also been shown to be selective cyclin coupling (3CC) involving the condensation of alde- dependant kinase inhibitors,6 GABA,7 and benzodiaze- hydes, 2-aminopyridine, and isocyanides (Scheme 1).10 8 pine receptor agonists, and bradykinin B2 receptor This robust approach allows for the preparation of a antagonists.9 Drug formulations containing imid- diverse range of products and suited our requirements azo[1,2-a]pyridines currently available on the market in the continuation of a synthetic program aimed at include alpidem 3 (anxiolytic), zolpidem 4 (hypnotic), kinase inhibitors. and zolimidine 5 (antiulcer).1 The 3CC reaction is carried out in the presence of an While there are a number of synthetic routes to the acid catalyst, usually scandium(III) triflate,11 although imidazo[1,2-a]pyridine ring system, the most common there have been reports describing the use of ammonium chloride,12 acetic acid,10a perchloric acid,10e or Montmo- rillonite clay K1013 to catalyze the reaction. The use of N X N solid supports with a range of acid catalysts in the R2 N N 1 3CC reaction has also been described, including the R 14 4 use of excess immobilized glyoxylic acid, toluenesulf- R 1 2 3 4 1 X=CH 3 R =R =Cl, R =R =C3H7 15 2 X=N N 1 2 3 4 onic acid (using an isonitrile resin), and catalytic 4 R =R =R =R =CH3 10d R3 O scandium triflate (using resin bound aldehydes). N Reactions were typically carried out at room tempera- SO2Me ture (for reactions catalyzed by scandium triflate, acetic N acid, toluenesulfonic acid or perchloric acid) or using 5 Figure 1. NH2 R1CHO N R1 N N Keywords: Multicomponent reaction; Imidazo[1,2-a]pyridines; Zinc R2NC chloride; Montmorillonite K10. catalyst HN R2 * Corresponding author. Tel.: +27 11 605 2455; fax: +27 11 608 3200; e-mail: [email protected] Scheme 1. 0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2007.04.008 4080 A. L. Rousseau et al. / Tetrahedron Letters 48 (2007) 4079–4082 conventional heating (for reactions catalyzed by glyoxy- ple solvent washes of the clay were required to recover lic acid or ammonium chloride). This, however, required the product. As both catalysts are substantially cheaper long reaction times of 20–72 h.10–15 The use of micro- than scandium triflate (the clay can also be recycled), wave reactors has been reported for reactions catalyzed these methods were used to prepare a collection of imi- by scandium triflate11a and Montmorillonite clay,13 with dazo[1,2-a]pyridines. This is the first reported use of zinc reduced reaction times. chloride in the 3CC synthesis of imidazo[1,2-a]pyridines. In our initial investigations, we hoped to find a general Without further optimization of the reaction conditions method that would be widely applicable to a range of for individual reactions, a range of imidazo[1,2-a]pyri- substrates without extensive optimization, thus allowing dines were prepared from 2-aminopyridine using either for the rapid preparation of a series of imidazo[1,2- zinc chloride or Montmorillonite clay K10 as the cata- a]pyridines for biological testing. The reaction of lyst (Table 2) in a microwave or an automated parallel 2-aminopyridine, 2,6-dimethylphenylisocyanide, and reactor. Both catalysts are applicable to a range of sub- benzaldehyde was selected as a test reaction (Scheme 1, strates, with some limitations. R1 = Ph, R2 = 2,6-dimethylphenyl), and the reaction conditions (catalyst, temperature, solvent, and reaction Reactions with most aldehydes were catalyzed success- time) varied (Table 1). fully with zinc chloride to afford the corresponding imidazo[1,2-a]pyridines. However, the reactions with Scandium triflate at ambient temperature gave the nicotinaldehyde did not go to completion when zinc desired product in good yield, but required long reaction chloride was used as the catalyst, and the only product times as reported11 (entry 1). In addition to this, scan- isolated from the reaction was the intermediate imine. dium triflate is an expensive catalyst, and a loading of Attempts to convert the imine isolated from this reac- 5 mol % corresponds to 25 mass % with respect to 2- tion (possibly still complexed with zinc chloride) to the aminopyridine. At room temperature, the reaction cata- desired product using a variety of catalysts afforded only lyzed by Montmorillonite clay KSF was very slow (entry low yields of the product, suggesting that zinc chloride 2), and only 20% of the desired product was isolated forms a stable complex with this imine once it is formed. after 4 days. When the same reaction was carried out In order to overcome this problem, Montmorillonite in a microwave reactor16 (with a change to a higher boil- clay K10 was used as the catalyst in the reactions with ing solvent), there was some improvement, but with a nicotinaldehyde, affording the desired imidazo[1,2-a]- substantial increase in uncharacterized by-product for- pyridines as shown in entries 5–8 of Table 2. The reac- mation (entry 3). In addition, multiple solvent washes tions with furfural were also carried out using the clay of the clay were required to recover the product. This catalyst, as we felt that coordination with zinc chloride reaction was repeated by first forming the imine, and would also be possible with these products. However, then treating the imine with 2,6-dimethylphenylisocya- a test reaction carried out with zinc chloride showed that nide in the presence of the catalyst under microwave reactions with furfural could, indeed, be catalyzed by conditions (entry 4). With multiple solvent washes of zinc chloride. the clay, this gave the desired product in 55% yield in 1 h. However, a one-pot reaction would be preferential. In general, the use of p-methoxybenzaldehyde and 3,4- dimethoxybenzaldehyde gave low yields of the desired Therefore, test reactions were carried out separately products. These oxygenated products were found to be with both Montmorillonite clay K10 and a novel cata- relatively unstable and there was also evidence for lyst for this reaction, zinc chloride, using both conven- decomposition on silica, which contributed to the low tional heating and microwave irradiation (Table 1, yields. This could not be attributed to the presence of entries 5–8). Both catalysts afforded adequate yields of zinc chloride, however, as low yields were also obtained the desired product in short reaction times. For reac- with Montmorillonite K10. tions catalyzed by zinc chloride, the work-up simply involved removal of solvent under reduced pressure, while Isobutyraldehyde afforded reasonable yields of the for reactions catalyzed by Montmorillonite clay, multi- desired imidazo[1,2-a]pyridines, indicating that zinc Table 1. Variation of reaction conditions for the 3CC reaction (R1 = phenyl, R2 = 2,6-dimethylphenyl) Entry Catalyst Solvent Conditions Time (h) Yield (%) 1 Sc(OTf)3 MeOH/DCM Ambient temperature 48 78 2 Montmorillonite KSF MeOH/DCM Ambient temperature 96 20a 3 Montmorillonite KSF 1,4-Dioxane Microwave 2 35b 4 Montmorillonite KSF 1,4-Dioxane Microwave 1 55c 5 ZnCl2 (5 mol %) 1,4-Dioxane Microwave 1 62 6 ZnCl2 (5 mol %) 1,4-Dioxane Reflux 5 65 7 Montmorillonite K10 (1 mass equiv) 1,4-Dioxane Microwave 1 61 8 Montmorillonite K10 (1 mass equiv) 1,4-Dioxane Reflux 5 72 a Incomplete reaction. b Formation of multiple products. c Imine formation carried out first. A. L. Rousseau et al. / Tetrahedron Letters 48 (2007) 4079–4082 4081 Table 2. Imidazo[1,2-a]pyridines prepared Entry Imidazo[1,2-a]pyridine Reaction conditions Yield Entry Imidazo[1,2-a]pyridine Reaction conditions Yield (%) (%) N N Ph ZnCl2, 1,4-dioxane, 62 13 OMe ZnCl2, 1,4-dioxane, 28 N N microwave, 1 h reflux 5 h 1 HN Montmorillonite clay K10, 72 HN 1,4-dioxane, reflux, 5 h N N Ph OMe N 2 N ZnCl2, 1,4-dioxane, 60 14 ZnCl2, 1,4-dioxane, 15 HN microwave, 1 h HN reflux 5 h N N Ph OMe N N 3 ZnCl2, 1,4-dioxane, 14 15 ZnCl2, 1,4-dioxane, 22 HN microwave, 1 h HN reflux 5 h N O N O N N Ph OMe N N 4 ZnCl2, 1,4-dioxane, 78 16 ZnCl2, 1,4-dioxane, 27 HN microwave, 1 h HN reflux 5 h N N OMe N N N 5 Montmorillonite clay K10, 54 17 ZnCl , 1,4-dioxane, 34 NH NH OMe 2 1,4-dioxane, microwave, 1 h reflux 5 h Montmorillonite 41 clay K10, 1,4-dioxane, reflux, 5 h N N OMe N N 6 N Montmorillonite clay K10, 57 18 ZnCl2, 1,4-dioxane, 9 NH 1,4-dioxane, microwave, 1 h NH OMe reflux 5 h N OMe N N N OMe 7 Montmorillonite clay K10, 20 19 N ZnCl2, 1,4-dioxane, 36 HN 1,4-dioxane, microwave, 1 h reflux 5 h N O HN N O N N OMe N N N 8 Montmorillonite clay K10, 59 20 ZnCl2, 1,4-dioxane, 75 HN 1,4-dioxane, microwave, 1 h HN OMe reflux 5 h N N N N O 9 ZnCl , 1,4-dioxane, reflux 5 h 30 21 ZnCl , 1,4-dioxane, 20 NH 2 HN 2 Montmorillonite clay K10, 25 reflux 5 h 1,4-dioxane, reflux, 5 h (continued on next page) 4082 A.
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