Clean and Efficient Synthesis of Isoxazole Derivatives in Aqueous Media

A series of 5-arylisoxazole derivatives were synthesized via the reaction of 3-(dimethyl-amino)-1-arylprop-2-en-1-ones with hydroxylamine hydrochloride in aqueous media without using any catalyst. This method has the advantages of easier work-up, mild reaction conditions, high yields, and an environmentally benign procedure.


Introduction
The need to reduce the amount of toxic waste and byproducts arising from chemical processes requires increasing emphasis on the use of less toxic and environmentally compatible materials in the design of new synthetic methods [1]. One of the most promising approaches is the use of water as the reaction medium [2]. Compared to organic solvents the aqueous medium is less expensive, less dangerous, and more environmentally friendly. In recent years, there has been increasing recognition that water is an attractive medium for many organic reactions [3][4][5]. Many important types of heterocycles, such as furans, pyridines, quinolines, indoles, triazines, acridines, pyrazines, and pyrimidines have been synthesized in aqueous media [6][7][8][9][10][11][12][13][14][15]. The synthesis of new and other important type of heterocyclic compounds in water continues to attract wide attention among synthetic chemists.

OPEN ACCESS
Nitrogen-containing heterocyclic building blocks are of great importance to both medical and organic chemists, and their synthesis continues to represent a challenge from both academic and industrial perspectives [16]. Isoxazole derivatives are an important class of heterocyclic pharmaceuticals and bioactive natural products because of their significant and wide spectrum of biological activities, including potent and selective antagonism of the NMDA receptor [17] and anti-HIV activity [18]. Many syntheses of isoxazoles have been developed [19,20]. However, these syntheses are usually carried out in organic solvents. As part of our current studies on the development of new routes to heterocyclic systems in aqueous media [21][22][23][24][25][26][27][28], we now report an efficient and clean synthetic route to isoxazole derivatives via the reaction of 3-(dimethylamino)-1-arylprop-2-en-1-ones with hydroxylamine hydrochloride in aqueous media.

Results and Discussion
When an equivalent mixture of an 3-(dimethylamino)-1-arylprop-2-en-1-one derivative 1 and hydroxylamine hydrochloride (2) was stirred at 50 °C in aqueous media, 5-arylisoxazole derivatives 3 were obtained in good yields (Scheme 1). The results are summarized in Table 1.  As shown in Table 1, this protocol could be applied to the 3-(dimethylamino)-1-arylprop-2-en-1-ones with both electron-withdrawing groups (such as halide groups) and electron-donating groups (such as methyl or methoxyl groups). Polysubstituted 3-(dimethylamino)-1-arylprop-2-en-1-ones could also be used in this synthesis. We concluded that the electronic nature of the substituent on the aromatic ring of 3-(dimethylamino)-1-arylprop-2-en-1-ones had no significant effect on this reaction. This synthesis was confirmed to follow the group-assisted-purification chemistry process [29][30][31], which can avoid traditional chromatography and recrystallization purification, that is, all the pure products can be obtained only by suction filtration without further purification. All the products 3 were identified from their IR, 1 H-NMR, and HRMS spectra.
Although the detailed mechanism of this reaction remains to be fully clarified, the formation of 5-arylisoxazoles 3 could be explained by the reaction sequence presented in Scheme 2.

Experimental
All reagents were purchased from commercial suppliers and used without further purification. Melting points are uncorrected. IR spectra were recorded on Varian F-1000 spectrometer in KBr with absorptions in cm −1 . 1 H-NMR and 13 C-NMR spectra were recorded on a Varian Inova-300 MHz or Varian Inova-400 MHz in CDCl 3 solution. J Values are in Hertz. Chemical shifts are expressed in parts per million downfield from internal standard TMS. High-resolution mass spectra (HRMS) were obtained using Bruker microTOF-Q instrument.

General Procedure for the Synthesis of 3-(Dimethylamino)-1-arylprop-2-en-1-ones 1a-l
A solution of substituted acetophenone (2 nmol) in N,N-dimethylformamide dimethyl acetal or N,N-dimethylacetamide dimethyl acetal (10 mL) was refluxed for 20 h during which time some methanol was formed and removed through a reflux condenser. After cooling, the precipitate was collected by suction to give compounds 1.

General Procedure for the Synthesis of Isoxazole Derivatives 3a-l
3-(Dimethylamino)-1-arylprop-2-en-1-one 1 (1 nmol), hydroxylamine hydrochloride 2 (1 nmol) and water (5 mL) were added to a 25-mL round-bottom flask. The mixture was then stirred at 50 °C for 2 h. After completion of the reaction, the mixture was then cooled to room temperature. The precipitate was collected by suction filtration to give products 3 without further purification.

Conclusions
In conclusion, we have developed an efficient synthesis of isoxazole derivatives via the reaction of 3-(dimethylamino)-1-arylprop-2-en-1-ones with hydroxylamine hydrochloride in aqueous media without using any catalyst. This method has the advantages of an easier work-up, mild reaction conditions, high yields, and an environmentally benign procedure.