Fast and Efficient Synthesis of 4-Arylidene-3-phenylisoxazol-5-ones

A convenient and easy synthesis of 4-arylidene-3-phenylisoxazol-5-ones by the three-component reaction of hydroxylamine, ethyl benzoylacetate and aromatic aldehydes in the presence of DABCO in refluxing ethanol is reported.


Introduction
Isoxazoles and its derivatives represent useful synthetic building blocks 1 .They are an important class of heterocycles compounds, which are employed in the area of pharmaceuticals and therapeutic such as antimicrobial, antifungal 2 , anticancer 3 , ulcerogenic 4 and anti-inflammatory 5 .Therefore, various synthetic methods have been reported to access the isoxazole derivatives 6 .
Multicomponent coupling reactions (MCRs) involving domino processes, in which several reactions are combined into one synthetic operation have emerged as a powerful valuable synthetic tool in organic synthesis 7 .MCRs have some advantages over classical multistep syntheses, including cost-effective, requiring a minimum of time, simple procedures, high degrees of atom economy, the possibility for combinatorial surveying of structural variations and environmental friendliness 8 .The preparation of heterocyclic systems using MCRs often involves classical carbonyl condensation chemistry.Among various carbonyl structures, 1,3-dicarbonyl compounds constitute important synthetic building blocks and intermediates, incorporating multiple functionalities that can be involved either as nucleophilic or electrophilic species in a variety of synthetic transformations 9 .Thus, the high synthetic potential of these easily accessible reagents has found numerous applications, especially for the synthesis of heterocyclic structures 10 .
Recently one-pot synthesis of these compounds catalyzed by pyridine is reported 13 .However, the use of toxic reagent (pyridine), long reaction time, low yields are among the drawbacks of this reported method.We herein described a facile synthesis of 4-arylidene-3phenylisoxazol-5-one derivatives by three-component condensation of ethyl benzoylacetate, hydroxylamine and aldehyde, in ethanol using DABCO, as weak and safe base, at reflux condition (Scheme 1).

Experimental
A mixture of 2 mmol ethyl benzoylacetate (1), 2 mmol hydroxylamine hydrochloride (2) and 1.1 mmol DABCO in 4 mL ethanol was refluxed for 3 min, after then 2 mmol aromatic aldehyde (3) was added and the mixture was further refluxed for 1-15 min.After completion of the reaction (monitored by TLC), the reaction mixture was cooled to room temperature and the resulting crystal product (4) was collected by filtration and washed with water.The product was found to be pure and no further purification was necessary.

Results and Discussion
In our initial experiments toward the development of this reaction, we have reacted 4-methoxybenzaldehyde (1.0 mmol) with hydroxylamine hydrochloride (1.0 mmol) and ethyl benzoylacetate (1.0 mmol) in ethanol, but no resulting product has been found at room temperature and reflux conditions.It was observed that the same reaction proceeded when we use DABCO in ethanol at room temperature yielding the corresponding 4-arylidene-3phenylisoxazol-5-one in 43% yield after 30 min.When we attempted the same reaction in ethanol at reflux condition, the reaction proceeded to completion within 4 min and yielded the corresponding 4-arylidene-3-phenylisoxazol-5-one in 90% yield.Encouraged by this result we have explored the efficiency of various bases on the reaction.Among the bases tested for this condensation reaction to yield 4-(4-methoxybenzylidene)-3-phenylisoxazol-5one, are triethyl amine (65%), pyridine (77%) 13 , piperidine (75%), DBU (78%) and DABCO (82%).However DABCO gave excellent yields.
In order to determine the most appropriate choice of solvent system for this DABCOpromoted synthesis of 4-arylidene-3-phenylisoxazol-5-ones, we have screened the solvents such as methanol (70%), ethanol (82%) and water (52%).Of the solvents tested in the screening we got the maximum yield of the product in shorter reaction times, when we use ethanol as solvent.
In order to determine the scope of this reaction, we have synthesized differently substituted 4-arylidene-3-phenylisoxazol-5-ones by varying differently substituted aldehydes including both electron-donating and electron-withdrawing groups.It is observed that the reaction gave good yields of products with faster reaction rate when the aldehydes bearing electron-donating group is used compared to the aldehydes with electronwithdrawing groups.The corresponding results are tabulated in Table 1.The plausible mechanism for the synthesis of 4-arylidene-3-phenylisoxazol-5-ones in the presence of DABCO involves the initial reaction of DABCO with hydroxylamine hydrochloride for repudiation of hydroxylamine which attacked to ethyl benzoylacetate to the formation of oxime then ring closing of oxime gives 3-phenylisoxazol-5-one.Reaction of DABCO with aldehyde leading to the formation of intermediate (A), which attacked by the 3-phenylisoxazol-5-one gives another intermediate (B) with subsequent elimination of H 2 O leads to the desired product (Scheme 2).All the unknown products were characterized by 1 H, 13