Arylideneisoxazole-5(4<i>H</i>)-One Synthesis by Organocatalytic Three-Component Hetero-Cyclization

An expedient, green, practical, and vastly efficient three-component cyclo-condensation is catalyzed by triphenylphosphine. The 4-arylideneisoxazol-5(4H)-one heterocycles are generated in good to excellent isolated reaction yields. Experiments show that the best results were obtained using 15 mol% of the catalyst under aqueous conditions at room temperature. The sonochemical synthesis of 4-arylideneisoxazol-5(4H)-one derivatives were also performed by using 10 mol% of triphenylphosphine in water as an eco-friendly reaction medium. In the comparative method, ultrasonication showed shorter reaction times and also requires smaller amounts of catalyst. While aryl/heteroaryl aldehydes were used, the variation of β-keto-ester was also explored. The β-keto-esters (ethyl acetoacetate/ethyl 4-chloro-3-oxobutanoate/ethyl 3-oxohexanoate) under optimized reaction conditions reacted well and afforded the corresponding 4-arylideneisoxazol-5(4H)-ones in shorter reaction times. The approach is simple, approximately compatible with the pot-, atom-, and step-economy.