An expedient and green ultrasound-promoted synthesis of fused α-pyrones from Baylis–Hillman acetates using basic ionic liquid [bdmim]OH

Abstract

α-Pyrone derivatives have also been used as synthetic intermediates in organic synthesis [28,29,[31][32][33][34].Thus, several protocols for the synthesis of α-pyrones and related compounds by numerous approaches have been reported. Such as gold (I)-catalyzed reaction of terminal alkynes and propiolic acids [35], palladium catalyzes annulation of alkynes [36,37], K 2 CO 3 -catalyzed Michael addition-lactonization of 1,2-allenic ketones [38], palladium-catalyzed coupling of 3-halo-(2Z)-enoic acids, 3-halo-(2Z)-enoates with allenylstannanes [39,40], from Baylis-Hillman adducts [40][41][42][43] and several others [44][45][46][47][48][49][50][51][52][53][54][55][56][57][58]. Some of these methods, despite their elegance, have drawbacks such as use of toxic catalysts and solvents, low yielding of products, harsh reaction conditions, lack of regioselectivity and suffering from multi-step reaction. As a result, development of a simple and green protocol for the α-pyrones is highly required.On the other hand, one of the most important principles of the green chemistry is elimination of hazardous solvents in chemical synthesis and avoiding use of toxic catalysts. Ionic liquids have been widely used as greener solvents, suitable for a range of organic reactions [59] and providing possibilities such as control of product distribution, enhanced rate, ease of product recovery and immobilization of catalyst [60][61][62][63][64][65][66][67][68][69][70]. Basic ionic liquids have been increasingly used as environmentally friendly solvents and catalysts with high activity and selectivity and easily recovered materials instead of traditional bases such as KOH, NaOH, K 2 CO 3 , NaHCO 3 , NaOAc, triethylamine, or tetrabutylammonium acetate [71][72][73]. In addition, ultrasound irradiation as a green source of energy has also been utilized to accelerate organic reactions, leading to desired products in shorter reaction time, higher yield and milder condition [74][75][76][77][78].Abstract Fused α-pyrones were obtained in a convenient and green protocol from Baylis-Hillman acetates and various cyclic dicarbonyl compounds in the presence of basic ionic liquid 1-butyl-2,3-dimethylimidazolium hydroxide ([bdmim]OH) under ultrasonic irradiation. This method provided the desired α-pyrones in good to high yields (82-98 %) and lower reaction times (5-15 min).