Thanks are due to University of Aveiro and FCT/Ministério da Educação e Ciência (MEC) for the financial support of the QOPNA research unit (FCT UID/QUI/00062/2013) and CIQ-UP (Pest-C/QUI/UI0081/2013) through national funds and, where applicable, cofinanced by the FEDER, within the PT2020 Partnership
Agreement, and to the Portuguese NMR Network, as well as to the Instituto Politécnico de Bragança. H. M. T. A. and C. F. R. A. C. L. are grateful to FCT for their PhD (SFRH/BD/86277/2012) and Postdoctoral (SFRH/BPD/77972/2011) grants, respectively.
The synthesis and reactivity of 2-[(1E,3E)-4-arylbuta-1,3-dien-1-yl]-4H-chromen-4-ones as dienes in Diels–Alder (DA) reactions with several electron-poor and electron-rich dienophiles under microwave irradiation was studied. The optimized reaction conditions were achieved with N-methylmaleimide as the dienophile and Sc(OTf )3 (OTf = triflate) as a Lewis acid under microwave-assisted and solvent-free conditions. The Lewis
acid improved the reaction yields as it prevented the adducts obtained from undergoing a second DA reaction; thus, the formation of a bisadduct was avoided. The α, :γ,δ-diene of the starting chromones was the most reactive, and the computational results confirmed the experimental findings. Theoretical calculations also provided a rationale for the unexpected lack of reactivity shown by some dienophiles. The adducts prepared were dehydrogenated with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ); however, the aza adducts were sensitive to the highly energetic reaction conditions necessary for the aromatization.
