Synthesis of Coumarins via [4+2] Cyclization of Siloxy Alkynes and Salicylaldehydes

A new approach for the synthesis of coumarins from siloxy alkynes and salicylaldehydes is disclosed. Unlike the previous benzannulation reactions of siloxy alkynes that all proceed by electron-inversed Diels-Alder mechanism, this process represents a new [4+2] cyclization. In the presence of the superior HNTf ₂catalyst, a wide range of coumarins were efficiently synthesized. This process was also extended to the synthesis of 2-quinolones. The polarized electron-rich triple bond might react sequentially with the aldehyde and hydroxy group by polarity switching in the stepwise formation of the C-C and C-O bonds. Control experiments provided insights into the mechanistic understanding.