Development of a new method for consecutive activation of conjugated alkynes based on intramolecular silyl migration

A new method for consecutive α- and β-activation of propiolates toward electrophiles has been developed, which is mediated by suitable tertiary amines (e.g., DABCO) involving intramolecular silyl migration as a key step. Methyl 3-trimethylsilylpropiolate was reacted with aromatic aldehyde in the presence of DABCO in refluxing benzene to give a highly functionalized olefin product, in which new carbon-carbon bonds were formed at both α- and β-positions of the starting propiolate. On the other hand, when using aliphatic aldehyde the reaction course was dramatically changed to afford a propargyl TMS ether as a sole product. However, we suppose that these reactions have a common reaction pathway partly, including ammonium ylide-alkylidene carbene equilibrium, and that the former products arise from the ylide form and the latter from the carbene form. These domino reactions were successfully applied for an intramolecular version by use of substrates having both formyl group and TMS-propiolate structure derived from salicylaldehyde, leading to a new formylcoumarin-forming reaction.