Brønsted acid-mediated selective α-alkenylation of 3,4-dihydro-2H-pyrans†

A protocol for the synthesis of α-alkenylated tetrahydropyrans (THPs) with 3,4-dihydro-2H-pyran derivatives and potassium alkenyltrifluoroborates was achieved in the presence of a Brønsted acid and a Lewis acid under mild reaction conditions. The oxonium ion intermediates, produced from various readily available 3,4-dihydro-2H-pyran derivatives, were attacked by the nucleophile to form the desired α-alkenylated products. This strategy could be an alternative approach to modify a series of modules that are of broad utility in natural products and fine chemicals.