The intramolecular ionic Diels−Alder reaction of α-acetylenic acetals as a precursor of the propargyl cation has been investigated in the presence of Lewis acids and in protic acids. The reaction of diene-tethered α-acetylenic acetals (1−2) with formic acid yielded the regioselective intramolecular ionic Diels−Alder reaction products, bicyclodienal (9) and bicyclodienone (11) derivatives, in good yields