Treatment of the acetylenic epoxides I having electron-donating groups at the acetylenic terminus with a catalytic amount of BF3.OEt(2) afforded 6-endo products with inversion of stereochemistry at the propynyl position in a highly stereoselective manner, whereas the acetylenic epoxides I possessing electron-withdrawing substitutents at the acetylenic terminus provided under similar acidic condition the corresponding tetrahydrofuran derivatives in a highly selective way.