Gassman's intramolecular [2+2] cationic cycloaddition.: Formal total syntheses of raikovenal and epi-raikovenal

The first intramolecular version of Gassman's cationic [2 + 2] cycloaddition employing vinyl acetals tethered to an unactivated olefin and its application in the formal syntheses of raikovenal and epi-raikovenal are described.