The products of the one-pot assembly of acetylene and ketones in the KOH/DMSO system, 7-methylene-6,8-dioxabicyclo[3.2.1]octanes, undergo an acid-catalyzed (CF3COOH, room temperature) rearrangement to rarely substituted cyclopentadienes in good-to-excellent yields. The mechanism of the rearrangement has been supported by the isolation and corresponding transformations of two intermediates.