Pure (E)-alkenoyl dihydropyrans are stereoselectively assembled from 2-acetyl-3,4-dihydropyrans (products of one-pot self-organization of acetylene gas and ketones) and aromatic aldehydes (52-86% yields). Furthermore, (E)-alkenoyl dihydropyrans undergo ring opening (acid hydrolysis) to afford 5-hydroxy-1,6-diketones (44-67% yields) and acetyl-2-(E)-styryl-2,3-dihydroxycyclopentanes (19-32% yields), both as single diastereomers.