The influence of a protected alcohol group adjacent to the ene or enophile component on diastereoselectivity in both thermal and Lewis acid-catalyzed 5-(3,4) ene reactions of a series of 1,6-dienes 1−7 has been studied. The results indicate that its effect can be considerable, and in one example, with a gem-dimethyl group on the connecting chain and a large silyl protecting group on the hydroxyl, the diastereocontrol was almost perfect, with three stereogenic centers and one double bond geometry set up in one step, e.g., 4 → 10. This new finding was exploited in a synthesis of epijasmonoid natural products, (±)-methyl cucurbate (19) and (±)-methyl epijasmonate (18) starting from aldehyde 24, where the key step was a highly diastereocontrolled 5-(3,4) ene cyclization 23 → 22.