Studies on Oxidopyrylium [5 + 2] Cycloadditions: Toward a General Synthetic Route to the C12-Hydroxy Daphnetoxins

12-Hydroxydaphnetoxins, members of the structurally fascinating daphnane diterpene family, exhibit a wide range of significant biological activities. A general route to the BC-ring system of 12-hydroxy daphnetoxins is reported based on d-ribose. Depending on the choice of protecting groups and solvent, the oxidopyrylium−alkene [5 + 2] cycloaddition originating from A provides cycloadduct diastereomer B or C with good to excellent selectivity.