5-Bromo-2-pyrone: An easily prepared ambiphilic diene and a synthetic equivalent of 2-pyrone in mild, thermal, Diels-Alder cycloadditions

5-Bromo-2-pyrone, a solid prepared easily in one-flask according to a new route (eq. 1), has been found to undergo smooth and highly regiocontrolled 2+4-cycloadditions between 25–100°C with both electron-poor and electron-rich dienophiles; subsequent radical debrominations produced halogen-free bicyclic lactones, including a precursor to a vitamin D steroid, thus showing 5-bromo-2-pyrone to be a practical and effective synthetic equivalent of 2-pyrone in thermal Diels-Alder cycloaddition.