Woodward–Hoffmann rules

…of statements now called the Woodward-Hoffmann rules, accounts for the failure of certain cyclic compounds to form from apparently appropriate starting materials, though others are readily produced; it also clarifies the geometric arrangement of the atoms in the products formed when the rings in cyclic compounds are broken.

These Woodward-Hoffman rules were probably the most important theoretical advance of the 1960s in organic chemistry. At the time of his death, Woodward was working on the synthesis of erythromycin.