Woodward and Hoffmann on Secondary Orbital Interactions. How to Make a Fine Two-Course Meal from Leftovers**

In 1965, R. B. Woodward and Roald Hoffmann published five communications in the Journal of the American Chemical Society that formed the basis for what has been known as the Woodward and Hoffmann (W−H) rules. The last two of these communications applied secondary molecular orbital interactions – that is, interactions that involved atomic orbitals removed from the primary reaction centers – to explain the so-called Alder endo-exo rule of maximum overlap of orbitals for regiochemistry in the Diels-Alder reaction and the then recently noted preference of the chair orientation over the boat orientation in the Cope reaction. This publication presents the back stories of these latter two W−H communications, based on a comprehensive examination of Hoffmann's laboratory notebooks, more than 100 hours of interviews with Hoffmann, interviews with other chemists, and documents found in both Hoffmann's and Woodward's archives.