Activation hardness and cycloadditions of even linear polyenes

Modes of thermal and photochemical cycloaddition of even linear polyenes are examined using the simple concept of activation hardness. The results are in agreement with the well-known Woodward–Hoffman rules. [R.B. Woodward and R. Hoffmann, The Conservation of Orbital Symmetry (Academic Press, New York, 1989)]. © 1994 John Wiley & Sons, Inc.