Interaction entre chromophores non‐conjugués: Diméthylidène‐2,3‐époxy‐5,6(exo)‐norbornane et diméthylidène‐2,3‐époxy‐5,6(exo)‐oxa‐7‐norbornane

Preparation of the title compounds 7 and 17 is described. A hypsochromic shift of the V ← N transition of the s‐cis‐butadiene chromophore in the UV. spectra of the epoxydienes 7 and 17 is observed relative to 2,3‐dimethylidene‐norbornanes (20, 29), and to 2,3‐dimethylidene‐7‐oxa‐norbornanes (22, 23). This effect is discussed in terms of geometry changes of the s‐cis‐butadiene and in terms of variation of the hyperconjugation of this chromophore with the σ‐skeleton of the bicyclic system.