Regiodivergent Synthesis of Bis(4-oxycoumarin)-based Dioxabicycles: Exploration of [4 + 4] (Heterocyclo)reversion/addition and 1,5-Hydrogen Shift Photochromism.

Two isomeric dioxabicyclic molecular skeletons were constructed by employing the concepts of divergent synthesis. A base-mediated and an acid-catalyzed pseudo-three-component reaction of two equivalents of 4-hydroxycoumarin and (Z)-3-chloro-3-phenylacrylaldehyde yielded the corresponding bis(4-oxycoumarin)-based 2,6- and 2,8-dioxabicycles, respectively. The prepared colorless 2,6-dioxabicycles turned red upon UV irradiation and underwent the reverse reaction when exposed to visible light. The photochromism was proposed to proceed via a sequential [4 + 4] (heterocyclo)addition/reversion and 1,5-hydrogen shift on the basis of photogenerated product-trapping experiments.