Three-step synthesis of flavanostilbenes with a 2-cyclohepten-1-one core by Cu-mediated [5 + 2] cycloaddition/decarboxylation cascade

The first synthesis of flavanostilbenes with a 2-cyclohepten-1-one core was carried out by applying an effective strategy in three steps from abundant polymerized flavanol resources. A key regio- and stereoselective Cu-mediated [5 + 2] cycloaddition/decarboxylation cascade was explored and applied without the use of protecting groups, and water as an environmentally friendly solvent contributed to the cascade. The intramolecular [5 + 2] cycloaddition mechanism, involving oxidation and dearomatization of the flavanol unit as a diene, was proposed and supported by the synthesis of the intermediate. The regioselectivity of the cyclization was found to be dependent on the substitution effects of the stilbene units by the exploration of substrate scope.