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Development of an oxa-3+3 annulation of vinyliminium salts with resorcinols as a 1,3-diketo equivalent is described. This annulation constitutes a cascade of Knoevenagel condensation–oxa-electrocyclization leading to a direct access to chromenes. A series of attempts was made to demonstrate its synthetic utility in natural product synthesis, culminating in a total synthesis of (±)-rhododaurichromanic acid A that also featured an intramolecular Gassman-type cationic 2+2 cycloaddition.
Tang et al. (Fri,) studied this question.