Acid‐Promoted Cascade Cyclization to Produce 2‐(4′‐Alkoxyaryl)‐3,4‐Fused Tricyclic Dihydrobenzopyrans via a Vinylidene para‐Quinone Methide Intermediate

We developed a novel method for synthesizing 2-(4-hydroxyaryl)-3,4-fused tricyclic dihydrobenzopyrans with 2,3-syn and 3,4-syn motif based on the acid-promoted cascade cyclization via vinylidene para-quinone methide intermediates. Using easily prepared linear substrates, TFA-promoted cascade cyclization proceeded in the presence of triethylsilane, affording a series of five to seven-membered ring-fused dihydrobenzopyran derivatives in moderate to excellent yield in a highly diastereoselective manner. The developed method provided new access to potent and selective ER agonists.