Base-Mediated Ring-Contraction of Pyran Systems Promoted by Palladium and Phase-Transfer Catalysis

A study of the ring-contraction of a model 3H-naphtho[2,1-b]pyran is described to elucidate and optimize the ring-contraction of naphthopyrans. Two efficient base-mediated protocols to access multiple naphthofurans, naphthodifurans, and a benzo-fused indole in generally good yields are reported. Furthermore, a protocol to selectively prepare (hetero)aryl-substituted naphthofurans via a Suzuki-coupling–ring-contraction process is presented. An additional protocol that allows Suzuki cross-coupling reactions to be performed on bromo-substituted naphthopyrans without the ring-contraction side reaction is reported.