Fused Multifunctionalized Chromenes from Tetraynes and α,β-Unsaturated Aldehydes

The synthesis of fused, highly substituted chromenes in a one-pot-three-step cascade hexadehydro-Diels-Alder reaction of tetraynes followed by an intermolecular α,β-unsaturated aldehyde rearrangement is reported. The target compounds were prepared by the condensation of malonates with 3-bromo-1-propyne, and the resulting 2,2-di(1-propyn-3-yl)malonates underwent cross-coupling with phenylethynyl bromides to afford 2,2-di(5-phenyl-2,4-pentadiynyl)malonates, which underwent intramolecular cyclization to produce tricyclic chromenes. The overall transformation involves the formation of four new C-C bonds and one new C-aryl-O-C-aryl bond by both intramolecular cyclization and intermolecular rearrangement reactions. These constitute a key strategy for the construction of a highly substituted natural chromene core and a robust method for producing other chromenes.