Indium(III)-Catalyzed Benzannulation of 3-Formyl-4-pyrones with Terminal Aryl Alkynes: Regioselective Synthesis of Functionalized Salicylaldehydes with ortho -Terphenyl Frameworks.

An effective and unprecedented In(III)-catalyzed [4 + 2] benzannulation of 3-formyl-4-pyrones with simple arylacetylenes is reported, which enables their conversion into structurally diverse salicylaldehyde-bearing ortho -terphenyl frameworks. This transformation is achieved via an In(III)-catalyzed C-O bond extrusion and proceeds without the requirement for an external oxidant. This protocol directly provides aromatic compounds from 4-pyrones. Isotopic substitution experiments suggest that the reaction proceeds via a stepwise sequence involving the addition of an alkyne to the pyran moiety followed by cyclization, oxidation, decarboxylation, and aromatization. The synthetic utility of this protocol is demonstrated by the transformation of the obtained product into molecules with bisindole, chromene, and oxime moieties.