An Unusual Conversion of 2-(Alkynonyl)Alkynylbenzenes to Isocoumarins by a Retro-Favorskii-like Degradation

We report the discovery of an anomalous reaction of 2-(alkynonyl)alkynylbenzenes under AgI catalysis for the selective formation of isocoumarins. This reaction is previously undocumented for 2-(alkynonyl)alkynylbenzenes in terms of the reaction mechanism and the product formed. Water (H2O18) labeling studies suggested a possible mechanistic pathway in which the initial formation of a pyrylium ion is followed by hydrative dealkynylation, that is, water incorporation and alkyne expulsion, similar to a retro-Favorskii reaction.