Formal homologous aldol reactions: interrupting the Nazarov cyclization via carboalkoxylation of alkynes.

Reactions between 1,4-pentadien-3-ones and aryl acetylenes in the presence of BF3·OEt2 furnish α-phenacyl cyclopentanones via a domino electrocyclization/carboalkoxylation reaction sequence. The overall process underscores a new mode of interrupted Nazarov trapping, where two new carbon-carbon bonds are installed with concomitant formation of carbonyl functionality.