[2+2] Photocycloaddition of Tetronates

Previously unknown in photochemistry was the [2+2] photocycloaddition of tetronates. The facial and simple diastereoselectivity of the reaction is excellent, as exemplified by the intramolecular photocycloaddition of the tetronate 1 to give the tetracycle 2. If the alkoxy group of the tetronic acid is not incorporated in the cyclic skeleton, the cyclobutane in the cycloaddition product may be induced to undergo ring opening in a retroaldol reaction.