Preparative Visible-Laser Photochemistry: Qinghaosu-Type 1,2,4-Trioxanes by Molecular Oxygen Trapping of Paterno-Bouchi Triplet 1,4-Diradicals

Article abstract of DOI:10.1021/jo00256a015

The visible-laser photochemistry of 3,4-dihydro-4,4-dimethyl-2H-pyran-2-one (1) with benzoquinone (BQ) and with phenanthraquinone (PQ) in an argon atmosphere afforded the expected regioisomeric oxetane products 2 and 3.Under an oxygen atmosphere, trapping of the Paterno-Bouchi preoxetane intermediates by molecular oxygen was observed for BQ, leading to the regioisomeric and stereoisomeric 1,2,4-trioxanes 4-BQ and 5-BQ.Regioselectivity, favoring the acetal-type structures 2 and 4, appears to follow diradical stability.Anomeric effects are invoked to rationalize the stereochemical course of the trioxane trapping products.The lactone-acetal structure 4 represents the first qinghaosu-type (artemisin) 1,2,4-trioxane.