Self-Sensitized Photooxygenation of 2H-Pyrans: Characterization of Unexpected Products Assisted by Computed Structural Elucidation and Residual Dipolar Couplings.

The UVA (350 nm) irradiation of an alpha-pyran in the presence of oxygen led to the unexpected formation of a tetraoxygenated compound whose structure could not be unambiguously determined on the basis of conventional H-1-C-13 correlated experiments. 1,1-ADEQUATE (adequate double quantum transfer experiment) and 1,n-ADEQUATE combined with computer-assisted structure elucidation software led to two structural possibilities involving the formation of either an epoxide or an oxetane. Residual dipolar couplings allowed not only the identification of the compound as a spiroepoxide but also the determination of its relative configuration. To account for its formation, we propose a bisepoxide intermediate that, as opposed to most alpha,beta-epoxyketones under irradiation, undergoes O-C-beta cleavage probably due to the presence of an extra oxygen substituent in the beta position. 1,2-Acyl migration would then proceed stereoselectively to the final product obtained as a single diastereomer.