Synthesis of bridged bicyclic ethers and fused oxetanes from pyran-4-ones via tandem solvent trapping and Norrish type II cyclization

Polyalkyl pyran-4-ones 1a-c were irradiated in methanol or ethanol. Althoug h the expected solvent trapping products 3 could be observed, extended irra diation times allowed exclusive formation of secondary photoproducts 4 and 5 in combined yields of 37-64%. These bicyclic compounds are believed to ar ise from gamma-hydrogen abstraction by the excited enone chromophore of 3, followed by closure of the resulting biradical through one of two possible pathways. Moderate stereoselectivity was observed in the radical coupling t o produce 4, whereas the analogous closure to 5 was completely diastereosel ective. Tautomerization of the enol precursors to 5 also occurred with comp lete selectivity for protonation from the exo face. Overall, this process c onverts simple, planar heterocycles and alkanols into complex products in a single transformation.