Synthesis of Bridged Bicyclic Ethers and Fused Oxetanes from Pyran-4-ones via Tandem Solvent Trapping and Norrish Type II Cyclization¹

Abstract

Polyalkyl pyran-4-ones 1a-c were irradiated in methanol or ethanol. Although the expected solvent trapping products 3 could be observed, extended irradiation times allowed exclusive formation of secondary photoproducts 4 and 5 in combined yields of 37-64%. These bicyclic compounds are believed to arise 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 to produce 4, whereas the analogous closure to 5 was completely diastereoselective. Tautomerization of the enol precursors to 5 also occurred with complete selectivity for protonation from the exo face. Overall, this process converts simple, planar heterocycles and alkanols into complex products in a single transformation.