The importance of ground state conformation in the photoisomerizations of protonated enones

The conformations of several protonated enones have been determined by low temperature 13 C nuclear magnetic resonance spectroscopy. Information on the ground state conformation has been used to understand the differences in the photochemistry of these systems. Photoisomerizations of 4 and 5 have been examined. In the case of 4, irradiation leads only to the formation of the Z isomer 16; protonated dihydrofuran 13 was not observed. In the case of 5, both the Z isomer 17 and protonated dihydrofuran 14 were observed. The protonated dihydrofuran 14 was produced from the start of the reaction. It is concluded from these and previous results that for the photocyclization to occur the starting cation must exist in an s-cis conformation in the ground state.