Photoinduced Cycloaddition and Ene Reactions of 2,7-Cyclooctadienone: Experimental and Computational Studies of a Cyclopentyl Oxyallyl Intermediate

The photochemical reactions of cyclooctadienone 6 have been studied as a mode of entry to a cyclopentyl−oxyallyl system. Irradiation of cis, cis -2,7-cyclooctadienone 6 results in isomerization to cis, trans -2,7-cyclooctadienone 24 . The strained dienone 24 is stable at −70 °C, but undergoes conrotatory cyclization at temperatures above −30 °C to the cis -bicyclo[3.3.0]oxyallyl system 25 . The density functional theory (DFT) structural optimizations for 6, 24, and 25 at the 6-31G* B3LYP level yielded energies, selected geometric parameters, and charge/spin population data. The oxyallyl intermediate reacts with vinyl ethers in a stepwise fashion to give bridged and fused tricyclic products with defined stereochemistry at the ring junctures. Simple alkenes react with 25 in a concerted ene reaction to give bicyclic products. An explanation is offered to harmonize the traditional zwitterionic view of oxyallyl systems with the current computational assessment of oxyallyls as diradical species.