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 degrees C, but undergoes conrotatory cyclization at temperatures above -30 degrees 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.