The bond length of a carbon-carbon sp2-sp2 sigma-bond without the perturbing effects of pi-interactions has been estimated by high level calculations on two prototypical systems: a 90 degrees -twisted form of butadiene and the tub conformer of cyclooctatetraene. The former system yields a value of 1.4818 A, considerably longer than previous estimates. The corresponding bond length in the latter is slightly shorter due to some pi-electron delocalization.

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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.