Interaction Between Oxygen Lone-Pair Orbitals of Ether and α,β-Unsaturated Ketone Functions in 3,5,6-Trimethylidene-7-oxabicyclo(2.2.1)heptan-2-one and 3,6-Dimethylidene-7-oxabicyclo(2.2.1)heptane-2,5-dione: A PE-Spectroscopic Investi