Selectivity in cycloadditions—-XII : The directive effect of enol ethers and thioenol ethers in cycloadditions of nitrile oxides

Cycloadditions of nitrile oxides to 2,3-dihydrofuran are highly regioselective whereas the regioselectivity of the cycloadditions to 2,3-dihydrothiophen is only moderate. The directing effect of oxygen and sulfur in these cycloadditions could be evaluated at 2.8 and 1.1 Kcal mol^-1 respectively. The related acyclic sulfur dipolarophiles, (E)-propenyl methyl and phenyl sulfides, similarly undergo cycloadditions with moderate regiochemistry.The different regioselectivities and reactivities of the dipolarophiles can be related to differences in energies and shapes of their highest occupied orbitals, which are also responsible for the diverging behaviour observed in the electrophilic reactions and 2 + 2 cycloadditions of enol and thioenol ethers.