Abstract
Singlet-oxygenation of 3-hydroxyphenyl-substituted dihydrofurans fused with a cyclohexane 1a−c exclusively gave the corresponding syn/anti-stereoisomeric mixtures of dioxetanes 2a−c. The syn/anti-π-facial selectivity in the 1,2-cycloaddition of singlet oxygen (1O2) was found to be remarkably sensitive to the solvent as well as the reaction temperature. In fact, the solvent effect was so conspicuous that inversion of the syn/anti-π-facial selectivity was observed in different solvents, such as chloroform and toluene. An LSER (linear solvation energy relationships) analysis suggested that the Lewis-acidity/basicity and HBD (hydrogen-bond donor)/HBA (hydrogen-bond acceptor) ability as well as dipolarity/polarizability of the solvent played an important role in this change in syn/anti-π-facial selectivity. An investigation of the temperature-dependency of the singlet-oxygenation suggested that the syn/anti-π-facial-selective 1,2-cycloaddition of 1O2 to 1 was a conformationally-(entropy-) controlled process.
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