Regio- and stereoselectivity in the Paternò–Büchi reaction on furan derivatives

The regiochemistry of the attack of a carbonyl group on furan, 2,3-dihydrofuran, 2-methylfuran, and 2-furylmethanol was explained assuming a frontier orbitals control of the reaction. The prefential formation of the endo adduct in the reaction between 2,3-dihydrofuran and benzaldehyde and the preferential formation of the exo isomer in the reaction between furan and benzaldehyde were explained considering the relative stability of the adducts. In the case of 2-methylfuran, the reaction with benzaldehyde gave the product deriving from the attack on the less hindered side of the molecule. The regiochemistry was explained considering the relative stability of the possible biradical intermediates. The exo stereoselectivity was explained on the basis of the stability of the adducts. 2-Furylmethanol gave a mixture of regioisomeric products where the most abundant one was that deriving from the attack on the most hindered side of the molecule. The regioselectivity was explained on the basis of the relative stability of the biradical intermediates.