On the stereoselectivity of the Paternò–Büchi reaction between carbonyl compounds and 2-furylmethanol derivatives. The case of aliphatic aldehydes and ketones

The Paternò–Büchi reaction between 2-furylmethanol derivatives and aliphatic aldehydes and ketones induced by irradiation through Vycor at 8°C shows high regioselectivity but no stereoselectivity. This behaviour can be rationalised by assuming that this type of compound reacts through both singlet and triplet excited states. Ab initio calculations are in agreement with the formation of the 1,4-biradical. The more stable biradical accounts for the observed regioselectivity. The lack of stereoselectivity was discussed on the basis of two hypotheses. The allylic strain proposed by Adam does not account for the observed results. On the contrary, hydrogen bond interaction between (triplet excited) carbonyl oxygen and hydroxy group is able to describe the observed behaviour.