Stereoselectivity in the Paternò-Büchi Reaction on Chiral Allylic Alcohols,for A Discussion of the Hydroxy Directing Effect

The stereoselectivity of the photochemical reaction between acetophenone and 2,2,5-trimethyl-4-hexen-3-ol was explained on the basis of a DFT study. Conformational analysis of the starting material showed that two conformations were the most stable ones. Assuming that the attack of the excited carbonyl group occurred on the same side of the hydroxy group (due to hydrogen bond or to the formation of a complex), the possible conformations of the biradical intermediate was examined. Only the precursors of the threo stereoisomer was obtained in agreement with experimental results.