Synergistic effects between Lewis and Brønsted acids: application to the Prins cyclization

Brønsted and Lewis acids can catalyze the Prins cyclization, an efficient method for the synthesis of tetrahydropyrans from homoallylic alcohols and carbonyl compounds. Synergistic effects between weak Brønsted and Lewis acids in these reactions have been analyzed by density functional theory [M06-L/def2-QZVP/IEFPCM(CH2Cl2)//M06-L/6-311+G(2df,2p)]. In order to characterize the reactivities of the employed Lewis acids, methyl anion and hydroxide affinities were determined. On the basis of our calculations, we found that the coordination of Lewis acids to carboxylic and sulfonic acids results in a significant increase in the Brønsted acidities of the latter.