The condensation of carbonyl compounds with electron-rich arenes: Mercury, thallium, gold or a proton?

Gold(III) chloride, mercury(II) perchlorate, thallium(III) perchlorate and p-toluenesulfonic acid were found to efficiently catalyze the condensation of two furans with aldehydes or acetone. The olefinic unit of alpha,beta-unsaturated carbonyl compounds reacts faster than the carbonyl group, other olefins do not react selectively. The first (addition) step is rate-limiting, the second (substitution) step is much faster.