Synthesis of ageratochromene dimer

Reflux of a benzene solution of 2,2-dimethyl-6,7-dimethoxy-4-chromanol in the presence of a catalytic amount of p-toluenesulfonic acid leads to ageratochromene dimer (II) rather than to ageratochromene (I). Treatment of I under the same conditions also produces II which is identical with the dimer of ageratochromene isolated by Kasturi and Manithomas from Ageratum conyzoides L.