An efficient Brφnsted acid-catalyzed cascade reaction of non-activated dienes leading to the dimerization of 1-aryl-2-alkylbutadiene derivatives was developed.The 5-membered ring products could be obtained in an almost complete diastereoselctive fashion(Scheme 1).Usually the expected products from the dimerization of isoprene and derivatives are linear dimers,or contain four-,six-,or eight-membered rings,which are formed via [2+2],[2+4] or [4+4] cycloaddition reactions respectively [1-5].This makes the observed reactivity even more interesting as well as synthetically useful.In order for the 5-membered ring dimer to be formed,an alkyl group in position C2 and an aromatic ring in position C1 of the diene moiety are required.In their absence,linear dimers,six-membered rings and complex mixture of oligomers were obtained.The alkyl group seems to stabilize the carbocationic intermediates that occur during the cyclization to the 5-membered ring.Additionally this methodology was successfully applied to a variety of functionalized 1-aryl-2-alkylbutadiene derivatives.