ABSTRACT
Since the catalytic asymmetric Diels-Alder reactions are key processes for the synthesis of stereogenic centers in the following molecules, our investigation begin with a kinetic study ofthe stereoselectivity of the cycloaddition (reaction: 1 --. 3) catalyzed by Lewis acids. Among the Lewis acid catalysts, titanium chloride, zinc chloride and alkyl aluminum complex (2}, the diethyl aluminum chloride was chosen as a catalyst for its mild catalytic activity which yielded good kinetic behavior for the cycloaddition reaction. Table 1 shows that reactivity of the dienophile 1 in the uncatalyzed reaction is much slower than that in the catalyzed reaction. Therefore, it is reasonable to assume that the contribution of the parallel thermal reaction to the total rate of catalytic reaction is negligtble. In the kinetic study, the molar equivalent of the catalyst is kept less than that of the dienophile because excess diethyl aluminum chloride will catalyze undesirable side reactions. Moreover, the method of initial rate is employed in determining rate constants and activation parameters, minimizing the effect of these parameters due to the formation of by-products.
