Intermolecular Diels-Alder reactions Lewis acid catalysis

Lewis-acid catalysis is effective in intermolecular as well as intramolecular /zomo-Diels-Alder reactions. Thus, complex polycyclic compounds 93 have been obtained in good yield by the cycloaddition of norbornadiene-derived dienynes 92 by using cobalt catalyst, whereas no reaction occurred under thermal conditions [91] (Scheme 3.18). 

Lewis acid catalysis usually substantially improves the stereoselectivity of intramolecular Diels-Alder reactions, just as it does in intermolecular cases. For example, the thermal cyclization of 1 at 160°C gives a 50 50 mixture of two stereoisomers, but the use of Et2AlCl as a catalyst permits the reaction to proceed at room temperature, and endo addition is favored by 8 l.69... 

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