Diene Substitution on the Rate of Cyclization

The accelerating effect of electron-donating diene substituents on the rate of the intermolecular Diels-Alder reaction is well known (1). It is apparent than an alkoxy substituent on the diene also accelerates the intramolecular cyclization [8]. As might be expected, this effect is not observed with a less polarized dienophile [9]. 

Steric bulk encumbering the approach of the dienophile to the diene can substantially slow the cychzation [10]. Holding the diene cisoid by inclusion in a ring accelerates the reaction slightly [11]. Finally, there has been conflicting evidence about the reactivity of Z vs. E dienes. While it was initially observed (19) that Z and E dienes react at about the same rate, we (3) and others have fotmd E dienes to be more reactive. Two recent studies [12, 13], taken together, show that, in fact, Z vs. E diene reactivity is a function of the particular geometry involved. 

Influence of the Elements Bridging the Diene and Dienophile on the Rate of CycUzation 

In general, connecting the diene and the dienophile by a flexible bridge should make the reaction between them more facile. There are other factors, however, that may be as important, or more important, than this more favorable entropy ring strain, buttressing, and conformational preferences, especially of hetero atoms. 

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