Effect of Buttressing

It would be expected that substituents on the bridge that tend to hold the diene and dienophile closer to each other would enhance the rate of the intramolecular Diels-Alder reaction. This effect has been demonstrated quantitatively by Boeckman [18], who found that a gem-dimethyl substitution 

The other pertinent information one might seek would be the effort on the rate of cyclization of including a heteroatom in the place of a methylene in the bridge. It is apparent that ethers are about the same as their all-carbon counterparts [25], while amides, with and without the carbonyl group in the bridge, are substantially slower [26]. Both this effect and the ring size effects noted above will probably vary with the position of the heteroatom in the bridge. 

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In order to see the effect of buttressing and the effect of size of the peri substituent on the barrier to rotation, 1-chloro- (112) and 1,3-di-tm-butyl-9-isopropyltriptycene (113) were prepared (161). The barrier for 112 was 22.9 kcal/mol. Comparing this to the tetrachloro compound 111 (all substituents Cl), AG = 25.5 kcal/mol, a normal buttressing effect seems to operate, in contrast to what is observed for the 9-rm-butyltriptycenes as discussed in the previous section. The barrier to rotation in 113 was 20.8 kcal/mol, which is very low... 

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