Diels-Alder question

[Keaver]

Ok the question goes

 

the diels-alder mechanism is best described as

 

a. proceeding stepwise by cation intermediate

b. proceeding stepwise by radical intermediate

c. proceeding by a syn cycloaddition

d. proceeding by an anionic radical cycloaddition

e. Proceeding by a "no mechanism" electrophilic addition

 

 

I know that a and b are incorrect because if it was stepwise then the stereochemistry would not be retained the reaction wouldn't work out. I'm leanin toward E as the answer but I'm not positive.

Edited December 6, 2012 by Keaver