The Diels–Alder reaction between protoanemonin and butadiene has been theoretically studied by means of the semiempirical MNDO and AMI methods. With use of the results obtained, the experimentally observed site selectivity is discussed regarding the mechanism of the process and the nature of the transition states. It is concluded that this reaction probably proceeds via a diradicai intermediate. © 1991, American Chemical Society. All rights reserved.