The Diels-Alder reaction of lambertianic acid with maleic anhydride occurred in a stereoselective fashion and yielded diastereoisomeric (1R,2S,6R,7R)- and (1S,2R,6S,7S)-exo-adducts. The latter reacted with L-valinol to give the corresponding diterpenoid imides, 4-aza-9-oxabicyclo[2.2.1]dec-8-enes. Reactions of lambertianic acid with N-substituted maleimides in the presence of Lewis acids afforded diastereoisomeric adducts having both exo and endo configuration. Some transformations of the adducts were examined with a view to obtain cantharidin and dihydroisoindole analogs.