Studies toward the Total Syntheses of Cucurbitacins B and D

Synthetic efforts toward the convergent construction of the tetracyclic triterpenoids cucurbitacins B and D are described. The results of a Diels−Alder study examining the effects of steric and electronic variations of 2-methyl-2-cyclohexenone on the endo/exo-diastereoselectivity of the reaction are presented. The diastereomer of the core of the cucurbitacins, epimeric at C8, C9, and C10, 51, was synthesized via a highly regio- and stereoselective Diels−Alder reaction of the diene 4 and the novel dienophile 50.