Total syntheses of the highly cytotoxic and pentacyclic diterpenes, quasimarian and simalikalactone D are proposed. Starting from a simple sugar, we intend to prepare a monocyclic Alpha-methylene-3-oxycyclopentanone which contains a total of three chiral centers together with a remote diene function. The preparation of this monocyclic species will utilize modern aldol-reaction chemistry some of which has been developed recently in our laboratories. Intramoleuclar [4 + 2] cycloaddition of this monocyclic substance is expected to occur exclusively in the exo-mode-- closely following the stereochemical outcome of our own work on the sesquiterpene, quadrone, as well as work of others. The cycloaddition reaction yields a total of five chiral centers contained in a tricyclic framework which represents rings B, C, and E of the target natural products. Of particular importance is the securement of all three sites of oxygenation in ring C in their correct stereochemical arrangement. Further, the tricyclic substance contains both functionality and molecular geometry suitable and conducive to the annualitive elaboration of the rings A and D present in these pentacyclic diterpenes. A new reaction sequence resulting in the annulation of ring A will be utilized.