Project Summary Polycyclic terpenoid natural products are endowed with a broad range of medicinally relevant biological activities. Taxol and artemisinin are two premier examples of life-saving terpenoids; the former is used clinically to treat several different cancers, while the latter is a critical antimalarial agent used worldwide. Chemical synthesis approaches to natural products provide opportunities to make compounds that might be scarcely available from nature, to generate analogues that are only available by total synthesis, and to make probe molecules for increased understanding of the underlying biology. A balance of innovative strategy and new chemical methodology promises efficient syntheses that can serve to answer important biological questions that often can only be probed with small molecules. As part of our laboratory’s long-term goal to enhance efficiency in the synthesis of complex natural products to facilitate important studies in biology, the objective of the proposed research program is to develop concise and creative synthesis designs and empowering methodological advances to permit access to many bioactive diterpenoid natural products. The rationale for this work is that synthetic chemistry is critical to the development of natural product “hit molecules” into legitimate preclinical lead compounds by analogue production, by identification of structure-activity relationships, by the synthesis of chemical probe molecules for mechanism of action studies, and more. An efficient total synthesis of targeted natural products provides a platform from which to address each of these key areas of research. The proposed research is significant because chemical synthesis will provide access to a broad range of biologically important secondary metabolites and analogues with which to interrogate key processes; at the same time, the underlying synthesis designs and methodologies will lead to vertical advancement of the field of organic chemistry. These contributions are innovative by virtue of the chemistry-driven, multi-faceted investigations into the biological activity of diterpenoids with immunosuppressive, antiviral, antibiotic, and neurological activity; the development of new stereocontrolled polyene cyclization strategies to access polycyclic diterpenoid natural products; and the elaboration of new radical bicyclization strategies to make complex architectures relevant to bioactive natural products.