Thesis advisor: Marc L. Snapper / Intramolecular cycloadditions of cyclobutadiene provide rapid access to rigid polycyclic systems with high strain energies and unique molecular geometries. Further functionalization of these systems followed by strain-release fragmentation provides great opportunities to construct fused-medium-ring architecture, which are very common in natural products but challenging to achieve efficiently. An intermolecular cyclopropanation/acid-mediated rearrangement strategy has been previously developed to access the 5-7 bicyclic ring systems in a highly stereospecific manner. The application of this strategy is being studied for the synthesis of a biologically interesting guaiane natural product: torilin. In a complementary fashion, an intramolecular cyclopropanation/thermal rearrangement sequence is developed to access two different molecular frameworks of 5-7-5 tricyclic ring systems. A library of functionalized 5-7-5 tricyclic ring systems can be systematically built up from the same starting material for potential future use in high-throughput screening. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.