The rearrangement of a homoallyl cation to a cyclopropylcarbinyl cation is thought to play a role in the biogenesis of a variety of cyclopropane-containing natural products,¹ a hypothesis which has previously led to the design of successful biomimetic syntheses of several natural products.² The strategy underlying this approach to cyclopropane synthesis³ can be applied more broadly and would be particularly valuable if it could be extended to a set of contiguous cyclopropanes. This concept has led us to examine the rearrangement of certain homoallylic systems bearing a leaving group (triflate) at one terminus and a cation-stabilizing metal (tin) at the other. The effects of protecting groups of varying steric demand and of olefin geometry on the stereochemical outcome of the cyclization were examined. "Zipper" cyclization of (8R,5E,2Z)-1-tri-n-butylstannyl-9-trityloxy-nona-2,5-dien-8-ol (117) led to the stereoselective formation of three distinct bicyclopropane stereoisomers (110,111,112). The major diastereomer was isolated through derivatization and the absolute stereochemistry was verified by X-ray crystallography. The trans,syn,trans-bicyclopropane 118 was carried forward to complete a formal synthesis of the antifungal agent FR-900848 (49). The synthesis of a key precursor to halicholactone (188), neohalicholactone (189), and the solandelactones A-H (190-197) constructed around a transcyclopropane core is also described. The key steps in this synthesis are the stereoselective synthesis of trans-vinylcyclopropane 79, followed by a highly diastereoselective acetate aldol reaction leading to compound 269. / Graduation date: 2005