| Oxonium ylides have found limited exposure in synthesis, mainly due to difficulties associated with their high reactivity. These putative intermediates are formed by capture of a diazoketone-derived metallocarbene by a pendant ethereal oxygen, and they have been shown to undergo synthetically useful rearrangements when appropriately substituted. Typically observed are the Stevens [1,2]-shift or the [2,3]-sigmatropic shift, which have been shown to give a wide variety of carbo- and heterocyclic structures in good yield, with high levels of chemoselectivity. Chapter 1 is a review of oxonium ylides in synthesis, emphasizing the factors that influence the subsequent reactivity of these intermediates.; The heteroatom-directed Stevens [1,2]-shift rearrangement protocol has been expanded to allow the synthesis of oxygen-bridged, fused 5-7 bicyclic ring systems in Chapter 2, wherein the products were recovered in good yield, with occasionally interesting chemoselectivity. Products resulting from rearrangements of oxonium ylides, and in some cases sulfonium ylides, were isolated. The occurrence of products from unanticipated rearrangements was dependant on the configuration of the starting diazoketone.; A novel method for the synthesis of oxonium ylides is described in Chapter 3, employing iodonium ylides as diazoketone surrogates for generating metallocarbene intermediates. This protocol was found to be catalyst-dependant, and to afford the products in equal, and in some cases greater yield, than the corresponding diazoketone processes in the aliphatic series investigated. The diazoketone protocol was found to be more effective than the iodonium ylide-mediated method for the aromatic substrate tested.; Finally, the application of the sulfur-directed Stevens [1,2]-shift was employed in the attempted total synthesis of the natural product (+)-dactylol in Chapter 4. Several variations on the original synthetic scheme were also examined, due to the occurrence of undesired transannular processes late in the synthetic sequence. Initially, we rearranged the order of the overall sequence to decrease the likelihood of such transannular reactions from occurring, but this was found to be unworkable. Lastly, an entirely new method for preparing the diazoketone precursor was developed. This precursor is designed to potentially enable the synthesis of (+)-dactylol by either the original sulfur-directed Stevens [1,2]-shift, or by a [2,3]-sigmatropic rearrangement. |
