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Oxonium ylides are highly reactive intermediates that can be used for the synthesis of heterocyclic and carbocyclic frameworks. These putative intermediates are almost exclusively formed from diazocarbonyl derived metallocarbenes, by attack of a pendent oxygen atom. Oxonium ylides have been shown to furnish products derived from the [1,2]-shift (Stevens rearrangement) or the [2,3]-shift in an efficient manner. Chapter 1 is a review of oxonium ylides in synthesis focusing on factors that influence the reactivity of these intermediates. Previous work has shown that the sulfur-directed Stevens rearrangement is an efficient reaction for the generation of seven and eight-membered oxa-bridged carbocycles fused to five-membered rings. Chapter 2 describes an extension of this chemistry where six-seven and six-eight membered oxa-bridged carbocyclic products were furnished in good yields. The chemo- and diastereoselectivity of these rearrangements was dependent on the configuration of the acetal stereocentre present in the diazoketone precursor. A novel approach towards the synthesis of the tigliane and daphnane diterpenes is described in Chapter 3. The tricyclic framework was generated from a highly stereoselective oxygen-directed Stevens rearrangement of an oxonium ylide. This methodology provides a straightforward route to the construction of the five-seven-six ring system found in these classes of natural products. Finally, the sulfur-directed Stevens [1,2]-shift was employed in an attempted total synthesis of the natural product (+)-phorbol in Chapter 4. Several variations of the original synthetic scheme were examined, in an effort to construct the core 6-7 ring structure. The Stevens rearrangement was found to provide the 6-7 ring system of phorbol in excellent yield. The chemo- and diastereoselectivity of this rearrangement was dependent on the configuration of the acetal centre as well as the conformation of the six-membered ring in the starting material. In the future, this advanced intermediate could be used to achieve the total synthesis of (+)-phorbol.

A rhodium-catalyzed process has been developed in which mixtures of internal and terminal olefins are isomerized and hydroborated in one step yielding the corresponding terminal pinacolboronates. Homologation and subsequent oxidation regiospecifically affords the terminal aldehyde in what amounts to a one-pot CO free hydroformylation. Good overall yields are obtained in all substrates examined. In a related study, mechanistic aspects of the rhodium catalyzed hydroboration of vinyl arenes have been probed. A combination of substituent effects (Hammett study), deuterium labeling studies and heavy atom isotope effects has demonstrated mechanistic differences in the hydroboration of electron rich and electron poor substrates. The results of the study further demonstrate the differences in reaction mechansim for hydroborations mediated with catecholborane versus pinacolborane. The Corey-Chaykovsky reaction, in which an aldehyde and a sulfur ylide are coupled to yield an epoxide has proven to be a versatile and valuable method for the production of epoxides. The reaction between benzaldehyde and benzyldimethylsulfonium tetrafluoroborate has been subjected to a kinetic analysis. Activation parameters were determined for the reaction and a large negative ΔS‡ of -35 cal/mol/K was calculated for the epoxidation of benzaldehyde. A large carbon kinetic isotope effect of 1.026 and an inverse deuterium isotope effect of 0.93 were determined for the reaction. A large positive Hammett ρ of +2.50 was found for the epoxidation of various substituted benzaldehydes by competition experiments. These results aided in the identification of the rate limiting step as addition of the ylide species to benzaldehyde. In a separate, although related study, the mechanism of the collapse of hydroxysulfonium salts has been examined with regard for implications in the epoxidation of aldehydes. The anti-diastereomer reacted with complete retention of stereochemistry and no crossover, while the syn-diastereomer gave crossover products along with cis and trans epoxides. Deprotonation and re-protonation on the carbon of the alpha-hydroxy sulfonium ylide was responsible for production of the trans epoxide as demonstrated by deuterium labeling. / Thesis (Ph.D, Chemistry) -- Queen's University, 2007-08-29 17:56:36.642

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