Investigations on transition state geometry of aldol addition reactions

The stereochemical course of the aldol reaction has been studied with rationally designed and synthesized model systems. In aqueous medium, the acid-induced aldol reactions of keto-aldehyde model system afford syn-aldol product via synclinal transition structure containing the hydrogen bond between an enol hydrogen and an aldehyde oxygen. The base-induced aldol cyclizations show high anti-selectivity via antiperiplanar metal free transition structure to minimize a dipole-dipole repulsion between the enolate oxygen and aldehyde oxygens. Depending on the nature of the solvents, the lithium enolate of keto-aldehyde model system undergoes cyclization with different stereoselectivity.The enolsilane-aldehyde model compound has been successfully synthesized to examine the relative orientation of silyl enol ethers and carbonyl moieties in the transition states of the Mukaiyama aldol reaction. This study has revealed a modest preference for the antiperiplanar (open transition state) orientation in Mukaiyama aldol reaction of enolsilane-aldehyde model system with representative Lewis acids and fluoride sources. With Sn(II) Lewis acids, synclinal (closed transition state) orientations which suggest bidentate coordinations are favored to give syn aldol products. The syn selectivity is found to be dependent on the nature of the counter ion.The investigation on the stereochemical course of the Lewis acid-mediated aldol reaction of a silyl end ether and an acetal has been performed. The effects of various Lewis acids and solvents on the stereochemical course of the reaction imply that enolsilane-dimethyl acetal model system probably reacts via S$\sb{\rm N}$2-like mechanism. In the cyclization of enolsilane-diisopropyl acetal, the lower syn selectivity is observed.U of I OnlyETDs are only available to UIUC Users without author permissio