Asymmetric Cascade Reactions Of Sulfur Ylides:Construction Of Cyclopropanes And2,3-Dihydrobenzofurans

The cyclopropane and2,3-dihydrobenzofuran are common structural subunits in many natural isolates and pharmacological agents. Many of them are biologically active molecules and also serve as versatile building blocks in organic synthesis. The synthsis of these compounds, especially chiral ones, are the focus of research interests in the area of organic synthesis. Nowadays, the diversified development of methodology provides new ways and means for the synthesis of these compounds. In this thesis, we present asymmetric cascade reactions of sulfur ylides to construct cyclopropanes and2,3-dihydrobenzofurans. The details are as follows.1. An asymmetric cyclopropanation of stabilized sulfur ylides with β,γ-unsaturated a-ketoesters using the C2-symmetric urea as a catalyst via hydrogen-bonding activation to allow an access to enantioselective1,2,3-trisubstituted cyclopropane derivatives has been described. We investigated the influence of different catalysts, substituent effects, solvents, reaction temperature and the amount of catalysts in the reaction. This reaction gave the corresponding cyclopropane derivatives in up to90:10er in toluene at-40℃. The method could efficiently construct three stereocenters and the products could be transformed into the corresponding amino acids. We proposed a new active mode between the catalyst and sulfur ylides based on the NMR studies and DFT calculation. The products were characterized by NMR and MS, as well as HRMS. The absolute configuration of the product was determined to be (1S,2R,3R) by X-ray analysis.2. An asymmetric cascade reaction of camphor-derived sulfonium salts with aldimines to allow an access to dihydrobenzofuranyl derivatives has been described. The influence of substituent effects on nitrogen, solvents, bases, reaction temperature, concentrations and the amount of bases in the reaction have been investigated. This reaction gave the corresponding2,3-dihydrobenzofurans in up to98%ee and over95:5dr in toluene at0℃. The products could be transformed into the corresponding benzofuran-derived cyclic β-amino acids. The materials and products were characterized by NMR and MS, as well as elemental analysis. Furthermore, the absolute configuration of the product was unambiguously determined to be (R, R) by X-ray analysis.