Studies On The Nucleophilic Addition To Five/Six-Membered Ring Oxocarbenium Ions

This thesis consists of two parts. In the first part, the stereoselective addition of carbon nucleophiles to five/six-membered-ring oxocarbenium ions is briefly reviewed. In the second part, our own efforts toward the synthesis of THF and THP via oxocarbenium ions are described. The results obtained are outlined as follows:(1) a new protocol of one-pot synthesis of tri-substituted THF and THP rings was achieved by employing the gold-catalyzed cyclization of the corresponding 1,4 and 1,5-alkynols, and the nucleophilic addition to oxocarbenium ions. (2) The stereo selectivity for the reactions of C-5 susbstituted six-membered-ring oxocarbenium ions with various nucleophiles was studied. The reduction ofδ-lactone to cyclic acetals followed by the BF3·Et2O-catalyzed nucleophilic addition of allyltrimethylsilane and silylketene acetals gave rise to 2,6-trans-products in high selectivity. On the other hand, whenδ-lactone firstly reacted with carbonion nucleophiles and then was reduced by Et3SiH under the catalysis of BF3·Et2O, 2,6-cis-products were generated as the only products. As an extension of this work, the total synthesis of seven-membered nature product (+)-isolaurepan was achieved.