Studies On The Nucleophilic Substitution To Multi-Substituted Six-Membered Ring Oxocarbenium Ions

This thesis consists of three parts. In the first part, the recent progress made in the synthetic studies on tetrahydrofuran (THF) and tetrahydropyran (THP) rings involving the intramolecular C-O coupling is briefly reviewed. The second part describes our study on the reaction of six-membered ring oxocarbenium ions with nucleophilies:two tetrahydropyran acetates,(4-((tert-butyldimethylsilyl)oxy)-6-ethyl-5-methyltetrahydro-2H-pyran-2-yl acetate and4-((tert-butyldimethylsilyl)oxy)-6-ethyltetrahydro-2H-pyran-2-yl acetate were synthetized from furfural though a newly designed route, and their reactions with various nucleophiles under Lewis acid catalysis were investigated concerning the stereochemistry. We found that the reaction of two tetrahydropyran acetates with allyltrimethylsilane in the presence of BF3-OEt2resulted in the formation of2,6-trans17a and19a respectively with high selectivity. However, when the nucleophilies became tert-butyldimethylsilyl ketene acetals, the selectivities of2,6-trans to2,6-cis were much lower. The third part deals with our study on the cobalt-catalyzed aerobic oxidative cyclization of (3,y-unsaturated oximes. We found that Co(nmp)2could effectively catalyze the5-exo intramolecular addition of β,γ-unsaturated oximes under aerobic conditions, generating the corresponding isoxazolines in good yields.