STEREOCHEMICAL CONTROL IN REACTIONS OF NUCLEOPHILES WITH OXOCARBENIUM IONS FORMED BY INTRAMOLECULAR OPENING OF ACTIVATED EPOXIDES BY NEIGHBORING CARBONYL GROUPS

Tandem cyclization/reduction and cyclization/alkylation processes for the stereoselective synthesis of 2,5-disubstituted tetrahydrofurans, 2,6-disubstituted tetrahydropyrans, and 2,7-disubstituted oxepanes are described. In the presence of Lewis acids or TMSOTf, gamma,delta-, delta,epsilon- and epsilon,zeta-epoxy ketones and esters undergo cyclization to the corresponding oxocarbenium ions, which react in situ with a variety of organosilanes and organoaluminum reagents to give the substituted oxacyclic products. For the synthesis of substituted tetrahydrofurans, the stereochemical control of the addition process was much higher with TMSOTf than BF3.OEt2.