Synthesis and reactivity of dioxycarbenium ions from epoxy esters and cationic zirconium complexes
摘要:
Rearrangement of epoxy esters in the presence Of Cp2Zr(R)+ or CP2Zr(Cl)+ results in synthetically useful dioxycarbenium intermediates. Acetals, ortho esters, and tetrahydrofurans are prepared by ligand transfer from zirconium, intramolecular trapping, or further rearrangement reactions.
In situ prepared Cp2ZrCl⊕ catalyzes the formation of dioxolenium ions from α,β-unsaturated epoxy esters. As a consequence of this activation process, acrylate, methacrylate and crotonate derivatives undergo a rapid and stereoselective cationic [4+2] cycloaddition with a wide range of dienes. Ring-extended carboxylic acid derivatives are formed in 1–7 h at 0–21 °C and in 50–90% yield after saponification