The First and Second Cinchona Rearrangement. Two Fundamental Transformations of Alkaloid Chemistry

摘要

Stereochemistry, products, and driving forces of the "first and second Cinchona rearrangement" have been investigated and a unified theory is presented. The first cage expansion affords [3.2.2]-azabicyclic α-amino ether and is formulated via a configurationally stable bridgehead iminium ion and quasiequatorial nucleophilic attack. The second cage expansion affords β-functionalized [3.2.2]-azabicycles. In this case a nonclassical nitrogen-bridged cation is postulated to account for retention of configuration and potential reversibility of the cage expansion. The second rearrangement is favored for the so-called cinch bases (6′-R = H) in trifluoroethanol. Stereoelectronic factors, electron demand at C9, ground state conformation, and solvent type are crucial in all cases. A two-step protocol for preparing 9-epi-configured Cinchona alkaloids from 9-nat precursors is described.