β‑Aminocarbonates in Regioselective and Ring Expansion Reactions

The
reactivity of β-aminocarbonates as anisotropic electrophiles
has been investigated with several phenols. Products distribution
shows that the regioselectivity of the anchimerically driven alkylation
reaction depends on the nucleophiles. The results suggest that in
the presence of nucleophiles that are also good leaving groups, the
reaction takes place under thermodynamic control favoring the attack
on the most sterically hindered carbon of the cyclic aziridinium intermediate.
Furthermore, when an enantiomerically pure pyrrolidine-based carbonate
was used, the reaction with phenols proceeds via a bicyclic aziridinium
intermediate leading to the stereoselective synthesis of optically
active 3-substituted piperidines via ring expansion reaction. These
results were confirmed both by NMR spectroscopy and X-ray diffraction
analysis.