Intramolecular [3 + 2]-Cycloadditions of Azomethine Ylides Derived from Secondary Amines via Redox-Neutral C–H Functionalization

Azomethine
ylides are accessed
under mild conditions via benzoic acid catalyzed condensations of
1,2,3,4-tetra­hydro­iso­quinolines or trypto­lines
with aldehydes bearing a pendent dipolarophile. These intermediates
undergo intramolecular [3 + 2]-cycloadditions in a highly diastereoselective
fashion to form polycyclic amines with four new stereogenic centers.
Challenging substrates such as piperidine, morpholine, and thiomorpholine
undergo the corresponding reactions at elevated temperatures.