Highly enantioenriched tetrahydropyridines from chiral organosilanes: application to the synthesis of quinolizidine alkaloid (-)-217A.

The tetrahydropyridine and piperidine subunits are widely found in biologically active molecules and natural products. Alhough considerable synthetic efforts have been made to reach these systems, there are few direct methods available for the preparation of enantionenriched tetrahydropyridines. A Lewis acid-promoted intramolecular imine crotylation is described and results in direct access to enantioenriched 2,6-cis- and 2,6-trans-3-trans-trisubstituted tetrahydropyridines with high diastereoselectivity. An example of the application of this methodology was demonstrated in an expedient total synthesis of (-)-quinolizidine 217A. The absolute stereochemistry of this natural product was established by the total synthesis.