Synthesis of Chiral Hydroxylated Quinolizidines via Vinylogous Bischler-Napieralski Nitrilium Ion Cyclizations

Article abstract of DOI:10.1021/jo00087a026

Treatment of the amido esters 9 and 17 with PPSE (polyphosphoric acid trimethylsilyl ester) followed by NaBH4 in ethanol gave the quinolizidinones 11-14 and 19 via a vinylogous Bischler-Napieralski nitrilium ion cyclization-reductive lactamization two-step process.Subsequent ozonolysis and reduction afforded chiral hydroxylated quinolizidines in moderate to good yield.In contrast to five-membered-ring formation, six-membered-ring formation via nitrilium-ion cyclization requires a p-methoxy-substituted styryl terminator.The effect para-substituted styryl terminators have on the energy of activation and ΔH for the cyclization process has been calculated by semiempirical and ab initio methods.