A new synthesis of spiropyrrolidine-tetralones via an unexpected formal ring-contraction of 4-disubstituted piperidine to 3-disubstituted pyrrolidine

We have developed an efficient synthesis of novel racemic spiropyrrolidine-tetralones via an unexpected ring-contraction reaction of a 4-disubstituted piperidine to 3-disubstituted pyrrolidine. We suggest that intramolecular quaternization of the piperidine nitrogen of compound 7 occurs to form a bridged bicyclic quaternary ammonium salt intermediate 10. The ring opening of 10 with cyanide resulted in pyrrolidine 9. The synthesis of racemic spiropyrrolidine-tetralone 15 is described as well as the related spiropiperidine-indanone, 1b.