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.