Asymmetric Organocatalytic [4 + 1] Annulations Involving a Polarity Reversal Process: A Tandem Catalytic Approach to Highly Functionalized Spiropyrazolone Derivatives

A tandem catalytic strategy for the asymmetric synthesis
of spirocyclopentanone
pyrazolones bearing three contiguous stereocenters and two quaternary
carbons with good stereoselectivities has been developed. This strategy,
using pyrazolones as efficient C1 synthons and involving a polarity
reversal process, not only overcame the energy barrier of the dearomatization
process but also avoided nucleophilic addition of the hydroxy group
in the enol form tautomer. Futhermore, spirocyclopentanones could
be transformed into spirocyclohexamide pyrazolone with the Lawesson
reagent.