Organocatalytic Enantioselective Vinylogous Michael-Aldol Cascade for the Synthesis of Spirocyclic Compounds

Enantioselective
synthesis of pyrazolone-fused spirocyclohexenols
by the secondary amine-catalyzed cascade reaction of α,β-unsaturated
aldehydes with α-arylidene pyrazolinones is reported. This formal
[3 + 3] organocascade reaction proceeds through a vinylogous Michael-aldol
sequence to furnish the spiroheterocycles with three stereocenters
including an all-carbon quaternary center in good yields and selectivities.
The catalytic generation of α,β-unsaturated iminium ions
from enals and tandem dienolate/enolate formation from pyrazolinones
are the key for the success of this spiroannulation reaction.