Cyclization of 2-(4-R-piperidino)benzaldehydes with 5-methyl-2-phenyl-2,4-dihydro-3H-pyrazol-3-one proceeding via tert-amino effect mechanism is stereoselective. Relative configuration of (3R*,4aS*,5R*)-2,3,4,4a,5,6-hexahydro-1H-benzo[c]quinolizine was established on the base of NMR spectroscopy data and X-ray analysis.

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The cyclization via the mechanism of "tert-amino effect" of 2-(4-R-piperidino)benzaldehydes with cyclic active methylene components (Meldrum's acid, 1,3-cyclohexanedione, and N,N-disubstituted barbituric acids) proceeded stereoselectively giving spiro-joined 2,3,4,4a,5,6-hexahydro-1H-benzo[c]quinolizines with axially oriented hydrogen atoms in the positions 3 and 4a of the benzo[c]quinolizine ring.

The synthesis of 2,3,4,4a,5,6-hexahydro-6H-spiro[benzo[c]quinolizine-5,4′-pyrazol]-5′-ones has been achieved by the reaction of 2-piperidinobenzaldehydes with 2-aryl-5-methyl-2,4-dihydropyrazol-3-one via the “tert-amino effect” mechanism.

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The reactions of 2-(3,5-dimethylpiperidino)benzaldehyde with Meldrum's acid and cyclohexane-1,3-dione occur as a tandem of the Knoevenagel condensation and cyclization promoted by the tert-amino effect. The cyclization yields only one isomer with the axial hydrogen atoms in positions 4 and 4a of the benzoquinolizine ring.