Asymmetric Organocatalytic Access to Spiro-fused Heterocyclic Compounds: E1cB Elimination Mediates Formal [4 + 2] Annulation

Based on the intramolecular E1cB elimination, a novel
[4 + 2] cyclization
was designed and successfully applied to the asymmetric synthesis
of polycyclic compounds which contained both chromane and spirooxindole
moieties. In the reaction, regardless of the competitive pathways
resulting from multireactive sites of starting materials, products
could be obtained in good yields (up to 84%) and with excellent enantioselectivities
(most 98 to >99% ee) under the catalysis of a chiral bifunctional
thiourea-tertiary amine (1–5 mol %).