Construction of Dispiro-Indenone Scaffolds via Domino Cycloaddition Reactions of α,β-Unsaturated Aldimines with 2‑Arylidene-1,3-indenediones and 2,2′-(Arylmethylene)bis(1,3-indenediones)

The catalyst-free domino reaction
of α,β-unsaturated N-alkyl or N-arylaldimines with two molecules
of 2-arylidene-1,3-indanediones in dry acetonitrile resulted in polysubstituted
spiro­[indene-2,3′-indeno­[2′,1′:5,6]­pyrano­[2,3-b]­pyridines] in moderate to good yields and with high diastereoselectivity.
The reaction mechanism included sequential aza/oxa-Diels–Alder
reactions via both endo-transition states. On the other hand, the
catalyst-free domino reaction of α,β-unsaturated N-arylaldimines with 2,2′-(arylmethylene)­bis­(1,3-indenediones)
afforded the mixed diastereoisomeric dispiro­[indene-2,1′-cyclohexane-3′,2″-indene]
derivatives in satisfactory yields. The reaction mechanism of this
formal [3 + 3] cycloaddition was believed to proceed with sequential
nucleophilic 1,4-/1,2-additions.