DBU-Mediated Diastereoselective (3+2)-Cycloaddition of Isatin Ketonitrones and Coumarins to Construct Coumarin-Fused Spiropyrolidine Oxoindoles

The synthesis of novel dicyclic spiropyrrolidine oxoindole derivatives is described. This approach relies on a (3+2)-cycloaddition reaction between coumarins and isatin ketonitrone 1,3-dipoles, which were formed in situ by condensation of various substituted isatins with arylhydroxylamines. The corresponding pentacyclic products, featuring four contiguous stereocenters—including two quaternary carbon stereocenters fused within a single ring system—were obtained smoothly in moderate to excellent yields (22–98%), with high regioselectivity (α and exo type) and diastereoselectivity (>20:1 dr). Over 45 examples of the synthesized compounds were fully characterized using a range of spectroscopic techniques, including single-crystal X-ray diffraction, FTIR, NMR, and mass spectrometry.