Synthesis and structural elucidation of a novel bis-spirooxindole from isatin and ethylenediamine

The reactivity of isatin toward ethylenediamine displays an unexpected stoichiometric divergence, affording either the anticipated diiminoisatin or a previously unreported pentacyclic bis-spirooxindole. A 2:1 isatin-to-diamine ratio provides the diiminoisatin, whereas a 1:2 ratio leads to the formation of a highly symmetric bis-spirooxindole scaffold. The new spirocyclic product was fully characterized by HRMS, IR and extensive 1D/2D NMR analysis, and its structure was unequivocally established by single-crystal X-ray diffraction. In addition, the isolated diiminoisatin can be independently reduced to the same bis-spirooxindole. These results broaden the scope of isatin–diamine condensations and demonstrate their potential to generate structurally complex spirooxindole architectures under simple conditions.