Rongalite-promoted self-dimerization of 3-acylidene-2-oxindoles: a diastereoselective route to synthesis of bispirooxindoles.

A facile and efficient one-pot rongalite-mediated self-dimerization of 3-acylidene-2-oxindoles has been developed for the diastereoselective synthesis of highly functionalized dispirocyclopentanebisoxindoles. The reaction proceeds a domino sequence involving intermolecular Michael addition followed by intramolecular aldol cyclization under basic conditions. Rongalite, an inexpensive and readily available reagent (∼$0.03 per g), plays a crucial role in promoting the transformation, offering significant advantages such as operational simplicity, step economy, scalability to gram-scale synthesis, and potential for post-functionalization. This methodology provides an efficient route to structurally complex oxindole frameworks with high stereocontrol, demonstrating broad synthetic utility.