Synthesis of Spirocyclic 1-Pyrrolines from Nitrones and Arynes via a Dearomative [3,3']-Sigmatropic Rearrangement.

: A dearomative [3,3']-sigmatropic rearrangement that converts  N -alkenylbenzisoxazolines to spirocyclic pyrroline cyclohexadienones has been discovered using the dipolar cycloaddition of an  N -alkenylnitrone and an aryne to access these unusual transient rearrangement precursors. This cascade reaction affords spirocyclic pyrrolines that are inaccessible via dipolar cycloadditions of exocyclic cyclohexenones and provides a fundamentally new approach to novel spirocyclic pyrroline and pyrrolidine motifs that are common scaffolds in biologically-active molecules. Diastereoselective functionalization processes have also been explored to demonstrate the divergent synthetic utility of the unsaturated spirocyclic products.