Functionalization and Rearrangement of Spirocyclohexadienyl Oxindoles: Experimental and Theoretical Investigations

Abstract The preparation and functionalization of spirocyclohexa‐2,5‐diene oxindoles is described. The spirocyclic core of the title compounds was installed by using a SmI 2 ‐mediated cyclization of aryl iodobenzamides. Epoxidation with CF 3 CO 3 H was then carried out and was shown to occur with a high level of diastereocontrol: the reagent approaches the diene moiety syn to the amide group, which is likely to be as a consequence of hydrogen bonding between the amide CO bond and the peracid hydrogen. Carbanionic functionalization of the spirocyclohexa‐2,5‐diene oxindoles was then examined, leading to an unprecedented rearrangement of the strained spiro system into dearomatized phenanthridinones. Upon treatment with lithium diisopropylamide (LDA) at −40 °C, the dienes rearranged to provide a phenanthridinone lithium enolate intermediate that was trapped by electrophiles including alkyl halides and aldehydes. Interestingly, alkylation and hydroxyalkylation occurred with different regiocontrol. DFT calculations were performed that rationalize the observed skeleton rearrangement, emphasizing the role of LDA/diisopropylamine in this rearrangement. The proposed mechanism thus relies on a thermodynamically driven diisopropylamine‐mediated proton transfer with the cleavage of the diene–amide CO bond as the key step.