Synthesis of Spiropyrrolines via Photocycloaddition-Retro-Mannich Reactions

Significance

The synthesis of spiroindolopyrroline and spirocyclohexylpyrroline derivatives by sequential [2+2] photocyclization (de Mayo reaction) and retro-Mannich reactions is reported. The inability to isolate 2 suggests that the retro-Mannich fragmentation takes place immediately after the formation of 2 and excludes the involvement of radical attack of the photoexcited alkylidene malo­nate at the indole 3-position. The ready availability of starting materials 1 and 4, the latter by Birch reduction, is noted. Intermediate 6 served for further, as yet not completed, elaboration to a key compound en route to the complex indole alkaloid Koumine.