Acid-promoted retro-Mannich reaction of N-protected tropenones to 2-substituted pyrroles

N-protected tropenone undergoes highly regioselective retro-Mannich reactions to 2-pyrrolyl ketones in the presence of Lewis or Bronsted acids. Analogously, a pyrrole deriv. was obtained by using subsequent nucleophilic trapping with trimethylsilanecarbonitrile. The possible Grob fragmentation of 3-oxo-8-azabicyclo[3.2.1]oct-6-ene-8-carboxylates and 8-[(4-methylphenyl)sulfonyl]-8-azabicyclo[3.2.1]oct-6-en-3-one was also investigated. The retro-Mannich reaction of 3-oxo-8-azabicyclo[3.2.1]oct-6-ene-8-carboxylic acid Me ester (I) in the presence of 4-methylbenzenesulfonic acid gave 2-(2-oxopropyl)-1H-pyrrole-1-carboxylic acid Me ester (II).