A STEREOSELECTIVE TRANSFORMATION OF PSEUDOINDOXYLS INTO OXINDOLES IN A SINGLE OPERATION

The Aristotelia alkaloid (+)-aristotelone (1), a spiro-pseudoindoxyl derivative, is transformed in over 90 % yield into the oxindole (+)-3-epitasmanine (3) upon treatment with hot BF3.Et(2)O in CH2Cl2. This intriguing transformation possibly proceeds through the intermediate 3-hydroxyindolenine derivative (-)-serratoline (2) which could be isolated when the reaction was run under milder conditions. This rearrangement, for which there is little precedent, is highly stereoselective in that the lactam carbonyl group ends up on the same face of the molecule as the C=O-unit of the starting pseudoindoxyl. That this outcome is due to a kinetic control was demonstrated by showing that the epimeric starting material (-)-4 furnished exclusively the naturally occurring alkaloid (-)-tasmanine (5) under the same reaction conditions.