A STEREOSELECTIVE TRANSFORMATION OF PSEUDOINDOXYLS INTO OXINDOLES IN A SINGLE OPERATION

The Aristotelia alkaloid (+)-aristotelone (1), a spiro-pseudoindoxyl d erivative, is transformed in over 90 % yield into the oxindole (+)-3-e pitasmanine (3) upon treatment with hot BF3.Et(2)O in CH2Cl2. This int riguing transformation possibly proceeds through the intermediate 3-hy droxyindolenine 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 th at 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 du e to a kinetic control was demonstrated by showing that the epimeric s tarting material (-)-4 furnished exclusively the naturally occurring a lkaloid (-)-tasmanine (5) under the same reaction conditions.