SYNTHESIS OF ARISTOTELIA-TYPE ALKALOIDS .12. TOTAL SYNTHESIS OF (-)-TASMANINE - STEREOELECTRONIC FACTORS THAT CONTROL THE REARRANGEMENT OF 3H-INDOL-3-OL DERIVATIVES TO OXINDOLES (= 1,3-DIHYDRO-2H-INDOL-2-ONES) OR TO PSEUDOINDOXYLS (= 1,2-DIHYDRO-3H-INDOL-3-ONES)

The oxidative transformation of (+)-aristoteline ((+)-5) into its metabolites, the recently synthesized indole alkaloids (-)-serratoline ((-)-6), (+)-aristotelone ((+)-2), and (-)-alloaristoteline ((-)-22), was investigated in more detail. It was demonstrated that the diastereoface selectivity of the reaction of(+)-5 with 3-chloroperbenzoic acid can be altered by variation of the solvent as well as by addition of CF3COOH. The chemoselectivity of the 1,2-rearrangement of the intermediate 3H-indol-3-ol derivatives could be controlled as follows: treatment of 3H-indol-3-ols with aqueous polyphosphoric acid led to the pseudoindoxyl (= 1,2-dihydro-3H-indol-3-one) derivatives, whereas an analogous treatment of the corresponding 0-benzoates furnished exclusively the corresponding, constitutionally isomeric 2-oxindole (= 1,3-dihydro-2H-indol-2-one) products. Exploitation of these and related findings led to efficient total syntheses of the Aristotelia alkaloid (-)-tasmanine ((-)-1) and of the corresponding unnatural epimer (+)-12, as well as of the two pseudoindoxyls (+)-aristotelone ((+)-2) and (-)-2-epiaristotelone((-)-11). All these transformations were carried out with synthetic (+)-aristoteline ((+)-5) as the single indole alkaloid precursor.