A Synthesis of Chiral 1,1,3-Trisubstituted 1,2,3,4-Tetrahydro-β-carbolines by the Pictet–Spengler Reaction of Tryptophan and Ketones: Conversion of (1R,3S)-Diastereomers into Their (1S,3S)-Counterparts by Scission of the C(1)–N(2) Bond
The Pictet-Spengler cyclization of the imines (3) prepared by the condensation of L-tryptophan methyl ester (1) and aryl methylketones (2), using titanium(IV) isopropoxide as an iminating reagent, quantitatively proceeded, when treated with trifluoroacetic acid (TFA) or formic acid, to provide two diastereomers, that is (1S,3S)-1-aryl-3-isopropoxycarbonyl-1-methyl-1,2,3,4-tetrahydro-beta-carbolines