Efficient and Highly Selective Synthesis of Enantiopure cis- or trans-3,4-Disubstituted 1,2,3,4-Tetrahydroisoquinolines

An efficient synthesis of enantiopure 3,4-disubstituted 1,2,3,4-tetrahydroisoquinolines, by treatment of readily available (2R,1′S)- or (2S,1′S)-2-(1-aminoalkyl)epoxides with H3PO4·BF3 complex, under mild reaction conditions, is reported. Both enantiopure diastereoisomers (3S,4S)- and (3S,4R)-3-alkyl-4-hydroxymethyl-1,2,3,4-tetrahydroisoquinolines were available starting from the suitable syn- or anti-aminoepoxide, respectively. A mechanism based on an intramolecular Friedel−Crafts-type reaction has been proposed to explain these results.