The asymmetric Bischler-Napieralski reaction: preparation of 1,3,4-trisubstituted 1,2,3,4-tetrahydroisoquinolines

The Bischler-Napieralski reaction, which is used to prepare dihydroisoquinolines from phenylethylamides, is demonstrated by the reaction of (S)-1-alkyl-1,2-diphenylethylamides with POCl3-P2O5. The reaction generated 3-alkyl-4-phenyl-1,2-dihydroisoquinolines with stereochemical selectivities of 80-91% de depending on the alkyl and the acetamide substituents. These are the rst examples of the asymmetric Bischler-Napieralski reaction where cyclisation discriminates between two identical diastereotopic aryl groups. Reduction of the resultant dihydroisoquinoline products with LiAlH4 generated the corresponding 1,2,3,4-tetrahydroisoquinolines in a stereoselective manner, carrying three stereogenic centres at C(1), C(3) and C(4).