(S)-1,2,3,4-tetrahydroisoquinoline derived chiral auxiliary in diastereoselective alkylation

A novel chiral auxiliary (S)-2-(1,2,3,4-tetrahydroisoquinolin-3-yl)propan-2-ol 1 was synthesized from (S)-1,2,3,4-tetrahydroisoquinoline acid 2. Alkylation of the propanoylated chiral auxiliary (S)-1-(3-(2-hydroxypropan-2-yl)-3,4-dihydroisoquinolin-2(1H)-yl)-propan-1-one 3 with benzyl bromide or n-butyl iodide caused moderate to high conversion (42–99%) and moderate diastereomeric ratios (up to 89:11). The major diastereomer obtained in the reaction with benzyl bromide was (R,S). Some of the reactions were carried out with LiCl as an enolate coordinating agent, which affected conversion and diastereomeric ratio negatively.