The synthesis of diastereomerically pure (-)-(S) menthyl 2-methoxy-1-naphthalenesulfinate (5) is reported. The reaction of (5) with methylmagnesium iodode or benzylmagnesium chloride gives (+)-(R) methyl and (+)-(R) benzyl 2-methoxy-1-naphthyl sulfoxide, (7) and (8) respectively, in high enantiomeric purity (98% ee). Lithiated (8) undergoes addition to 6,7-dimethoxy-3,4-dihydroisoquinoline N-oxide (2) to give a 1-benzyl-1,2,3,4-tetrahydroisoquinoline derivative (9) with high product diastereoselection (d. r. 96 : 4). The stereochemistry of (9) was determined by a single crystal X-ray structure determination. © 1994.