An Enantioselective Access to 1-Alkyl-1,2,3,4-tetrahydroisoquinolines. Application to a New Synthesis of (−)-Argemonine

Potassium ferricyanide oxidation of salt 1 gave isoquinolinone 7 whose treatment with Grignard reagents resulted in a high-yield formation of substituted isoquinolinium salts 5. The selectivity of the reduction of these salts to give derivatives 6 has been studied. Particularly good selectivities (82−84%) were observed when R is a benzylic group. On the basis of these results, a practical and enantioselective synthesis of the natural alkaloid (−)-argemonine is presented.