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

Potassium ferricyanide oxidation of salt <b>1 </b>gave isoquinolinone <b>7 </b>whose treatment with Grignard reagents resulted in a high-yield formation of substituted isoquinolinium salts <b>5</b>. The selectivity of the reduction of these salts to give derivatives <b>6</b> 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.