A New Entry to Asymmetric Synthesis of 1-Substituted 1,2,3,4-Tetrahydro-β-carbolines Employing a Pyroglutamic Acid Derivative as a Chiral Auxiliary

β-Carboline which was protected at N-9 by an acyl group derived from l-pyroglutamic acid reacted with allyltributyltin or silyl enol ethers in the presence of an alkyl chloroformate in a highly diastereoselective manner to give 1-substituted 1,2-dihydro-β-carbolines. The compounds were readily transformed to the corresponding asymmetric 1-substituted tetrahydro-β-carbolines that are common partial structures in a variety of indole alkaloids.