Diastereoselective Tandem 6-exoCarbolithiation Intramolecular Ring Opening in (−)-8-Aminomenthol-Derived Perhydrobenzoxazines. A New Synthesis of Enantiopure 4-Substituted Tetrahydro Isoquinolines and 2-Azabenzonorbornanes

Aryllithiums prepared by bromine-lithium interchange in chiral 2-(o-bromophenyl)-substituted perhydro-1,3-benzoxazines participate in the intramolecular 6-exo carbolithiation reaction with unactivated double bonds attached to the nitrogen substituent of the heterocycle. When the reaction time is extended or no TMEDA is used, the cyclized lithium intermediates react intramolecularly with the N,O-acetal system leading to 2-azabenzonorbornane derivatives. The reactions are highly stereoselective and constitute a high-yielding synthesis of enantiopure 4-substituted tetrahydroisoquinolines or 7-substituted 2-azabenzonorbornanes.