Highly Stereoselective Total Syntheses of (±)-Chelidonine and of (±)-Norchelidonine by an Intramolecular o-Quinodimethane/Nitrostyrene-Cycloaddition

Conversion of 2-bromomethylstyrene 22 and benzocyclobutenyl carbamate 28 to the benzophenanthridine alkaloids (±)-chelidonine ( 1 , five steps, 25% from 28 ) and to (±)-norchelidonine ( 2 , six steps, 24% from 28 ) are described. The key step 29 → 31 involves a highly regio- and stereocontrolled intramolecular Diels-Alder reaction of the (E)-quinodimethane 30 .