Stereocontrolled ring opening of Axially prostereogenic biaryl lactones with hydrogen nucleophiles : directed synthesis of a dioncophylline a precursor and (optionally) its atropdiastereomer

The atropdiastereoselective torsion of a rapidly isomerizing axially prostereogenic biaryl unit, as present in the bridged naphthyl isoquinoline, (2R,4R)-3-benzyl-1,3,4,6-tetrahydro-8,9-dimethoxy-2,4- dimethyl-6-oxo-2H-naphtho[1',2':4,5]pyrano[3,2-h]isoquinoline, (4) is described, by ring opening of the lactone function with simple achiral hydrogen nucleophiles