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. Profiting from an internal asymmetric induction, the reaction may be led to (1R,3R,7M) -2-benzyl-1,2,3,4-tetrahydro-7-(2-hydroxymethyl-4,5-dimethoxynaphth-1-yl)-1,3-dimethylisoquinolin-8-ol (5a), an intermediate in the stereocontrolled total synthesis of the liana alkaloid dioncophylline A (1a), or, optionally, to its atropdiastereomer 5b.