Diastereospecific Synthesis of Tetrahydroisoquinolines via Radical Cyclization: Application in the Synthesis of ent-Tadalafil

An enantioselective synthesis of 1-substituted tetrahydroisoquinolines from L–Dopa methyl ester through intramolecular aryl radical cyclization is demonstrated. The strategy consists of bromination of (S)-2-amino-3-(2-bromo-4,5-dimethoxyphenyl)propanoate followed by condensation with various aldehydes to afford bromoimidate ester. Aryl radicals generated from bromoimidate ester under the radical generating conditions (ⁿBu₃SnH/AIBN) cyclizes via 6-endo mode to afford cis-1-substituted tetrahydroisoquinolines exclusively in 99% ee. The utility of this synthetic protocol is demonstrated in the synthesis of (6S, 12aS) Tadalafil (5 steps, 21%, 99% ee). (Figure presented.).