First-Generation Asymmetric Synthesis of the Selective Estrogen Receptor Degrader GDC-9545 (Giredestrant) Featuring a Highly Efficient Pictet–Spengler Reaction and a C–N Coupling Reaction

An asymmetric synthesis of the selective estrogen receptor degrader GDC-9545 (1) is described. The synthesis features a Friedel–Crafts indole functionalization and a strain-release aminoazetidine formation to construct the two key starting materials 2 and 4, respectively, a diastereoselective Pictet–Spengler reaction (98% yield, 95:5 dr) to assemble the tetrahydrocarboline core, and a highly efficient Pd-catalyzed C–N coupling (90% yield) using [t-BuBrettPhos Pd(allyl)]OTf as the catalyst and DBU as the base to furnish the final C–N bond. This expedient route produces GDC-9545·tartrate active pharmaceutical ingredient in a longest linear sequence of six steps in 37% overall yield with 99.0 area % HPLC purity without chromatographic purification.