Enantioselective Synthesis of SM-130686 Based on the Development of Asymmetric Cu(I)F Catalysis To Access 2-Oxindoles Containing a Tetrasubstituted Carbon

Two different catalytic enantioselective approaches to 3-aryl- and 3-alkenyl-3-hydroxy-2-oxindoles have been developed. First, enantioselective arylation and alkenylation reactions of isatins using aryltrimethoxysilanes and alkenyltrimethoxysilanes as nucleophiles can be catalyzed by a complex of CuF with structurally tuned Taniaphos (6) in the presence of a catalytic amount of ZnF2. Despite the wide substrate scope, this intermolecular reaction was not applicable to a catalytic enantioselective synthesis of SM-130686 (1), a highly potent, orally active growth hormone secretagogue containing a sterically congested chiral tetrasubstituted carbon. Therefore, we developed an intramolecular catalytic enantioselective arylation of α-keto amides, taking advantage of the robustness of arylboronate reagents under multiple synthetic conversions and silica gel column chromatography purification. A complex of CuF with Ph−BPE (12) catalyzed the enantioselective arylation of α-keto amide 19, affording product 20 in 85% ee. The addition of ZnF2 to this intramolecular reaction was not necessary. The first enantioselective synthesis of SM-130686 was achieved using this catalytic methodology. Because 2-oxyindoles are a versatile motif for biologically active compounds, the two types of Cu-catalyzed asymmetric reactions developed here will be useful for the synthesis of other natural products and pharmaceutical leads.