Switching of Enantioselectivity in the Cu-Catalyzed Asymmetric Decarboxylative Aldol Reaction of Tryptanthrin with β-Keto Acids: An Unexpected Counteranion Effect

Cu-bisoxazoline-catalyzed enantioselective decarboxylative aldol reaction of tryptanthrin with aryl-substituted beta-keto acids is developed, providing a straightforward approach to deliver a series of phaitanthrin A analogues. Both enantiomers of the products can be obtained with good to high enantioselectivity in the presence of a single chiral ligand by simply changing the copper salts. Based on the X-ray crystallographic analysis of chiral Cu(II)-bisoxazoline complexes, the tentative stereochemical models are presented to account for the observed counteranion-induced switching in enantioselectivity.

Automated summary

The study presents a Cu-bisoxazoline-catalyzed enantioselective decarboxylative aldol reaction for the synthesis of a series of phaitanthrin A analogues, with both enantiomers of the products obtainable with good to high enantioselectivity by simply changing copper salts. Tentative stereochemical models based on X-ray crystallographic analysis of chiral Cu(II)-bisoxazoline complexes explain the observed counteranion-induced switching in enantioselectivity.