Enantioselective Addition of Thiols to Imines Catalyzed by Thiourea–Quaternary Ammonium Salts

Asymmetric phase-transfer catalysis was first applied to the synthesis of chiral N,S-acetals by using amino acid-based bifunctional thiourea-ammonium salt catalysts. The reaction could be performed on the gram scale to give up to 93% ee and 99% yield with a catalyst loading as low as 0.1 mol % within 5 min.