Tetraalkylphosphonium Salts as Chiral Phase-Transfer Catalysts

Significance

The C ₂-symmetric chiral quaternary tetraalkylphosphonium salt (S)-3 is employed as a phase-transfer catalyst (PTC) in asymmetric ­aminations of β-keto esters. In the presence of 3 mol% of (S)-3, 1.2 equivalents of di-tert-butyl azodicarboxylate and 1-5 equivalents of K₂HPO₄ as the base, the corresponding amination products of cyclic five-membered β-keto esters are obtained in nearly quantitative yields and high enantioselectivities (er up to 97.5:2.5). This method is useful for the preparation of aldose reductase inhibitor AS-3201 (Ranirestat).