Quinine-Derived Thiourea and Squaramide Catalyzed Conjugate Addition of alpha-Nitrophosphonates to Enones: Asymmetric Synthesis of Quaternary alpha-Aminophosphonates

摘要

Conjugate addition of a-nitrophosphonates to enones was carried out in the presence of two sets of organocatalysts, viz. a quinine-thiourea and a quinine-squaramide. The quinine-thiourea provided the products possessing an a-quaternary chiral center in high enantioselectivities only in the case of electron rich enones. On the other hand, the quinine-squaramide was more efficient in that a wide variety of electron rich and electron poor enones underwent Michael addition of nitrophosphonates to afford the quaternary a-nitrophosphonates in excellent yields and enantioselectivities. The hydrogen bonding donor ability of the bifunctional catalyst, as shown in the proposed transition states, appears primarily responsible for the observed selectivity. However, a favorable p-stacking between the aryl groups of thiourea/squaramide and aryl vinyl ketone also appeared favorable. The reaction was amenable to scale up, and the enantioenriched quaternary a-nitrophosphonates could be easily transformed to synthetically and biologically useful quaternary a-aminophosphonates and other multifunctional molecules.