title: Quinine-derived thiourea and squaramide catalyzed conjugate addition of α-nitrophosphonates to enones: asymmetric synthesis of quaternary α-aminophosphonates.
abstract: Conjugate addition of α-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 α-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 α-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 π-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 α-nitrophosphonates could be easily transformed to synthetically and biologically useful quaternary α-aminophosphonates and other multifunctional molecules.
authors.affiliation: Department of Chemistry, Indian Institute of Technology Bombay , Mumbai 400 076, India.
authors.forename: Kalisankar
authors.initials: K
authors.lastname: Bera
authors.forename: Irishi N N
authors.initials: IN
authors.lastname: Namboothiri
mesh_terms: D000577:Amides / Q000737:chemistry*; D001686:Biological Phenomena; D002384:Catalysis; D007659:Ketones / Q000737:chemistry*; D015394:Molecular Structure; D063088:Phosphoramides / Q000138:chemical synthesis* / Q000737:chemistry*; D011803:Quinine / Q000737:chemistry*; D013237:Stereoisomerism; D013890:Thiourea / Q000737:chemistry*
chemical_list: D000577:Amides; D007659:Ketones; D063088:Phosphoramides; D011803:Quinine; D013890:Thiourea