title: Tunable diastereoselection of biased rigid systems by Lewis acid induced conformational effects: a rationalization of the vinylation of cyclic nitrones en route to polyhydroxylated pyrrolidines.
abstract: The diastereofacial selection in addition reactions to biased rigid systems can be modulated by the action of Lewis acids. As an example, the stereoselectivity of the nucleophilic addition of vinyl magnesium bromide (VMB) to cyclic nitrones in the presence of diethylaluminum chloride (DEAC) shows a strong dependence on the temperature and the carbon substituent adjacent at the reaction center; it is remarkable that whereas a high selectivity is obtained at higher temperatures, in the presence of DEAC, a trend to invert the stereochemical course of the reaction is observed at lower temperatures, provided the substituent at C3 of the pyrrolidine ring allows delivery of the vinyl moiety. This behavior and difference in selectivity is to be attributed to the high conformational barriers of the intermediate nitrone-DEAC-VMB complex. A clear inversion of the selectivity is observed at -78 degrees C for the reaction of the nitrone protected as an O-methyl derivative. The present study provides a rationalization for the stereocontrolled addition of nucleophiles to rigid systems (cyclic nitrones).
authors.affiliation: Laboratorio de Síntesis Asimétrica, Departamento de Química Orgánica, Instituto de Ciencia de Materiales de Aragón, Universidad de Zaragoza, CSIC, Campus San Francisco, 50009 Zaragoza, Aragón, Spain.
authors.forename: Ignacio
authors.initials: I
authors.lastname: Delso
authors.forename: Eduardo
authors.initials: E
authors.lastname: Marca
authors.forename: Vanni
authors.initials: V
authors.lastname: Mannucci
authors.forename: Tomás
authors.initials: T
authors.lastname: Tejero
authors.forename: Andrea
authors.initials: A
authors.lastname: Goti
authors.forename: Pedro
authors.initials: P
authors.lastname: Merino
mesh_terms: D002384:Catalysis; D015394:Molecular Structure; D009589:Nitrogen Oxides / Q000737:chemistry*; D011759:Pyrrolidines / Q000138:chemical synthesis* / Q000737:chemistry; D013237:Stereoisomerism; D014753:Vinyl Compounds / Q000737:chemistry*
chemical_list: D009589:Nitrogen Oxides; D011759:Pyrrolidines; D014753:Vinyl Compounds; C015388:nitrones