title: Accessing N-Stereogenicity through a Double Aza-Michael Reaction: Mechanistic Insights.
abstract: Further development of the chemistry and applications of chiral compounds that possess configurationally stable stereogenic nitrogen atoms is hampered by the lack of efficient strategies to access such compounds in an enantiomerically pure form. Esters of propiolic acid and chiral alcohols were evaluated as cheap and readily available Michael acceptors in a diastereoselective synthesis of N-stereogenic compounds by means of a double aza-Michael conjugate addition. Diastereomeric ratios of up to 74:26 and high yields were achieved with (-)-menthyl propiolate as a substrate. Furthermore, a detailed mechanistic investigation was undertaken to shed some light on the course of this domino transformation. Kinetic studies revealed that the protic-solvent additive acts as a Brønsted acid and activates the ester toward the initial attack of the tetrahydrodiazocine partner. Conversely, acidic conditions proved unfavorable during the final cyclization step that provides the product.
authors.affiliation: Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology, Zürich, Vladimir-Prelog-Weg 2, 8093 Zürich (Switzerland).|Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstrasse 40, 48149 Münster (Germany).
authors.forename: Sonja
authors.initials: S
authors.lastname: Kohrt
authors.affiliation: Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology, Zürich, Vladimir-Prelog-Weg 2, 8093 Zürich (Switzerland).|Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstrasse 40, 48149 Münster (Germany).
authors.forename: Nico
authors.initials: N
authors.lastname: Santschi
authors.affiliation: Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology, Zürich, Vladimir-Prelog-Weg 2, 8093 Zürich (Switzerland). cvengros@inorg.chem.ethz.ch.
authors.forename: Ján
authors.initials: J
authors.lastname: Cvengroš
keywords: Michael addition; Tröger base; asymmetric synthesis; diastereoselectivity; stereogenicity