Enantioselective Electrophilic Aromatic Nitration: A Chiral Auxiliary Approach

摘要

Enantioselective electrophilic aromatic nitration methodology is needed to advance chirality-assisted synthesis (CAS). Reported here is an enantioselective aromatic nitration strategy operating with chiral diester auxiliaries, and it provides an enantioselective synthesis of a C-3v-symmetric tribenzotriquinacene (TBTQ). These axially-chiral structures are much sought-after building blocks for CAS, but they were not accessible prior to this work in enantioenriched form without resolution of enantiomers. This nitration strategy controls the stereochemistry of threefold nitration reactions from above the aromatic rings with chiral diester arms. Dicarbonyl-to-arenium chelation rigidifies the reaction systems, so that remote stereocenters position the ester-directing groups selectively over specific atoms of the TBTQ framework. Closely guided by computational design, a more selective through-space directing arm was first predicted with density functional theory (DFT), and then confirmed in the laboratory, to outperform the initial structural design. This enantio- and regioselective TBTQ synthesis opens a new pathway to access building blocks for CAS.