Cyclophane-type imidazolium salts with planar chirality as a new class of N-heterocyclic carbene precursors.
Cyclophane-type imidazolium salts with planar chirality were synthesized from enantiopure 2-amino alcohols, of which the N(1) and N(3) positions were connected with a bridge. The structural profiles of the imidazolium salts and their derivative N-heterocyclic carbenes (NHCs) were investigated by means of several analyses. The chiral NHCs derived from these imidazolium salts were found to catalyze the asymmetric cross-annulation of an alpha,beta-unsaturated aldehyde with a ketone by means of the "conjugated" umpolung of the enal to give the target gamma-lactone with good to excellent enantioselectivity (up to 94% ee). Based on the expected structure of the NHCs and their intermediates, together with the absolute configuration of the products, a plausible mechanism for the stereocontrol was proposed.
Matsuoka
Yuki
Y
Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
Ishida
Yasuhiro
Y
Sasaki
Daisuke
D
Saigo
Kazuhiko
K