A bifunctional iminophosphorane (BIMP)-catalyzed, enantioselective
intramolecular oxa-Michael reaction of alcohols to tethered, low electrophilicity
Michael acceptors is described. Improved reactivity over previous
reports (1 day vs 7 days), excellent yields (up to 99%), and enantiomeric
ratios (up to 99.5:0.5 er) are demonstrated. The broad reaction scope,
enabled by catalyst modularity and tunability, includes substituted
tetrahydrofurans (THFs) and tetrahydropyrans (THPs), oxaspirocycles,
sugar and natural product derivatives, dihydro-(iso)-benzofurans,
and iso-chromans. A state-of-the-art computational study revealed
that the enantioselectivity originates from the presence of several
favorable intermolecular hydrogen bonds between the BIMP catalyst
and the substrate that induce stabilizing electrostatic and orbital
interactions. The newly developed catalytic enantioselective approach
was carried out on multigram scale, and multiple Michael adducts were
further derivatized to an array of useful building blocks, providing
access to enantioenriched biologically active molecules and natural
products.