Radical Reactions with Chiral Acylammonium Salts: Synthesis of Functionalized δ-Lactams through Giese-Initiated Organocascades

Abstract

Enantioselective Giese reactions employing chiral α,β-unsaturated acylammonium salts and subsequent diastereoselective trapping followed by lactamization deliver optically active δ-lactams. Alkyl iodides bearing tosylamides undergo radical initiation using triethylborane at low temperatures to provide carbon-centered radicals to initiate the described organocascade. Subsequent diastereoselective inter- or intramolecular trapping of the incipient α-radical leads to highly functionalized, enantioenriched mono- and bicyclic δ-lactams (up to 99:1 er, > 19:1 dr) bearing up to three stereogenic centers. Interestingly, benzotetramisole imparts diastereoselectivity that contradicts steric considerations alone. Results from DFT calculations rationalized the observed enantio- and diastereoselectivities, revealed an electrostatic interaction between the sulfone oxygens and the ammonium cation in the initial α-radical intermediate, and aided in application of this methodology to bicyclic δ-lactams through intramolecular trapping of the α-radical intermediate.