Enantioselective Hydrogen Atom Transfer Reactions: Synthesis ofN-Acyl-α-Amino Acid Esters

Late-Stage Modification of Oligopeptides by Nickel-Catalyzed Stereoselective Radical Addition to Dehydroalanine

Radical addition to dehydroalanine (Dha) represents an appealing, modular strategy to access non-canonical peptide analogues for drug discovery. Prior studies on radical addition to the Dha residue of peptides and proteins have demonstrated outstanding functional group compatibility, but the lack of stereoselectivity has limited the synthetic utility of this approach. Herein, we address this challenge by employing chiral nickel catalysts to control the stereoselectivity of radical addition to Dha on oligopeptides. The conditions accommodate a variety of primary and secondary electrophiles to introduce polyethylene glycol, biotin, halo-tag, and hydrophobic and hydrophilic side chains to the peptide. The reaction features catalyst control to largely override substrate-based control of stereochemical outcome for modification of short peptides. We anticipate that the discovery of chiral nickel complexes that confer catalyst control will allow rapid, late-stage modification of peptides featuring nonnatural sidechains.

Enantioselective Radical-Polar Crossover Reactions of Indanonecarboxamides with Alkenes

Highly efficient asymmetric intermolecular radical-polar crossover reactions were realized by combining a chiral N,N'-dioxide/Ni^II complex catalyst with Ag₂ O under mild reaction conditions. Various terminal alkenes and indanonecarboxamides/esters underwent radical addition/cyclization reactions to afford spiro-iminolactones and spirolactones with good to excellent yields (up to 99 %) and enantioselectivities (up to 97 % ee). Furthermore, a range of different radical-mediated oxidation/elimination or epoxide ring-opening products were obtained under mild reaction conditions. The Lewis acid catalysts exhibited excellent performance and precluded the strong background reaction.

Enantioselective Radical Cyclizations: A New Approach to Stereocontrol of Cascade Reactions

Stereocontrol in a cascade radical addition-cyclization-trapping reaction was achieved by a new approach, which utilizes a hydroxamate ester moiety as a coordinating chiral Lewis acid tether between two radical acceptors. A remarkable feature of this reaction is the construction of three bonds and tertiary and quaternary stereogenic centers through both inter- and intramolecular carbon-carbon bond-forming processes. The chiral Lewis acid mediated reaction of oxime ethers also proceeded smoothly with good enantio- and diastereoselectivities, indicating the usefulness of the cascade approach for the asymmetric synthesis of various gamma-lactams.

Enantioselective Construction of Quaternary Stereocenters

The stereoselective formation of C-C bonds is of great importance for the synthesis of enantiomerically pure natural products and pharmaceuticals. A broad repertoire of chiral auxiliaries, reagents, and catalysts can be utilized for the reliable generation of tertiary stereocenters. In contrast, the synthesis of organic compounds with quaternary stereocenters is a much more demanding and challenging task. Every enantioselective synthetic method can demonstrate its value through the generation of a fully substituted carbon center. In this Minireview examples of newer stoichiometric and catalytic methods are summarized which have proved their suitability for the enantioselective construction of quaternary stereocenters.