Copper-Catalyzed Asymmetric 1,3-Dipolar Cycloaddition of Imino Esters to Unsaturated Sultones

N-Terminal-Specific Dual Modification of Peptides through Copper-Catalyzed [3+2] Cycloaddition

Site-specific introduction of multiple components into peptides is greatly needed for the preparation of densely functionalized and structurally uniform peptides. In this regard, N-terminal-specific peptide modification is attractive, but it can be difficult due to the presence of highly nucleophilic lysine ϵ-amine. In this work, we developed a method for the N-terminal-specific dual modification of peptides through a three-component [3+2] cycloaddition with aldehydes and maleimides under mild copper catalysis. This approach enables exclusive functionalization at the glycine N-terminus of iminopeptides, regardless of the presence of lysine ϵ-amine, thus affording the cycloadducts in excellent yields. Tolerating a broad range of functional groups and molecules, the present method provides the opportunity to rapidly construct doubly functionalized peptides using readily accessible aldehyde and maleimide modules.

Electron-rich benzofulvenes as effective dipolarophiles in copper(i)-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylides

A series of benzofulvenes without any electron-withdrawing substituents were employed as 2π-type dipolarophiles for the first time to participate in Cu(i)-catalyzed asymmetric 1,3-dipolar cycloaddition (1,3-DC) reactions of azomethine ylides. An intrinsic non-benzenoid aromatic characteristic from benzofulvenes serves as a key driving force for activation of the electron-rich benzofulvenes. Utilizing the current methodology, a wide range of multi-substituted chiral spiro-pyrrolidine derivatives containing two contiguous all-carbon quaternary centers were formed in good yield with exclusive chemo-/regioselectivity and high to excellent stereoselectivity. Computational mechanistic studies elucidate the origin of the stereochemical outcome and the chemoselectivity, in which the thermostability of these cycloaddition products is the major factor.

Annulation Cascade of Sulfamate-Derived Cyclic Imines with Glycine Aldimino Esters: Synthesis of 1,3-Benzoxazepine Scaffolds

An efficient (3 + 2) cycloaddition triggered annulation is reported to access 1,3-benzoxazepine frameworks. With amine base, sulfamate-derived cyclic imines readily react with glycine aldimino esters to furnish benzo-fused seven-membered heterocyclic products in good yields. The cascade reaction involves the formation of one C-C, one C-N, and one C-O bond along with the cleavage of two C-N bonds and one S-O bond. The synthesis of o-tyrosine analogues has also been accomplished from annulation products.

Iridium-Catalyzed Diastereo- and Enantioselective [4 + 3] Cycloaddition of 4-Indolyl Allylic Alcohols with Azomethine Ylides

An unprecedented iridium-catalyzed asymmetric [4 + 3] cycloaddition of racemic 4-indolyl allylic alcohols with azomethine ylides is reported. The ability of acid promoter zinc triflate to perform multiple roles is the key factor for the success of this strategy. This method provides scalable and efficient access to biologically important azepino[3,4,5-cd] indoles in good yields with generally excellent diastereo- and enantioselectivities (up to >20:1 dr and >99% ee). Mild reaction conditions, easily accessible substrates and chiral catalyst, and broad substrate scope highlight the practicality of this methodology.