An Efficient Approach to the Securinega Alkaloids Empowered by Cooperative N-Heterocyclic Carbene/Lewis Acid Catalysis

N-Heterocyclic Carbene Catalyzed Reactions Involving Acetylenic Breslow and/or Acylazolium as Key Intermediates

N-heterocyclic carbene (NHC) organocatalysis has been developed as a powerful tool in modern synthetic chemistry. NHC catalytic activation of ynals and alkynoic acid derivatives provided versatile reactions that involve acetylenic Breslow and/or acylazolium as key intermediates, and diverse transformations have been established for access to molecules with unique skeletons in efficient fashions. Herein we summarize the recent achievements in NHC-catalyzed reactions involving acetylenic Breslow and/or acylazolium intermediates. Different reactions belonging to three catalytic modes, including (1) conjugate additions to acetylenic Breslow derived α,β-unsaturated acylazolium intermediates, (2) β-umpolung of ynals via acetylenic Breslow intermediates, and (3) conjugate additions to acetylenic acylazolium intermediates, are emphasized with examples and plausible mechanisms cited to guide a better understanding.

Ionic Hydrogen Bond-Assisted Catalytic Construction of Nitrogen Stereogenic Center via Formal Desymmetrization of Remote Diols

The control of noncarbon stereogenic centers is of profound importance owing to their enormous interest in bioactive compounds and chiral catalyst or ligand design for enantioselective synthesis. Despite various elegant approaches have been achieved for construction of S-, P-, Si- and B-stereocenters over the past decades, the catalyst-controlled strategies to govern the formation of N-stereogenic compounds have garnered less attention. Here, we disclose the first organocatalytic approach for efficient access to a wide range of nitrogen-stereogenic compounds through a desymmetrization approach. Intriguingly, the pro-chiral remote diols, which are previously not well addressed with enantiocontrol, are well differentiated by potent chiral carbene-bound acyl azolium intermediates. Preliminary studies shed insights on the critical importance of the ionic hydrogen bond (IHB) formed between the dimer aggregate of diols to afford the chiral N-oxide products that feature a tetrahedral nitrogen as the sole stereogenic element with good yields and excellent enantioselectivities. Notably, the chiral N-oxide products could offer an attractive strategy for chiral ligand design and discovery of potential antibacterial agrochemicals.

The Combination of Lewis Acid with N-Heterocyclic Carbene (NHC) Catalysis

In the last ten years, the combination of Lewis acid with N-heterocyclic carbene (NHC) catalysis has emerged as a powerful strategy in a variety of important asymmetric synthesis, due to the ready availability of starting materials, operational simplicity and mild reaction conditions. Recent findings illustrate that Lewis acid could largely enhance the efficiency and enantioselectivity, reverse the diastereoselectivity, and even influence the pathway of the same reaction partners. Herein, this review aims to reveal the recent advances in NHC-Lewis acid synergistically promoted enantioselective reactions for the expeditious assembly of versatile biologically important chiral pharmaceuticals and natural products.

A cooperative N-heterocyclic carbene/chiral phosphate catalysis system for allenolate annulations

The highly enantioselective NHC-catalyzed [3+2] annulation reaction with α,β-alkynals and α-ketoesters has been developed. A new mode of cooperative catalysis involving the combination of a chiral Brønsted acid and a C1-symmetric biaryl saturated-imidazolium precatalyst was required to generate the desired γ-crotonolactones in high yields and levels of enantioselectivity.