N-Heterocyclic carbene-catalyzed enantioselective hetero-[10 + 2] annulation

On the Number of π-Electrons Involved in Stepwise Cycloaddition Reactions

The development of higher-order cycloadditions has mainly been restricted by the requisite usage of highly conjugated and reactive π-systems. Recent years have witnessed organocatalysis as a potent mediator for several of the challenges associated herein, rendering higher-order cycloadditions a legitimate option for achieving the selective construction of specific molecular scaffolds. These developments reinvigorate the efforts to try to understand the underlying principles for cycloadditions involving a higher number of π-electrons than the "classical" cycloadditions; how do we properly address the impact which the addition of further π-electrons have on the reactivity of a system? Herein, computational investigations of two model higher-order cycloaddition systems have been performed to try to provide insights on changes in energetic barriers induced by the presence of benzofusions in a position which is unobstructive to the reactivity. With experimental substantiation as support, these studies might open up for a discussion on whether the π-electrons of benzofused systems simply act as spectator electrons, or play a tangible role on the observed reactivity to an extent where a distinct nomenclature is meritable.

Highly Enantioselective Construction of the Polycyclic Piperazin-2-ones via NHC-Catalyzed [12 + 2] Cycloadditions

NHC-catalyzed diastereoselective and enantioselective [12 + 2] higher-order cycloadditions of 5H-benzo[a]-pyrrolizine-3-carbaldehydes with cyclic sulfonic imines are described. Various optically pure polycyclic piperazin-2-ones were successfully constructed under mild reaction conditions with excellent diastereoselectivities and enantioselectivities. The generated chiral polycyclic piperazin-2-ones could be transformed to planar-chiral nine-membered cyclic amides via squaramide-catalyzed asymmetric ring-expansion reactions.

Carbene-Catalyzed Activation of 2-Aminobenzaldehyde for Access to Chiral Fluorescent Quinazolinone

A carbene-catalyzed reaction to synthesize a chiral quinazolinone with a new activation mode of an "aniline-like" N-H moiety is disclosed. Addition of the nitrogen atom of diphenyl o-aminobenzaldehydes via NHC activation to imines leads to chiral quinazolinones with high yields and optical purities. The acidity of the N-H moiety was extremely increased through the formation of an acyl azolium intermediate, which was investigated by DFT calculations. Moreover, the chiral quinazolinones were found to have high fluorescence quantum efficiency.

Dynamic kinetic resolution of γ,γ-disubstituted indole 2-carboxaldehydes via NHC-Lewis acid cooperative catalysis for the synthesis of tetracyclic ε-lactones

The ubiquity of ε-lactones in various biologically active compounds inspired the development of efficient and enantioselective routes to these target compounds. Described herein is the enantioselective synthesis of indole-fused ε-lactones...

Asymmetric higher-order [10+n] cycloadditions of palladium-containing 10π-cycloaddends

We uncovered an asymmetric higher-order [10 + 2] cycloaddition reaction between diverse activated alkenes and a new type of π-allylpalladium complex-containing dipole-type 10π-cycloaddend, which was generated in situ from 2-methylene-1-indanols via a dehydrative insertion and deprotonation strategy under double activation of Pd(0) and phosphoric acid. A similar strategy was applied to an asymmetric higher-order [10 + 8] cycloaddition reaction or [10 + 4] cycloaddition reaction by using a heptafulvene derivative or a cyclic enone, respectively, as the acceptor. A variety of polycyclic frameworks imbedding an indene core were generally furnished in moderate to excellent yields with high levels of enantioselectivity by employing a newly designed chiral phosphoramidite ligand.

A type of π-allylpalladium complex-containing 10π-cycloaddend generated from 2-methylene-1-indanols under double activation of Pd(0) and phosphoric acid undergoes asymmetric higher-order [10 + 2] cycloadditions with diverse activated alkenes.

Biomimetic enantioselective synthesis of β,β-difluoro-α-amino acid derivatives

Although utilization of fluorine compounds has a long history, synthesis of chiral fluorinated amino acid derivatives with structural diversity and high stereoselectivity is still very appealing and challenging. Here, we report a biomimetic study of enantioselective [1,3]-proton shift of β,β-difluoro-α-imine amides catalyzed by chiral quinine derivatives. A wide range of corresponding β,β-difluoro-α-amino amides were achieved in good yields with high enantioselectivities. The optically pure β,β-difluoro-α-amino acid derivatives were further obtained, which have high application values in the synthesis of fluoro peptides, fluoro amino alcohols and other valuable fluorine-containing molecules.

Organocatalytic Higher-Order [8+2] Cycloaddition for the Assembly of Atropoenantiomeric 3-Arylindolizines

We present an unprecedented atroposelective [8+2] cycloaddition reaction between pyridinium/isoquinolinium ylides and ynals. It is worth noting that this protocol represents a new example of the organocatalyzed atropoenantioselective higher-order cycloaddition reaction, providing various axial chiral 3-arylindolizines in good yields and high enantioselectivities. In addition, the obtained axially chiral 3-aryldolizines also provide many opportunities for structural transformations and potential drug discovery.