Organocatalytic Enantioselective 1,3-Dipolar [6+4] Cycloadditions of Tropone

Construction of central and axial chirality via Pd(II)/Bim-catalyzed asymmetric dearomative Michael reaction of polycyclic tropones

A highly enantioselective Pd/Bim-catalyzed dearomative Michael reaction applying polycyclic tropones as non-benzenoid aromatic Michael acceptors and arylboronic acids as aryl pronucleophiles has been developed. The bridged biaryls bearing central and axial chirality, including pentacyclic cyclohepta[b]indoles and 6,7-dihydrodibenzo[a,c][7]annulen-5-ones, are generally generated in good to high yields and excellent enantioselectivities and can be readily transformed into useful derivatives.

Enal-azomethine ylides: application in the synthesis of functionalized pyrroles

Rhodium-catalyzed [3 + 2] annulation of diazoenals and N-alkyl imines resulted in N-alkyl-pyrrole-3-carbaldehyde derivatives. The reaction involves thermal 6π-electrocyclization and aromatization of a new class of enal-azomethine ylides (EAYs). The EAYs derived from dihydroisoquinoline and 2H-azirine gave fused-pyrrole and pyridine derivatives, respectively. The synthetic importance of pyrrole products has been demonstrated by one-step synthesis of the biologically relevant pyrrolo[3,2-c]quinoline scaffold as well as pyrrolo[2,1-a]isoquinoline which is a core structure of lamellarin alkaloids.

Asymmetric [4 + 2], [6 + 2], and [6 + 4] Cycloadditions of Isomeric Formyl Cycloheptatrienes Catalyzed by a Chiral Diamine Catalyst

Novel asymmetric aminocatalytic cycloadditions are described between formyl cycloheptatrienes and 6,6-dimethylfulvene that lead to [4 + 2], [6 + 2], and [4 + 6] cycloadducts. The unprecedented reaction course is dependent on the position of the formyl functionality in the cycloheptatriene core, and each formyl cycloheptatriene isomer displays a distinct reactivity pattern. The formyl cycloheptatriene isomers are activated by a chiral primary diamine catalyst, and the activation mode is dependent on the position of the formyl functionality relative to the cycloheptatriene core. The [4 + 2] and [6 + 2] cycloadducts are formed via rare iminocatalytic inverse electron-demand cycloadditions, while the [4 + 6] cycloadduct is formed by a normal electron-demand cycloaddition. The reactivity displayed by the different formyl cycloheptatrienes was investigated by DFT calculations. These computational studies account for the different reaction paths for the three isomeric formyl cycloheptatrienes. The aminocatalytic [4 + 2], [6 + 2], and [4 + 6] cycloadditions proceed by stepwise processes, and the interplay between conjugation, substrate distortion, and dispersive interactions between the fulvene and aminocatalyst mainly defines the outcome of each cycloaddition.

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.

5-Substituted-furan-2(3H)-ones in [8 + 2]-Cycloaddition with 8,8-Dicyanoheptafulvene

This study demonstrates the use of organocatalytic Brønsted base activation of 5-substituted-furan-2(3H)-ones to generate 2π-components for the diastereoselective [8 + 2]-cycloaddition involving 8,8-dicyanoheptafulvene as an 8π-component. The use of dienolates in a higher-order cycloaddition reaction leads to the formation of biologically relevant polycyclic products bearing a γ-butyrolactone structural motif, thus broadening the synthetic potential of Brønsted base activated higher-order cycloadditions.

Three-Component, Diastereoselective [6 + 3] Annulation of Tropone, Imino Esters, and Arynes

A transition-metal-free, three-component, and diastereoselective [6 + 3] annulation reaction employing tropone, imino esters, and arynes allowing the synthesis of bridged azabicyclo[4.3.1]decadienes is demonstrated. The key nitrogen ylides for the [6 + 3] annulation were generated by the addition of imino esters to the arynes followed by a proton transfer. The nitrogen ylides undergo a regioselective addition to tropone to furnish the desired products in moderate to good yields with good functional group tolerance under mild conditions.

Aminocatalytic [8+2] Cycloaddition Reactions toward Chiral Cyclazines

An efficient and exceptionally stereoselective synthesis of chiral cycl[3.2.2]azines has been realized by means of the rational design and utilization of novel (E)-3-benzylidene-3H-pyrrolizines in iminium-ion-catalyzed [8+2] cycloaddition reactions. The presented protocol allows for the incorporation of diverse enals, including cinnamaldehydes, enolizable aldehydes, and substrates of extended conjugation. The obtained products contain both an electron-rich alkenyl pyrrole moiety and an electron-deficient carbaldehyde substituent, and both moieties can undergo selective transformations with retention of the stereochemical information established in the [8+2] cycloaddition.

Construction of bridged polycycles through dearomatization strategies

Bridged polycycles are privileged molecular skeletons with wide occurrence in bioactive natural products and pharmaceuticals. Therefore, they have been the pursing target molecules of numerous chemists. The rapid and convenient generation of sp3-rich complex three-dimensional molecular skeletons from simple and easily available aromatics has made dearomatization a highly valuable synthetic tool for the construction of rigid and challenging bridged rings. This review summarizes the-state-of-the-art advances of dearomatization strategies in the application of bridged ring formation, discusses their advantages and limitations and the in-depth mechanism, and highlights their synthetic value in the total synthesis of natural products. We wish this review will provide an important reference for medicinal and synthetic chemists and will inspire further development in this intriguing research area.

[8+2] vs [4+2] Cycloadditions of Cyclohexadienamines to Tropone and Heptafulvenes-Mechanisms and Selectivities

The cinchona-alkaloid-catalyzed cycloaddition reactions of 2-cyclohexenone with tropone and various heptafulvenes give [8+2] or [4+2] cycloadducts, depending on the substituents present on the heptafulvene. We report the results of new experiments with heptafulvenes, containing diester and barbiturate substituents, which in combination with computational studies were performed to elucidate the factors controlling [8+2] vs [4+2] cycloaddition pathways, including chemo-, regio-, and stereoselectivities of these higher-order cycloadditions. The protonated cinchona alkaloid primary amine catalyst reacts with 2-cyclohexenone to form a linear dienamine intermediate that subsequently undergoes a stepwise [8+2] or [4+2] cycloaddition. Both tropone and the different heptafulvenes initially form [8+2] cycloadducts. The final product is ultimately decided by the reversibility of the [8+2] cycloaddition and the relative thermal stability of the [4+2] products. The stereoisomeric transition states are distinguished by the steric interactions between the protonated catalyst and tropone/heptafulvenes. The [8+2] cycloaddition of barbiturate-heptafulvene afforded products with an unprecedented trans-fusion of the five- and six-membered rings, while the [8+2] cycloadducts obtained from cyanoester-heptafulvene and diester-heptafulvene were formed with a cis-relationship. The mechanism, thermodynamics, and origins of stereoselectivity were explained through DFT calculations using the ωB97X-D density functional.

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.