Rhodium-Catalyzed Enantioselective and Desymmetrizative Pauson-Khand Reaction: Access to Tricyclo[6.2.1.04,11]undecenes

Stereoselective construction of 5-6-5 aza-tricyclic scaffolds via catalytic asymmetric aza-Piancatelli/Diels-Alder reactions

A chiral Brønsted acid-catalyzed asymmetric cascade aza-Piancatelli rearrangement/intramolecular Diels-Alder reaction has been developed. This method enables the atom- and step-efficient synthesis of chiral aza-[5,6,5]-tricyclic derivatives with multiple contiguous stereocenters in a highly enantioselective manner from readily available N-pentadienylanilines and 2-furylcarbinols.

Theoretical Studies on the Competing Mechanism and Origin of Diastereoselectivity of NHC-Catalyzed Intramolecular [3 + 2] Annulations of Ynals

Chiral tricyclic 6,5,5-fused rings exhibit structural diversity and possess important biological activities in the synthesis of natural products. However, predicting the possible mechanism and origin of stereoselectivity in these reactions remains a challenge. In this article, we conducted a theoretical investigation into the NHC-catalyzed intramolecular [3 + 2] annulations of ynals to generate tricyclic 6,5,5-fused rings. Our calculations revealed that NHC could nucleophilically attack the carbonyl group of the ynal reactant, leading to the formation of a Breslow intermediate via a 1,2-proton transfer. Subsequently, an intramolecular Michael addition takes place, resulting in a 6-5 bicyclic intermediate. We then compared the competitive processes involving proton transfer and the Mannich reaction. The more energetically favorable process involves an HOAc-assisted proton transfer process, followed by the Mannich reaction. To ascertain the origin of the diastereoselectivity, we performed noncovalent interaction (NCI) and atom-in-molecule (AIM) analyses. This work is useful for understanding the general principles and detailed mechanisms of the synthesis of chiral 6,5,5-fused tricyclic scaffolds with unique diastereoselectivity.

Stereoselective transition metal-catalyzed [(2+2)+1] and [(2+2)+2] carbocyclization reactions using 1,6-enynes with 1,1-disubstituted olefins: construction of quaternary centers

Transition metal-catalyzed carbocyclization reactions provide a powerful method for the stereoselective assembly of complex, highly substituted (poly)cyclic scaffolds. Although 1,6-enynes are common substrates for these transformations, using polysubstituted alkene derivatives to construct functionalized cyclic products remains challenging due to their significantly lower reactivity. This Perspective highlights key developments in stereoselective semi-intramolecular metal-catalyzed [(2+2)+1] and [(2+2)+2] carbocyclizations of 1,6-enynes containing 1,1-disubstituted alkenes, which produce cycloadducts with quaternary stereogenic centers. The insights gleaned from these examples provide a blueprint for developing more general carbocyclization strategies with challenging polysubstituted olefins.

Pd-Catalyzed [2 + 2 + 2] Cyclization of Alkyne-cyclohexadienones and O-Akynyl Benzenesulfonamides for Construction of Fused Tricyclic Hydronaphthofurans

A palladium-catalyzed [2 + 2 + 2] cyclization of 1,6-enynes with unsymmetrical alkynes has been successfully accomplished, resulting in the formation of a series of fused tricyclic hydronaphthofurans with high stereo- and regioselectivity in a single step. This reaction demonstrates 100% atomic economy and exhibits a broad substrate scope.

Systematic Parameter Determination Aimed at a Catalyst-Controlled Asymmetric Rh(I)-Catalyzed Pauson-Khand Reaction

Transition metal-catalyzed carbocyclization reactions have revolutionized the synthesis of complex cyclic organic compounds. Yet, subtle substrate changes can significantly alter reaction pathways. The asymmetric Rh(I)-catalyzed Pauson-Khand reaction (PKR) exemplifies such a reaction, hindered by a narrow substrate scope and competing reactivity modes. In this study, we identified parameters predictive of the yield and enantioselectivity in the catalyst-controlled asymmetric PKR, using 1,6-enynes with a 2,2-disubstituted alkene. In this way, ring-fused cyclopentenones can be formed with chiral quaternary carbon centers. Using bisphosphine ligand parameters from palladium complexes, including the energy of the Pd lone pair orbital and the angle formed by the phosphorus aryl groups on the ligand, we established strong correlations with experimental ln(er) (R 2 = 0.99 and 0.91) for two distinct precursors. Solvent dipole moments correlated with ln(er) for high-dipole-moment precursors (R 2 = 0.94), while Abraham's hydrogen bond basicity is more relevant for low-dipole-moment precursors (R 2 = 0.93). Additionally, counterions were found to have a significant impact on the PKR reactivity and selectivity, as does the steric demand of the alkyne substituent of the enyne precursor. In the latter case, ln(er) correlates with Sterimol B1 values for products from different alkyne substituents (R 2 = 0.99). Furthermore, the computed C≡C wavenumber of the enyne precursor can be directly aligned with the yield of asymmetric PKRs.

N-Heterocyclic Carbene-Catalyzed Formal Intramolecular [3 + 2] Annulations of Cyclohexadienone-Tethered Ynals

A formal [3 + 2] annulation of cyclohexadienone-tethered ynals is enabled by an N-heterocyclic carbene (NHC) catalyst, affording a tricyclo[6.2.1.04,11]undecane framework. This study represents the first demonstration of using C═C double bonds as the reaction partner in the NHC-catalyzed annulation of ynals. This strategy is characterized by mild reaction conditions and 100% atom economy as well as high catalytic performance and efficiency.

Recent Progress in Asymmetric Domino Intramolecular Cyclization/Cascade Reactions of Substituted Olefins

The domino cyclization/coupling strategy is one of the most effective methods to produce cyclized and multi-functionalized compounds from olefins, which has attracted huge attention from chemists and biochemists especially for its considerable potential of enantiocontrol. Nowadays, more and more studies are developed to achieve difunctionalization of substituted olefins through an asymmetric domino intramolecular cyclization/cascade reaction, which is still an elegant choice to accomplish several synthetic ideas such as complex natural products and drugs. This review surveys the recent advances in this field through reaction type classification. It might serve as useful knowledge desktop for the community and accelerate their research.

Catalyst-Controlled Diastereoselective Synthesis of Bridged [3.3.1] Bis(Indolyl)-Oxanes and Oxepanes via Desymmetrization of Bis(Indolyl)-Cyclohexadienones

A catalyst-controlled divergent synthesis of bridged [3.3.1] bis(indolyl)-oxanes and cis-[6.7] fused bis(indolyl) oxepanes via diastereoselective desymmetrization of bis(indolyl)-cyclohexadienones is presented for the first time. The reaction is highly atom- and step-economic, furnishing sp³-rich functionalized bis(indolyl) derivatives in good to excellent yields with wide substrate scope. The reaction proceeds through Friedel-Crafts alkylation followed by catalyst-controlled selective C-C bond formation/rearrangement. Gram scale synthesis and synthetic utility to generate bis(indolyl) alkaloid-like molecular diversity were also illustrated.

Asymmetric synthesis of tricyclic 6,5,5-fused polycycles by desymmetric Pauson-Khand reaction

A new rhodium(I)-BINAP-catalyzed enantioselective desymmetric Pauson-Khand (PK) reaction of prochiral 1,6-enynes with CO has been developed. Diverse tricyclic 6,5,5-fused bowl type polycycles with three continuous chiral centers were synthesized in...