Asymmetric (4+n) Cycloadditions of Indolyldimethanols for the Synthesis of Enantioenriched Indole-Fused Rings

Switching between the [2π+2σ] and Hetero-[4π+2σ] Cycloaddition Reactivity of Bicyclobutanes with Lewis Acid Catalysts Enables the Synthesis of Spirocycles and Bridged Heterocycles

The exploration of the complex chemical diversity of bicyclo[n.1.1]alkanes and their use as benzene bioisosteres has garnered significant attention over the past two decades. Regiodivergent syntheses of thiabicyclo[4.1.1]octanes (S-BCOs) and highly substituted bicyclo[2.1.1]hexanes (BCHs) using a Lewis acid-catalyzed formal cycloaddition of bicyclobutanes (BCBs) and 3-benzylideneindoline-2-thione derivatives have been established. The first hetero-(4+3) cycloaddition of BCBs, catalyzed by Zn(OTf)2, was achieved with a broad substrate scope under mild conditions. In contrast, the less electrophilic BCB ester undergoes a Sc(OTf)3-catalyzed [2π+2σ] reaction with 1,1,2-trisubstituted alkenes, yielding BCHs with a spirocyclic quaternary carbon center. Control experiments and preliminary theoretical calculations suggest that the diastereoselective [2π+2σ] product formation may involve a concerted cycloaddition between a zwitterionic intermediate and E-1,1,2-trisubstituted alkenes. Additionally, the hetero-(4+3) cycloaddition may involve a concerted nucleophilic ring-opening mechanism.

In(OTf)3-catalyzed formal (4 + 3) cycloaddition reactions of 3-benzylideneindoline-2-thiones with 2-indolylmethanols

We report the In(OTf)3-catalyzed formal (4 + 3) cycloaddition of 3-benzylideneindoline-2-thiones with 2-indolylmethanols. This reaction not only broadens the synthetic utility of 3-benzylideneindoline-2-thiones as scarce indole-based sulfur-containing four-atom building blocks, but also provides a rapid and facile access to synthesize diindole-annulated tetrahydrothiepines.

Cooperative Catalysis-Enabled (4 + 3) Cycloaddition of 2-Indolylmethanols with In Situ-Generated ortho-Naphthoquinone Methides

A cooperative catalysis-enabled (4 + 3) cycloaddition of 2-indolylmethanols with ortho-naphthoquinone methides (o-NQMs), which were in situ-generated from enynones, has been established in the presence of silver/Brønsted acid cocatalysts. In the reaction pathway, the key o-NQM intermediates were formed through Ag(I)-catalyzed cyclization of enynones, while the indole-based carbocation intermediates were generated via Brønsted acid-catalyzed dehydration of 2-indolylmethanols. By this approach, a wide range of seven-membered cyclohepta[b]indoles were synthesized in good yields with high efficiency under mild reaction conditions, which serves as a useful strategy toward constructing indole-fused seven-membered rings. Moreover, the catalytic asymmetric version of this (4 + 3) cycloaddition has been realized under the cooperative catalysis of Ag(I) with chiral phosphoric acid, which offered chiral cyclohepta[b]indole with a good enantioselectivity (75% ee). This work not only represents the first cooperative catalysis-enabled (4 + 3) cycloaddition of 2-indolylmethanols but also provides a good example for o-NQM-involved cycloadditions, which will contribute to the chemistry of 2-indolylmethanols and enrich the research contents of cooperative catalysis.

Enantioselective Tsuji-Trost α-Fluoroallylation of Amino Acid Esters with Gem-Difluorinated Cyclopropanes

A novel enantioselective Tsuji-Trost-type cross coupling reaction between gem-difluorinated cyclopropanes and N-unprotected amino acid esters enabled by synergistic Pd/Ni/chiral aldehyde catalysis is presented herein. This transformation streamlined the diversity-oriented synthesis (DOS) of optically active α-quaternary α-amino acid esters bearing a linear 2-fluoroallylic motif, which served as an appealing platform for the construction of other valuable enantioenriched compounds. The key intermediates were confirmed by HRMS detection, while DFT calculations revealed that the excellent enantioselectivity was attributed to the stabilizing non-covalent interactions between the Pd(II)-π-fluoroallyl species and the Ni(II)-Schiff base complex.

Design and Catalytic Asymmetric Synthesis of Furan-Indole Compounds Bearing both Axial and Central Chirality

In the chemistry community, catalytic asymmetric synthesis of furan-based compounds bearing both axial and central chirality has proven to be a significant but challenging issue owing to the importance and difficulty in constructing such frameworks. In this work, we have realized the first catalytic asymmetric synthesis of five-five-membered furan-based compounds bearing both axial and central chirality via organocatalytic asymmetric (2+4) annulation of achiral furan-indoles with 2,3-indolyldimethanols with uncommon regioselectivity. By this strategy, furan-indole compounds bearing both axial and central chirality were synthesized in high yields with excellent regio-, diastereo-, and enantioselectivities. Moreover, theoretical calculations were conducted to provide an in-depth understanding of the reaction pathway, activation mode, and the origin of the selectivity.

Asymmetric Organocatalyzed Cyclization Cascade Reactions of 3,3-Difluoro-2-aryl-3H-indoles and Enamides

The direct functionalization of β-C(sp2)-H bonds in enamides has garnered increasing attention within the realm of organic synthesis. However, these remarkable advancements are predominantly dependent on transition metals; limited success has been achieved via organocatalytic catalysis. Herein, we report a CPA-catalyzed β-C(sp2)-H functionalization of enamides cascade intramolecular cyclization to synthesize the chiral dihydropyrimido[1,6-a]indoles bearing gem-difluoromethylene. Moreover, this methodology enables the synthesis of diverse chiral dihydropyrimido[1,6-a]indoles with outstanding enantioselectivities in moderate to high yields.

Atroposelective Synthesis of Planar-Chiral Indoles via Carbene Catalyzed Macrocyclization

Indole-based planar-chiral macrocycles are widely found in natural products and bioactive molecules. However, in sharp contrast to the preparation of indole-based axially chiral structures, the enantioselective catalysis of indole-based planar-chiral macrocycles is still a formidable task so far. Here we report an N-heterocyclic carbene (NHC)-catalyzed intramolecular atroposelective macrocyclization of 3-carboxaldehyde indole/pyrroles, featuring with broad substrate scope and good functional group tolerance, and allowing for a rapid access to diverse indole/pyrrole-based planar-chiral macrocycles with various tether-lengths (10-16 members) in good yields and with excellent enantioselectivities. Importantly, the indole-based macrocyclic structures with both planar and axial chirality were directly and efficiently obtained through this protocol with excellent enantioselectivities and diastereoselectivities. In addition, these synthesized planar-chiral macrocycles offer many possibilities for chemists to develop new catalysts or ligands, as well as new reactions.

Organocatalytic Enantioselective Nucleophilic Addition of Indole Imine 5-Methides

Despite the enormous developments in the asymmetric transformations of indole imine methides (IIMs), the remote asymmetric induction involving IIMs remains challenging due to the spatial interaction requirement between the substrate and catalyst. Herein we report the first catalytic asymmetric nucleophilic addition to indole imine 5-methide (5-IIM), the only topological isomer of IIMs whose asymmetric addition remains unknown. Despite the challenging remote stereocontrol, high efficiency and respectable enantioselectivity were achieved to provide access to a range of enantioenriched indole-containing triaryl alkanes.

A General Protocol toward Synthesis of 3-Methylindoles Using Acenaphthoimidazolyidene-Ligated Oxazoline Palladacycle

An efficient catalytic strategy toward the synthesis of N-substituted 3-methylindoles from inactive o-dihaloarenes and N-allylamines was developed by using a 1,3-bis(2,6-diisopropylphenyl)acenaphthoimidazol-2-ylidene (AnIPr)-ligated oxazoline palladacycle. It enabled a very broad substrate scope tolerating different functional groups, electronic properties, and steric bulkiness and afforded desired products in good to excellent yields. Importantly, it showed great potential to synthesize several bioactive compounds and key intermediates of natural products in high yields.

Enantioselective Palladium(II)-Catalyzed Desymmetrizative Coupling of 7-Azabenzonorbornadienes with Alkynylanilines

A PdII -catalyzed, domino enantioselective desymmetrizative coupling of 7-azabenzonorbornadienes with alkynylanilines is disclosed herein. This operationally simple transformation generates three covalent bonds and two contiguous stereocenters with excellent enantio- and diastereo-selectivity. The resulting functionalized indole-dihydronaphthalene-amine conjugates served as an appealing platform to streamline the diversity-oriented synthesis (DOS) of other valuable enantioenriched compounds. DFT calculations revealed that the two stabilizing non-covalent interactions contributed to the observed enantioselectivity.

The rapid construction and biological evaluation of densely substituted pyrrolo[1,2-a]indoles via a BF3·OEt2-assisted cascade approach

Lewis-acid cascade reactions promoted by BF3·OEt2 are reported for the synthesis of highly substituted pyrrolo[1,2-a]indoles and congeners of benzofuro[2,3-b]indoles. These reactions are highly regio- and diastereoselective towards generating up to five contiguous stereogenic centers, including two vicinal quaternary centers. Furthermore, an established cascade approach and the mechanism proposed herein are well supported by quantum chemistry calculations. In addition, a self-dimerization intermediate was trapped and isolated to establish a strategy for potential access to both pyrrolo and benzo indole derivatives, leaving sufficient freedom for broadening. Furthermore, in-silico molecular docking and all atomistic molecular dynamic (MD) simulation analysis suggests that the synthesized pyrrolo[1,2-a]indole derivatives stably bind at the active site of the mycobacterial secreted tyrosine phosphatase B (MptpB) enzyme, an emerging anti-mycobacterial drug target. Deep learning-based affinity predictions and MMPBGBSA-based energy calculations of the docked poses are presented herein.

Organocatalytic Diastereoselective (4 + 1) Cycloaddition of o-Hydroxyphenyl-Substituted Secondary Phosphine Oxides

The first organocatalytic diastereoselective (4 + 1) cycloaddition of o-hydroxyphenyl-substituted secondary phosphine oxides (SPOs) has been established, which makes use of o-hydroxyphenyl substituted SPOs as suitable four-atom phosphorus-containing 1,4-dinucleophiles and 3-indolylformaldehydes as competent 1,1-dielectrophiles under Bro̷nsted acid catalysis. The reaction mechanism was suggested to involve the formation of 3-indolylmethanol intermediates and vinyliminium intermediates, which played an important role in controlling the reactivity and diastereoselectivity of the (4 + 1) cycloaddition under Bro̷nsted acid catalysis. By this approach, a series of benzo oxaphospholes bearing P- and C-stereocenters were synthesized in moderate to good yields (50%-95% yields) with excellent diastereoselectivities (all >95:5 dr). This reaction not only represents the first organocatalytic diastereoselective (4 + 1) cycloaddition of o-hydroxyphenyl-substituted SPOs but also provides an efficient and diastereoselective method for the construction of phosphorus-containing benzo five-membered heterocyclic skeletons bearing both P-stereocenter and C-stereocenter.

Indole-Fused Benzoxepine Synthesis via Visible-Light-Driven Aerobic Dehydrogenative [5 + 2] Annulation

A photocatalyzed oxidative dehydrogenative annulation between 2-(1H-indol-2-yl)phenols and alkenylphenols is presented. Various indole-fused benzoxepines can be obtained at room temperature using atom-efficient strategies. This method not only avoids the use of stoichiometric amounts of oxidants but also exhibits excellent atom economy by generating H2O as the only theoretical byproduct.

Yb(OTf)3-Catalyzed Formal (4 + 3) Cycloaddition Reactions of 3-Benzylideneindoline-2-thiones with Donor-Acceptor Cyclopropanes

A Yb(OTf)3-catalyzed formal (4 + 3) cycloaddition reaction of donor-acceptor cyclopropanes with 3-benzylideneindoline-2-thiones as sulfur-containing 4π components has been successfully achieved. A series of functionalized 5,10-dihydro-2H-thiepino[2,3-b]indole derivatives were synthesized with good yields and moderate to good diastereoselectivity. The reaction described herein represented the inaugural (4 + 3) cycloaddition of 3-benzylideneindoline-2-thiones.