Catalytic Asymmetric [4 + 1] Cyclization of Benzofuran-Derived Azadienes with 3-Chlorooxindoles.

A chiral guanidine-catalyzed asymmetric [4 + 1] cyclization of benzofuran-derived azadienes with 3-chlorooxindoles has been established, which constructed chiral spirooxindole frameworks with in situ generation of a five-membered ring with high diastereoselectivities (up to >95:5 dr) and good enantioselectivities (up to 94:6 er). This reaction represents the first catalytic asymmetric [4 + 1] cyclization of benzofuran-derived azadienes, which will enrich the research field of catalytic asymmetric cyclizations of such reactants. In addition, this reaction provides a useful strategy for the enantioselective construction of five-membered ring-based chiral spirooxindole scaffolds.

Organocatalytic [4 + 2] cyclizations of para-quinone methide derivatives with isocyanates.

The first [4 + 2] cyclizations of para-quinone methide derivatives with isocyanates have been established under the catalysis of Brønsted acids or Brønsted bases, which efficiently constructed benzoxazin-2-one frameworks in generally high yields (up to 95%). This reaction will not only enrich the research contents of para-quinone methide-involved cyclization reactions but also provide a useful method for constructing biologically important benzoxazin-2-one frameworks.

Catalytic Asymmetric [4+2] Cycloaddition of in Situ Generated o-Quinone Methide Imines with o-Hydroxystyrenes: Diastereo- and Enantioselective Construction of Tetrahydroquinoline Frameworks.

A catalytic asymmetric [4+2] cycloaddition of ortho-quinone methide imines in situ generated from o-aminobenzyl alcohols with o-hydroxystyrenes has been established under the catalysis of chiral phosphoramide, which afforded chiral tetrahydroquinolines in moderate to good yields, good enantioselectivities, and excellent diastereoselectivities (up to 82% yield, 93:7 er, all >95:5 dr). In this catalytic asymmetric [4+2] cycloaddition, the hydrogen-bonding interaction between chiral phosphoramide and two substrates was proposed to play a crucial role in controlling the enantioselectivity. This reaction not only provides a useful approach for constructing chiral tetrahydroquinoline frameworks, but also demonstrates the great practicability of ortho-quinone methide imines in catalytic asymmetric cycloadditions.

Metal-Catalyzed Oxa-[4+2] Cyclizations of Quinone Methides with Alkynyl Benzyl Alcohols.

An oxa-[4+2] cyclization of quinone methides with alkynyl benzyl alcohols has been realized in the presence of a metal catalyst, and the reaction afforded spiroacetal products in overall high yields (up to 99%) and good diastereoselectivities (up to >95:5 dr). By carrying out the reaction under gold catalysis and utilizing alkynyl benzyl alcohols as electron-rich reaction partners, this approach provides a useful strategy for settling the challenges in oxa-[4+2] cyclization of para-quinone methide derivatives. This reaction serves as a good example for metal-catalyzed oxa-[4+2] cyclizations of quinone methides. In addition, it also offers a useful method for the construction of spiroacetal skeletons.

Application of Naphthylindole-Derived Phosphines as Organocatalysts in [4 + 1] Cyclizations of o-Quinone Methides with Morita-Baylis-Hillman Carbonates.

An organocatalytic [4 + 1] cyclization of o-QMs with MBH carbonates has been established using naphthylindole-derived phosphine (NIP) as an organocatalyst. By using this approach, a series of 2,3-dihydrobenzofuran derivatives have been synthesized in high yields and excellent diastereoselectivities (up to 99% yield, >95:5 dr). This reaction not only has established the first [4 + 1] cyclization of o-QMs with MBH carbonates but also represents the first application of naphthylindole-derived phosphines as organocatalysts in catalytic reactions. In addition, this reaction has also provided a useful method for constructing 2,3-dihydrobenzofuran scaffolds.

A catalytic asymmetric interrupted Nazarov-type cyclization of 2-indolylmethanols with cyclic enaminones.

A chiral phosphoric acid-catalyzed asymmetric interrupted Nazarov-type cyclization of C3-alkenyl-substituted 2-indolylmethanols has been established by using cyclic enaminones as nucleophiles, which afforded chiral cyclopenta[b]indole derivatives with excellent diastereoselectivities and moderate to good enantioselectivities. This reaction will not only enrich the research contents of indolylmethanol-involved catalytic asymmetric transformations, but also advance the chemistry of catalytic asymmetric interrupted Nazarov-type cyclizations. In addition, this reaction will also provide a useful method for synthesizing chiral cyclopenta[b]indole derivatives.

Design and Enantioselective Construction of Axially Chiral Naphthyl-Indole Skeletons.

The first enantioselective construction of a new class of axially chiral naphthyl-indole skeletons has been established by organocatalytic asymmetric coupling reactions of 2-naphthols with 2-indolylmethanols (up to 99 % yield, 97:3 e.r.). This approach not only affords a new type of axially chiral heterobiaryl backbone, but also provides a new catalytic enantioselective strategy for constructing axially chiral biaryl scaffolds by making use of the C3-electrophilicity of 2-indolylmethanols.

Gallium Bromide-Promoted Dearomative Indole Insertion in 3-Indolylmethanols: Chemoselective and (Z/E)-Selective Synthesis of 3,3'-Bisindole Derivatives.

Gallium bromide (GaBr3)-promoted dearomative indole insertion in 3-indolylmethanols has been established, which chemoselectively constructs a biologically important 3,3'-bisindole framework bearing an all-carbon quaternary center in high yields and excellent (Z)-selectivities (up to 99% yield, all >95:5 Z/E). The reaction pathway was suggested to include a tandem sequence of Michael addition/C-C bond cleavage/nucleophilic addition, wherein the strong acidity of GaBr3 played a crucial role in the key step of C-C bond cleavage. This reaction not only provides a new strategy for dearomatization of indoles, but also represents a new reaction category for 3-indolylmethanols, which involves a rarely reported late-stage C-C bond cleavage of 3-indolylmethanol derivatives. In addition, this approach also offers an efficient method for the synthesis of biologically important 3,3'-bisindole derivatives.

Enantioselective Construction of Spiro[indoline-3,2'-pyrrole] Framework via Catalytic Asymmetric 1,3-Dipolar Cycloadditions Using Allenes as Equivalents of Alkynes.

The first catalytic asymmetric 1,3-dipolar cycloadditions (1,3-DCs) of isatin-derived azomethine ylide with allenes have been established, which efficiently assembly isatins, amino-esters and 2,3-allenoate into enantioenriched spiro[indoline-3,2'-pyrrole] derivatives with a quaternary stereogenic center in generally high enantioselectivities (80-98% ee). In this allene-involved 1,3-DC, an unexpected spirooxindole framework bearing an intra-annular C═C double bond was generated, which is quite different from previously reported 1,3-DCs of allenes. This approach not only confronted the great challenge in using allenes as dipolarophiles of 1,3-DCs, but also provided a unique strategy of using allenes as equivalents of alkynes to construct spiro[indoline-3,2'-pyrrole] structure. Besides, this reaction also represents the first catalytic asymmetric ketone-involved 1,3-DCs of allenes, which will also greatly enrich the research contents of 1,3-DCs, the chemistry of allenes as well as the synthetic methods of spirooxindoles.

Catalytic asymmetric formal [3+3] cycloaddition of an azomethine ylide with 3-indolylmethanol: enantioselective construction of a six-membered piperidine framework.

A catalytic asymmetric formal [3+3] cycloaddition of 3-indolylmethanol and an in situ-generated azomethine ylide has been established to construct a chiral six-membered piperidine framework with two stereogenic centers. This approach not only represents the first enantioselective cycloaddition of isatin-derived 3-indolylmethanol, but also has realized an unusual enantioselective formal [3+3] cycloaddition of azomethine ylide rather than its common [3+2] cycloadditions. Besides, this protocol combines the merits of a multicomponent reaction and organocatalysis, which efficiently assembles a variety of isatin-derived 3-indolylmethanols, aldehydes, and amino esters into structurally diverse spiro[indoline-3,4'-pyridoindoles] with one all-carbon quaternary stereogenic center in high yields and excellent enantioselectivities (up to 93 % yield, >99 % enantiomeric excess (ee)). Although the diastereoselectivity of the reaction is generally moderate, most of the diastereomers can be separated by using column chromatography followed by preparative TLC.

Organocatalytic asymmetric inverse-electron-demand 1,3-dipolar cycloaddition of N,N'-cyclic azomethine imines.

The first organocatalytic asymmetric inverse-electron-demand 1,3-dipolar cycloaddition (IED 1,3-DC) of N,N'-cyclic azomethine imines has been established in the presence of chiral phosphoric acid. This approach assembles N,N'-cyclic azomethine imines and o-hydroxystyrenes into chiral N,N-bicyclic pyrazolidin-3-one derivatives with the creation of two stereogenic centers, one of which is quaternary, in excellent diastereoselectivities and good enantioselectivities (up to >95:5 dr, 88:12 er). The investigation of the activation mode of the reaction revealed that the dual hydrogen-bonding interaction between the two substrates and the catalyst together with the conjugative effect initiated by the o-hydroxyl group played a crucial role in the designed IED 1,3-DC. This study will not only greatly enrich the underdeveloped research potential of catalytic asymmetric IED 1,3-DCs but will also facilitate the design of other enantioselective IED 1,3-DCs based on different activation modes.

Organocatalytic atroposelective synthesis of axially chiral N,N'-pyrrolylindoles via de novo indole formation.

The first organocatalytic atroposelective synthesis of axially chiral N,N'-pyrrolylindoles based on o-alkynylanilines was successfully established via de novo indole formation catalyzed by chiral phosphoric acid (CPA). This new synthetic strategy introduced CPA-catalyzed asymmetric 5-endo-dig cyclization of new well-designed o-alkynylanilines containing a pyrrolyl unit, resulting in a wide range of axially chiral N,N'-pyrrolylindoles in high yields with exclusive regioselectivity and excellent enantioselectivity (up to 99% yield, >20 : 1 rr, 95 : 5 er). Considering the potential biological significance of N-N atropisomers, preliminary biological activity studies were performed and revealed that these structurally important N,N'-pyrrolylindoles had a low IC50 value with promising impressive cytotoxicity against several kinds of cancer cell lines. DFT studies reveal that the N-nucleophilic cyclization mediated by CPA is the rate- and stereo-determining step, in which ligand-substrate dispersion interactions facilitate the axial chirality of the target products.

Asymmetric Synthesis of Axially Chiral Naphthyl-C3-indoles via a Palladium-Catalyzed Cacchi Reaction.

Atropoisomeric biaryl motifs are widely found in natural products and bioactive compounds as well as chiral catalysts and ligands. Various efficient approaches have been disclosed for the construction of chiral six-six biaryl skeletons. In contrast, the enantioselective synthesis of axially chiral arylindoles through the strategy of de novo construction, other than the asymmetric functionalization of indoles, remain a challenging task. Herein we report an efficient Pd(0)/(S)-Segphos-catalyzed atroposelective Cacchi reaction of 2-alkynylanilines with sterically congested naphthyl halides, which afforded an array of naphthyl-C3-indoles in high yields with good to excellent atroposelectivities. The addition of water and the modulation of the manipulation procedure by premixing the palladium complex and the naphthyl halide were the keys to success. The conformational stability of the obtained axially chiral naphthyl-C3-indole containing a synthetically more-valuable free NH moiety is revealed through kinetic experiments.

Brønsted Acid-Catalyzed (4 + 3) Cyclization of N, N'-Cyclic Azomethine Imines with Isatoic Anhydrides.

A Brønsted acid-catalyzed (4 + 3) cyclization of N, N'-cyclic azomethine imines with isatoic anhydrides has been discovered, which constructs seven-membered nitrogenous heterocyclic frameworks with overall high yields (up to 98% yield). This reaction represents a rarely reported (4 + 3) cyclization of N, N'-cyclic azomethine imines, which involves the reassembly of a C-N bond. In addition, this reaction has also accomplished the unprecedented (4 + 3) cyclization of isatoic anhydrides.

Catalytic chemoselective [3+3] cycloadditions of azomethine ylides with quinone monoimides leading to the construction of a dihydrobenzoxazine scaffold.

A chemoselective [3+3] cycloaddition of in situ generated azomethine ylides with quinone monoimides has been established, which efficiently led to the construction of dihydrobenzoxazine frameworks with biological relevance, with excellent chemoselectivities and high yields (up to >95 : 5 cr and 98% yield).