A highly enantioselective [3+2] cyclization of 3‐vinylindoles with isatin‐derived 3‐indolylmethanols has been established in the presence of chiral phosphoric acid, which constructed the enantioenriched cyclopenta[b]indole scaffold linking a 3‐vinylindole moiety in generally good yields, considerable diastereoselectivities and high enantioselectivities (up to 93%, >95:5 dr, >99.5:0.5 er). More importantly, this reaction utilized N‐protected 3‐vinylindoles as competent reactants, which confronted the challenge in realizing a successful monoactivation mode of chiral phosphoric acid to vinyliminium in 3‐indolylmethanol‐involved enantioselective transformations. Besides, this protocol also settled the challenge in realizing a highly enantioselective [3+2] cyclization of 3‐vinylindoles with 3‐indolylmethanols, which would serve as a useful strategy for constructing an enantioenriched cyclopenta[b]indole scaffold linking a 3‐vinylindole moiety.
A novel synthetic method to access fused indolin‐3‐ones with a tetrasubstituted carbon stereocenter has been developed via NHC‐catalyzed umpolung formal [3+3] cycloaddtion of enals with isatogens. This methodology could be also applied for the quick construction of the 6‐5‐5 tricyclic pyrrolo[1,2‐a]indole skeleton which is frequently found as a core structure of many indole alkaloids.
A direct diastereoselective synthesis approach of important 9H‐pyrrolo[1,2‐a]azepin‐9‐amines was established via base‐promoted [4+3] annulation between donor–acceptor reagents derived from 1H‐pyrrole‐2‐carbaldehydes and alkyl 2‐aroyl‐1‐chlorocyclopropanecarboxylates. This transition metal‐free domino reaction proceeded quickly under mild basic conditions, affording potentially bioactive azepine derivatives in moderate to high yields with high diastereoselectivities (up to >20:1).
The first catalytic asymmetric [3+2] cycloaddition of 2‐indolylmethanols with 3‐vinylindoles has been established, which makes use of C‐3 unsubstituted 2‐indolylmethanol as a C3 building block in the presence of a chiral phosphoric acid. By using this strategy, biologically important cyclopenta[b]indole frameworks were efficiently constructed with regiospecificity in high yields (up to 99%), excellent diastereo‐ and enantioselectivities (up to >95:5 dr, 96:4 er). This reaction not only represents the first catalytic asymmetric [3+2] cycloaddition of C‐3 unsubstituted 2‐indolylmethanols, but also has confronted the great challenges in 2‐indolylmethanol‐involved enantioselective transformations.
The pyrrolo[3,2,1‐ij]quinoline heterocyclic core is found in the structure of a variety of compounds with interesting applications and then, new efficient and flexible strategies to construct this skeleton are required. Here, a new diastereoselective tetrafluoroboric acid (HBF4)‐catalyzed three‐component coupling reaction of 1H‐indole‐7‐carbaldehyde derivatives, anilines and electron‐rich alkenes to give pyrrolo[3,2,1‐ij]quinolines is described. The reaction involves an unusual [4+2]‐heterocyclization between an in situ formed imine and an alkene. The new catalytic method, where water is the only by‐product, is efficient, robust and flexible, and allows for multigram‐scale synthesis.
A Brønsted acid‐catalyzed [3+2] cyclodimerization of 3‐alkyl‐2‐vinylindoles has been established, which led to the diastereoselective construction of pyrroloindole frameworks in high yields (up to 98% yield, all >95:5 dr). This reaction not only represents the first application of 3‐alkyl‐2‐vinylindoles as nitrogen‐carbon‐carbon (NCC) building blocks, but also reveals a new [3+2] cyclodimerization of 2‐vinylindoles. Control experiments demonstrated that both the free N H group and the C‐3 alkyl group in the indole moiety play a crucial role in performing the [3+2] cyclodimerization. A preliminary attempt also demonstrated the feasibility of performing heterodimerization.
A straightforward and efficient method for the synthesis of nitrogen‐functionalized cyclopentane derivatives via [3+2] cycloaddition of enamines with donor‐acceptor cyclopropanes in the presence of catalytic amounts of various Lewis acids at room temperature has been developed; furthermore, the corresponding β‐amino acid was synthesized by monodecarboxylation and hydrogenolysis. An enantioenriched synthesis of nitrogen‐functionalized cyclopentane derivatives through dynamic kinetic asymmetric transformation of racemic donor‐acceptor cyclopropanes has also been achieved employing a copper complex [Cu(OTf)2‐ L1 ] as the catalyst affording an enantiomeric ratio up to 8:1.
A convenient gold‐catalyzed strategy for the synthesis of imidazo[1,2‐a]pyridine derivatives has been developed via gold carbene complexes. This transformation opens a new synthetic route to a variety of 3‐carbonyl‐substituted imidazo[1,2‐a]pyridines using air as oxidant affording the products in good yields.
Various 3‐azabicyclo[3.1.1]heptane derivatives were synthesized from Morita–Baylis–Hillman adduct‐derived 1,3‐dienes bearing a 4,4‐diaryl moiety through a thermal intramolecular [2+2] cycloaddition approach. By using the same approach, bicyclo[3.1.1]heptane, 3‐azabicyclo[3.2.0]heptane, and 3‐oxabicyclo[3.1.1]heptane derivatives could also be synthesized. A structurally similar dimethylallyl derivative underwent an intramolecular ene reaction to afford the pyrrolidine derivative.
trans‐2‐Aroyl‐3‐arylcyclopropane‐1,1‐dicarboxylates upon treatment with aluminium(III) chloride (AlCl3) underwent ring‐opening, fragmentation, recombination and lactonization to give highly substituted 2‐pyrones. Alternatively, when treated with titanium(IV) chloride (TiCl4), the cyclopropane diesters underwent a Nazarov cyclization to afford 1‐indanones with high diastereoselectivity.
A copper‐catalyzed cascade reaction involving trifluoromethylation of acrylamides coupled with ring closure and indole dearomatization is reported. This facile transformation was highly regioselective and proceeded under mild conditions, allowing efficient access to trifluoromethyl‐substituted spiro[indole‐3,3′‐pyrrolidine] derivatives, which are of increasing interest to the pharmaceutical industry.
The reaction of N‐[2.2]paracyclophanyl‐substituted amides or amines with phenyliodine diacetate (PIDA) and protic nucleophiles affords mixed para‐substituted [2.2]paracyclophane derivatives in moderate to good yields. As protic nucleophiles carboxylic acids and alcohols as well as pyridine hydrobromide can be used. 4‐Hydroxy[2.2]paracyclophane reacts in an analogous manner.
2‐Alkoxy‐2,3‐dihydrofurans were found to be versatile benzannulation reagents. Indoles can be synthesized in good to excellent yields via the [4+2] annulation of 2‐butoxy‐2,3‐dihydrofuran with pyrroles catalyzed by copper bromide. With the same protocol, carbazoles can also be obtained when indoles are used as starting material with the aid of p‐toluenesulfonic acid. This type of benzannulation reagent can also be used to synthesize benzofuran, benzothiophene and naphthalene derivatives in the presence of a catalytic amount of triflic acid in moderate yields. This benzannulation protocol features wide substrate scope and mild reaction condition. Moreover, most of examples have a good regioslectivity.
The Wittig reaction of isatin derivatives with Morita–Baylis–Hillman bromides of cinnamaldehydes afforded 3‐dienylidene‐2‐oxindoles. These trienes were converted into the corresponding spirooxindoles in a stereoselective manner in refluxing toluene in good yields. The diastereomeric spirooxindoles could be obtained stereoselectively by adding a catalytic amount of palladium(II) acetate via the palladium‐catalyzed isomerization of EEE‐trienes to ZEE‐trienes followed by a more facile 6π‐electrocyclization process. The obtained spirooxindoles could be further functionalized by palladium‐catalyzed oxidative arylation, thionation with Lawesson’s reagent, catalytic hydrogenation and Friedel–Crafts‐type reaction.
An efficient method for the synthesis of indole derivatives from readily available pyrimidyl‐substituted anilines and diazo compounds via rhodium(III)‐catalyzed C H bond activation has been developed. This cyclization reaction displays excellent functional group compatibility and regioselectivity, which overcomes some drawbacks of the classical indole synthetic methods and provides a facile approach for the construction of multi‐substituted indole derivatives. The redox‐neutral intermolecular annulation procedure comprises tandem C H bond activation, cyclization, and condensation steps, releasing water and nitrogen as by‐products.
The catalytic asymmetric [4+2] annulations of isatins with but‐3‐yn‐2‐one catalyzed by the Cinchona alkaloids‐derived oragnocatalyst (DHQD)2PHAL have been developed in the presence of 3.0 equivalents of D ‐diethyl tartrate in the mixed solvent (diphenyl ether/diethyl ether=1/1) or a slightly modified one, affording the corresponding substituted spiro[indoline‐3,2′‐pyran]‐2,4′(3′H)‐diones in good to excellent yields with high enantioselectivities under mild conditions.
We recently developed a completely new type of Rh‐catalyzed [5+2] cycloaddition by using 3‐acyloxy‐1,4‐enyne (ACE) as the 5‐carbon building block. In this update, we show that ACE can undergo intramolecular [5+2] cycloaddition with either an alkene or an allene in the presence of an appropriate rhodium catalyst and ligands to afford bicyclic compounds with multiple stereogenic centers. In most cases, cis‐fused bicyclo[5.3.0]decadienes are prepared highly diastereoselectively.
A straightforward approach for the chemodivergent synthesis of quinolines is described through site‐selective coupling of ortho‐aminoaryl ketones with α‐enolic dithioesters (DTEs) under solvent‐free conditions. The operationally and user‐simple one‐pot methodology is based on the trifunctional nature of DTEs. Both the carbonyl and the thiocarbonyl moiety in α‐enolic dithioesters were employed for the efficient construction of three differently substituted quinolines in a chemoselective manner simply by variation of an easy to handle acid catalyst.
The regioselectivity of the [3+2] cyclocondensation of trifluoromethyl‐α,β‐ynones with hydrazines can be readily tuned to preferentially afford either 3‐ or 5‐trifluoromethylpyrazoles through variation of the reaction conditions. Under catalysis with copper(II) acetate (2.0 mol%), cyclocondensation proceeded smoothly to yield 3‐trifluoromethylpyrazoles with high regioselectivity. In contrast, when the reaction was conducted in dimethyl sulfoxide under catalyst‐free conditions, the formation of 5‐trifluoromethylpyrazoles was predominantly observed.
Indole and indoline derivatives were selectively and temperature dependently synthesized via the intramolecular cross‐coupling reaction between the amino and aromatic bromine functionalities of 2‐bromophenethylamine derivatives in the presence of 10% palladium on carbon (Pd/C), 1,1′‐bis(diphenylphosphino)ferrocene (DPPF), and sodium tert‐butoxide (NaO‐t‐Bu) in mesitylene at 140 and 200 °C, respectively. The neutralization using acetic acid after formation of the indoline derivatives effectively promoted their aromatization, and the corresponding indole derivatives were obtained at 140 °C. Furthermore, various aryl groups were also introduced to the N‐1 position of the indole, pyrrole, and carbazole rings by their direct intramolecular arylation with aryl halides and a one‐pot protocol for N‐arylindole synthesis from 2‐bromophenethylamine was developed.
