Synthesis of Tetrahydroindolizino[8,7-b]indole Derivatives in the Presence of Fe(OTf)3 or CF3SO3H through Intramolecular Dearomatization of Indole

CuI-Catalyzed Dearomatization/Peroxidation/Cyclization Cascade of Pyrrole-Tethered Indoles

A mild CuI-catalyzed dearomatization/peroxidation/cyclization cascade of pyrrole-tethered indoles has been reached, providing peroxide-incorporated indolizino[8,7-b]indole derivatives in acceptable to good yields (46-76%). Dehydrogenated peroxide can be obtained by the use of a FeCl3/TBHP (tBuOOH)/2,2,2-trifluoroethanol (TFE) system at 50 °C.

FeCl3-catalyzed oxidative diselenylation of pyrrole-tethered indoles

A mild and efficient FeCl3-catalyzed oxidative diselenylation of pyrrole-tethered indoles has been achieved by using RSeSeR as the source of selenium and m-chloroperoxybenzoic acid (mCPBA) as the oxidant (17-70% yields). In addition, this selenylation reaction system can be expanded to the functionalization of tryptophan derivatives and melatonin at the C-2 position of the indole moiety (55-58% yields).

CuI-Catalyzed Selenylation of Pyrrolo[2,1-a]isoquinolines and Other Electron-Rich Heteroarenes

We have established a mild CuI-catalyzed selenylation of pyrrolo[2,1-a]isoquinoline derivatives in the presence of mCPBA (m-chloroperoxybenzoic acid) at ambient temperature. Corresponding organoselenides have been prepared readily in 53-92% yields. This process can also be expanded to the modification of pyrroles, azaindole, and indoles, delivering the desired heterocyclic selenides in moderate to good yields.

p-TsOH-Mediated Intramolecular C2-Arylation on NH-Indoles: Access of 5,10-Dihydroindeno[1,2-b]indoles

5,10-Dihydroindeno[1,2-b]indole has served as an important starting precursor for BARAC-fluor reagent in medicinal chemistry. Herein, an unprecedented p-TsOH assisted intramolecular C2-arylation of NH-indoles via C(sp2)-CN/C(sp2)-H coupling, offering a series of 5,10-dihydroindeno[1,2-b]indoles with moderate to good yields, has been showcased under redox-neutral conditions. Furthermore, successful scalability and synthetic applications highlight the practical nature of the method.

POCl3/Sulfoxide-Promoted Synthesis of Indolizino[8,7-b]indoles

A mild chlorocyclization of pyrrole-tethered indoles has been realized using POCl3 as the chlorine source and tetramethylene sulfoxide as the promoter. A variety of chlorinated indolizino[8,7-b]indole derivatives have been constructed efficiently under this reaction system in moderate to good yields (19 examples, up to 93% yield).

Diastereoselective Synthesis of Indoline- and Pyrrole-Embedded Tetracycles via an Unprecedented Dearomative Indole-C3-Alkylation/Aza-Friedel-Crafts Cascade Reaction

Dearomative indole C3-alkylation─intramolecular iminium trapping cascade reaction of indole-C3-tethered nucleophiles is a well-known blueprint for accessing 2,3-fused indolines. In exploring this strategy, synthetic chemists have utilized diverse classes of electrophilic reagents. However, the tethered nucleophiles have mainly been limited to heteronucleophiles and enolates; exploitation of tethered arenes/heteroarenes remains unknown. We herein describe the first examples of pyrrole-intercepted dearomative indole C3-allylation and benzylation of indole-tethered pyrroles toward the synthesis of 2,3-cis-fused tetracyclic indolines featuring a C3 all-carbon quaternary stereocentre. Our methodology capitalizes on the capability of NaO^tBu/Et₃B combination to direct the intermolecular alkylation to take place regioselectively at the indole C3 position over the other reactive sites (indole N and C2 and pyrrole C2 positions) and leverages the high nucleophilicity of the pyrrole template for the concomitant aza-Friedel-Crafts ring closure that traditionally would require an additional acid-catalyzed synthetic step. This cascade reaction is accomplished with broad substrate scope and excellent yields and chemo-, regio-, and diastereoselectivities.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Unbiased C3-Electrophilic Indoles: Triflic Acid Mediated C3-Regioselective Hydroarylation of N-H Indoles

The direct dearomative addition of arenes to the C3 position of unprotected indoles is reported under operationally simple conditions, using triflic acid at room temperature. The present regioselective hydroarylation is a straightforward manner to generate an electrophilic indole at the C3 position from unbiased indoles in sharp contrast to previous strategies. This atom-economical method delivers biologically relevant 3-arylindolines and 3,3-spiroindolines in high yields and regioselectivities from both intra- and intermolecular processes. DFT computations suggest the stabilization of cationic or dicationic intermediates with H-bonded (TfOH)n clusters.

Copper-catalyzed [4+1] cycloannulation of 2-aminochalcones with ethyl diazophenylacetates via ester rearrangement

Copper-catalyzed [4+1] cycloannulation of 2-aminochalcones with ethyl diazophenylacetates was developed, exhibiting a wide range of substrates and good tolerance of functional groups.

KOH-mediated stereoselective alkylation of 3-bromooxindoles for the synthesis of 3,3′-disubstituted oxindoles with two contiguous all carbon quaternary centres

The stereoselective synthesis of 3,3′-disubstituted oxindoles having all-carbon quaternary stereocenters has been achieved using KOH as a base with an excellent diastereomeric ratio (98 : 2).