Indium-Catalyzed Annulation of 2-Aryl- and 2-Heteroarylindoles with Propargyl Ethers: Concise Synthesis and Photophysical Properties of Diverse Aryl- and Heteroaryl-Annulated[a]carbazoles

Abstract

Treatment of 2-aryl- and 2-heteroarylindoles with propargyl ethers in the presence of a catalytic amount of indium nonafluorobutanesulfonate [In(ONf)(3)] gave aryl- and heteroaryl-annulated[a]carbazoles in good yields. The synthetically attractive feature is reflected by its applicability to a wide range of 2-aryl- and 2-heteroarylindoles. In the annulation reaction, propargyl ethers act as C3 sources (HC[triple bond]C-CH(2)OR). Among these, two carbon atoms are incorporated into the product as members of a newly constructed aromatic ring and the remaining carbon atom forms a methyl group on the aromatic ring, where the methyl group is always located next to the C3 position of the indole nucleus. The methyl group can be easily removed through SeO(2) oxidation followed by decarbonylation with RhCl(CO)(PPh(3))(2)-Ph(2)P(CH(2))(3)PPh(2) as a catalyst. The new annulation strategy is applicable also to symmetrical dimers such as bithiophene and bifuran derivatives. Mechanistic studies suggest that the first step is addition reaction initiated by regioselective nucleophilic attack of the C3 of 2-aryl- and 2-heteroarylindoles to the internal carbon atom of the C[triple bond]C bond in propargyl ethers. The next stage is ring-closing S(N)2 process kicking out the alkoxy group and then aromatization via a 1,3-hydrogen shift is the final step. The two carbon-carbon bond-forming reactions achieved in one-pot contribute largely to the reduction in the number of steps for the synthesis of aryl- and heteroaryl-annulated[a]carbazoles. Furthermore, utilization of the Fischer indole synthesis for efficient supply of the substrates, 2-aryl- and 2-heteroarylindoles, is another important factor shortening the overall process. The development of the annulation with a wide substrate scope provided a unique opportunity to evaluate photophysical properties of a series of aryl- and heteroaryl-annulated[a]carbazoles. Almost all the compounds evaluated in this study were found to emit purple to green light in the visible region. Some interesting structure-property correlations are also described.

Lewis Acid Mediated One‐Pot Synthesis of Aryl/Heteroaryl‐Fused Carbazoles Involving a Cascade Friedel–Crafts Alkylation/Electrocyclization/Aromatization Reaction Sequence

“…[11] A one-pot synthesis of carbazolonaphthoquinones was realized in a three-step tandem radical alkylation/cyclization/aromatization reaction sequence. [12] Very recently, highly efficient one-pot syntheses of carbazoles and dihydrobenzo [a]carbazoles were reported that involve Pd-mediated domino reactions.…”

An Efficient Synthesis of Substituted Quinolines via Indium(III) Chloride Catalyzed Reaction of Imines with Alkynes

“…For imines 1 with aryl groups including o-bromo, m-chloro, o-chloro and o-methyl, the desired products were also obtained in 80, 82, 81 and 83% yields, respectively (Table 2, entries 7-8 and 12-13). Subsequently, the scope of alkynes in this reaction was further investigated, and it was found that substituted phenylacetylenes with electron-donating or electron-withdrawing groups were perfectly suitable substrates for this transformation, and the expected products were obtained in moderate to excellent yields (Table 2, entries [14][15][16][17]. Interestingly, aliphatic alkyne like 1-hexyne with benzaldimine 1a also reacted smoothly to give 4-butyl-2-phenylquinoline 3r in 75% yield (Table 2, entry 18).…”

Applications of aminocarbazoles in heterocyclic synthesis

“…[44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] Aryl and heteroaryl-annulated carbazoles, such as pyridocarbazoles, indolocarbazoles, and pyrrolocarbazoles have attracted growing attention since they are distributed in numerous natural products with diverse useful bioactivities. [60][61][62][63][64][65][66][67][68] Aminocarbazoles have been an important synthon to construct pyridocarbazoles, 69 which are well known for their antitumor properties. 70,71 Also, aminocarbazoles have been used to prepare pyrimido [5,4-b]carbazole derivatives.…”