Indole-to-Carbazole Strategy for the Synthesis of Substituted Carbazoles under Metal-Free Conditions

Abstract

An efficient indole-to-carbazole strategy has been developed under metal-free conditions. This carbazole formation was highly promoted by NHI with high regioselectivity through formal [2 + 2 + 2] annulation of indoles, ketones, and nitroolefins. It thus conveniently enabled the assembly of a large number of diversified carbazole products with good tolerance of a broad range of functional groups.

Straight access to highly fluorescent angular indolocarbazoles via merging Au- and Mo-catalysis

“…6a-c By contrast, carbazole synthesis has experienced an outstanding development. 8,9 Into this regard, we have described the Au(III)-catalysed 10 cyclization of 1,1-bis(indol-3-yl)-3-alkyn-2-ols B, prepared from readily accessible α,α-bis(indol-3-yl) ketones A, 11 affording a wide range of 1-(indol-3-yl)carbazoles C through a selective 1,2rearrangement involving an alkyl migration over the alkenyl migration (Scheme 1a). 12 Due to the elevated functional group tolerance of this reaction, carbazoles bearing 2-nitroaryl substituents could be easily accessed opening the possibility to subsequently carry out Cadogan reductive cyclization, such as the Mo-catalysed one developed by our group and widely used by other authors (Scheme 1b).…”

Salicylic Acid‐Promoted Three‐Component Annulation of Benzimidazoles, Aryl Nitroalkenes and Elemental Sulfur

“…On the basis of the results from above experiments and previous reports, [10][11]15] a tentative reaction mechanism was proposed (Scheme 4). With the assistance of Brønsted acid, aza-Michael addition of benzimidazole 1 a to β-nitrostyrene 2 a proceeds to afford the intermediate B bearing an imine cation moiety.…”

“…On the basis of the results from above experiments and previous reports, [10][11]15] In summary, we have developed a facile Brønsted acid-promoted three-component reaction that give a platform for the synthesis of benzo [4,5]imidazo [2,1-b] thiazoles from readily available benzimidazoles, nitroalkenes, and elemental sulfur. This metal-free protocol provides a straightforward entry to complex S,Nheterocycles from simple raw materials with a range of compatible functionalities.…”

Efficient synthesis of polyfunctionalized carbazoles and pyrrolo[3,4-c]carbazoles via domino Diels–Alder reaction

“…Because indoles are readily available materials, the direct extension of indoles to carbazole skeletons has a great advantage [19][20][21][22][23][24][25][26][27]. Therefore, the Diels-Alder reaction of activated 2-vinylindolines or 3-vinylindolines with diverse dienophiles has become the most attractive strategy for the synthesis of carbazole deriv- atives [28][29][30][31][32][33][34][35][36][37][38][39][40]. In recent years, by using the one-pot domino synthetic strategy of in situ-generated 2-vinyl-or 3-vinylindolines and sequential Diels-Alder reaction with activated dienophiles, we have successfully developed several efficient synthetic protocols for diversely functionalized tetrahydrocarbazoles and the corresponding carbazole derivatives [41][42][43][44][45][46][47].…”