Brønsted Acid‐Catalyzed Straightforward Synthesis of Benzo[b]carbazoles from 2,3‐Unsubstituted Indoles

Abstract

Abstract. Described is a general and efficient synthesis of valuable benzo [b]carbazoles by Brønsted acid-catalyzed reaction between simple C2,C3-unsubstituted indoles and o-(-(hydroxy)benzyl)benzaldehyde acetals. Highly selective migration processes are involved as key steps in the overall cascade sequence that implies the one-pot formation of two new bonds and a cycle in a regioselective fashion.

Annulative π‐Extension (APEX): Rapid Access to Fused Arenes, Heteroarenes, and Nanographenes

“…This chemistry has the potential to enable the synthesis of adipocyte fatty acid binding protein (A‐FABP) inhibitors (Scheme ) . Various π‐extended fused carbazoles 91 can be prepared by the Brønsted acid catalyzed naphthoannulations of indoles when ortho ‐[α‐(hydroxy)benzyl]benzaldehyde acetals 90 are used as the π‐extending reagents (Scheme ) . Even the benzothiophene‐fused carbazole can be obtained.…”

Three-Component Cascade Synthesis of Carbazoles through [1s,6s] Sigmatropic Shift under Metal-Free Conditions

“…Due to the importance of carbazoles in these fields, a series of methods for the synthesis of these compounds have been developed during the past several decades. − Among them, the indole-to-carbazole strategy represents an efficient approach for carbazole synthesis since it starts from simple substrates without prefunctionalization and offers convenient opportunities to achieve desirable synthetic convergence and function-installing flexibility. Generally, the indole-to-carbazole process mainly includes three approaches: (i) with the assistance of the transition-metal (TM) catalyst, such as palladium, rhodium, manganese, and gold, indoles react with two molecules of alkynes or alkenes through [2 + 2′+2′] annulation to produce substituted carbazoles (Scheme a); (ii) indoles without prefunctionalization react with highly reactive coupling partners such as 1,4-dicarbonyl compounds, or their alkyne-derivatives, allene-derivatives and dihydrofuran through [2 + 4] annulation to afford the carbazole products; (iii) 2,3-unsubstituted indoles couple with two different molecules through [2 + 2′+2″] annulation to give unsymmetrical carbazoles derivatives under TM-free conditions . Among these strategies, the third approach is the most attractive method for carbazole synthesis since different substituents can be introduced in one-pot under TM-free conditions.…”

Bismuth(III) Triflate Catalyzed Three-Component Reactions of Indoles, Ketones, and α-Bromoacetaldehyde Acetals Enable Indole-to-Carbazole Transformation

“…The direct conversion of 2,3-unsubstituted indoles into carbazoles has an intrinsically high synthetic potential because simple indoles are readily available. At present, this transformation can generally be realized through the following three approaches (Figure ): (i) [2 + 4] annulation, in which indoles are reacted with a carbon-based 1,4-biselectrophile, such as a 1,4-dicarbonyl compound, or their alkyne-, allene-, dihydrofuran-, and donor-and-acceptor-cyclopropane-type variations, with the aid of an acid or transition-metal catalyst to create carbazole scaffolds (see Figure S1 in Supporting Information (SI)); (ii) [2 + 2′ + 2′] annulation, in which indoles are reacted with two molecules of alkene or alkyne in the presence of a transition-metal catalyst to form carbazoles; and (iii) [2 + 2′ + 2″] annulation, in which indoles are reacted with two different molecules, and each of them contribute two carbons to construct a carbazole ring . Among these three approaches, the third is considered the most attractive route for carbazole synthesis not only because it uses easily available substrate and transition-metal-free reaction conditions but also because it synthesizes carbazoles with a high potential of molecular diversity and complexity.…”