Metal-Free Synthesis of 1,4-Disubstituted Carbazoles from Sulfonylindoles

Significance

Reported is a two-step procedure for the synthesis of 1,4-disubstituted carbazoles from sulfonylindoles and acetal-protected β-nitro ketones. Treatment of these reagents with KF on basic alumina leads to 3-(2-nitroalkyl)indoles, which, without the need for intermediate purification, undergo acid-mediated cyclization to carbazoles in moderate yields (over two steps). This ­cyclization clearly involves acetal deprotection, Friedel-Crafts reaction, and final elimination of ­nitrous acid. The method is practically simple and utilizes mild conditions. However, both starting materials require preparation.