Domino Nitro Reduction-Friedländer Heterocyclization for the Preparation of Quinolines

Abstract

The Friedländer synthesis offers efficient access to substituted quinolines from 2-aminobenzaldehydes and activated ketones in the presence of a base. The disadvantage of this procedure lies in the fact that relatively few 2-aminobenzaldehyde derivatives are readily available. To overcome this problem, we report a modification of this process involving the in situ reduction of 2-nitrobenzaldehydes with Fe/AcOH in the presence of active methylene compounds (AMCs) to produce substituted quinolines in high yields. The conditions are mild enough to tolerate a wide range of functionality in both reacting partners and promote reactions not only with phenyl and benzyl ketones, but also with β-keto-esters, β-keto-nitriles, β-keto-sulfones and β-diketones. The reaction of 2-nitroaromatic ketones with unsymmetrical AMCs is less reliable, giving a competitive formation of substituted quinolin-2(1H)-ones from the cyclization of the Z Knoevenagel intermediate which appears to be favored when certain large groups are adjacent to the AMC ketone carbonyl.

Facile access to 3-sulfonylquinolines via Knoevenagel condensation/aza-Wittig reaction cascade involving ortho-azidobenzaldehydes and β-ketosulfonamides and sulfones

“…In turn, diversely substituted quinoline-3-sulfones are available through a range of synthetic methodologies suggested recently. Cyclization strategies [35][36][37][38][39][40][41][42][43][44][45] as well as cycloaddition/cyclocondensation techniques [46][47][48][49][50][51] represent those with heteroring construction. Alternative approaches rely on a peripheral modification of various substrates, such as 3-bromoquinolines [52][53][54][55], quinoline-3-boronic acids [56], and diazonium salts [57].…”