Overview on iron/acetic acid mediated reactions II.A.1. Introduction
Reductive cyclization is a powerful tool for the construction of C-N bonds and for the synthesis of N-heterocycles. Intermolecular and intramolecular reductive cyclization methods are widely used for the synthesis of potential bio-active molecules such as Indole, quinoline, carbazoleand acridinone derivatives.1Thisprocess involves the reduction of nitro group to amino group followed by condensation with carbonyl group. The general methods used for reductive cyclization are Zn/NH4Cl,2 Zn/AcOH,3Na2S2O4,4SnCl2.2H2O,5 Fe/HCl,6 Fe/NH4Cl,7 Fe/AcOH,8 TiCl3/HCl,9 TiCl3/NH4OAc,10TiCl4/Sm,11 SnCl2/HCl12 systems. Moreover catalytic hydrogenation systems such as Pd/C,13 PtO2,14 Raney Ni,15 Pd/NaBH4,16 Raney Ni/NH2NH217are also employed for reductive cyclization. These methods suffer many shortcomings including, the metals used for this transformation are very expensive (Pd/Pt), the use of flammable hydrogen gas, use of high pressure equipment, formation of mixture of products, incompatibility of reduction-sensitive functional groups, etc. The one-pot selective reductive cyclization of nitro compounds by using iron powder in acetic acid has been reported as an efficient method for synthesis of various N-heterocycles in good to excellent yields.18 The Fe/AcOH system has many advantages such as Fe powder is commercially available, less expensive, non-toxic, and environmental friendly. In addition to this functional group tolerance, short reaction times, simple work up procedure make this system more attractive. Therefore, we selected Fe/AcOH system for the construction of biologically active nitrogen containing heterocyclic frameworks via reductive cyclization.
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II.A.2. Recent advances in iron/acetic acid mediated reactions
In 2002, Basavaiah et al. reported an efficient method for the synthesis of (2-oxo-1,2-dihydroquinolin-3-yl)methyl acetate derivatives from Baylis –Hillman adducts by using iron/
acetic acid system (Scheme II.A.2.1).19
Scheme II.A.2.1
In 2004, Basavaiah et al. reported a convenient operationally simple, one-pot procedure for the synthesis of 1,2,3.4-tetrahydroacridine derivatives by using iron/ acetic acid system (Scheme II.A.2.2).20
Scheme II.A.2.2
In 2005, Warnmark and co-workers reported the synthesis of 2-pyridone fused 2.2’-bipyridine derivatives from2.2’-bipyridine derivatives by using iron/ acetic acid system. The precursors are prepared via Ullman coupling (Scheme II.A.2.3).21
Scheme II.A.2.3
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In 2006, Basavaiah et al.reported an easy, one-pot procedure for the synthesis of 3-benzoyl quinoline derivatives from Baylis –Hillman adducts by using iron/ acetic acid system (Scheme II.A.2.4).22
Scheme II.A.2.4
In 2007, Basavaiah et al.reported an efficient one-pot method for the synthesis of heterocyclic frame works containing azocine moiety by using iron/ acetic acid system. This procedure involves alkylation followed by reductive cyclization(Scheme II.A.2.5).23
Scheme II.A.2.5
In 2010, Basavaiah and co-workers reported a simple facile and one-pot procedure for the synthesis of tricyclic heterocyclic systems containing [1,8]naphthyridin-2-ones by using iron/
acetic acid system (Scheme II.A.2.1).24
Scheme II.A.2.6
II.A.3. Recent iron/acetic acid mediated protocols from our group
For the past half a decade, our group has investigated the applications of conventional iron acetic acid system for the generation of versatile heterocyclic compounds.25 In 2009, Ramesh et al. reported a new and simple method for C-alkylation of Baylis- Hillman alcohols with
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various indoles in the presence of catalytic amount of molecular iodine in acetonitrile at room temperature, followed by iron acetic acid mediated reductive cyclization produced indolylquinoline derivatives (Scheme II.A.3.1).26
Scheme II.A.3.1
In 2010, Ramesh et al. reported an iron/acetic acid-mediated carbon degradation protocol for the synthesis of various substituted quinoline derivatives(Scheme II.A.3.2).27
Scheme II.A.3.2
In 2010, Ramesh et al. reported an unprecedented route for the synthesis of 3,3′-biindoles by reductive cyclization of 3-[2-Nitro-1-(2-nitrophenyl)ethyl]-1H- indoles mediated by iron/acetic acid. Both symmetrical and unsymmetrical 3,3′-biindoles could be generated by this method. Mild conditions, high yields of the products, and environmentally acceptable reagent are the merits of the procedure (Scheme II.A.3.3).28
Scheme II.A.3.3
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In 2011, Ramesh et al. reported a simple and facile route for the synthesis of 2H-1,4- benzoxazin -3-(4H)-ones via reductive cyclization of 2-(2nitrophenoxy) acetonitrile adducts in the presence of Fe/acetic acid (Scheme II.A.3.4).29
Scheme II.A.3.4
In 2011, Janreddy et al. reported an efficient method for the Synthesis of carbazol-4-ones, 3,4-dihydrocyclopental-indol-1-one, and indole derivatives by a Fe/AcOH-mediated intramolecular reductive N-heteroannulation of 3-hydroxy-2-(2-nitrophenyl)enones. The same protocol has been extended to access indolo carbazolone and indoloquinolone derivatives(Scheme II.A.3.5).30
Scheme II.A.3.5
In 2012, Ramesh et al. reported a protocol for the synthesis of Indolylquinolines, Indolylacridines, and Indolylcyclopenta[b]quinolines from the Baylis−Hillman Adducts. This protocol explain an in situ [1,3]-sigmatropicrearrangement of indole nucleus toaccess Indolylacridines and Indolylcyclopenta[b]quinoline derivatives(Scheme II.A.3.6).31
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