Abstract

Anthranilic acid is a benzenoid compound that has adjacent active important two reactive groups namely carboxylic acid group (-COOH) and amino group (-NH2). Consequently fascinating anthranilic acid derivatives have been prepared through both the active functional groups. The literature review reveals an experiential and holistic approach of various manufacturing procedures and skills used for the production of anthranilic acid analogues and anthranilic scaffold along with the biological activities of some of the promising derived anthranilic acid molecules. Pharmacology of anthranilic acid derivatives has been reported as alpha-glucosidase inhibitors, urease suppressors, keto-aldo reductase undecaprenyl pyrophosphate synthase (UppS) suppressors, mitogen activated protein kinase (MAPK), COX(Cyclooxygenase) inhibitors The analogues of anthranilic acid have been reported as an excellent pharmaceutical aids such as anticancer, antidiabetic, antiviral, anti-inflammatory, analgesic and antimicrobial agents. The present review unveils the importance of anthranilic scaffold as a beginning material for the development of potent drug for their various pharmacological activities.

Keywords

Anthranilic acid derivative, Heterocycles, Synthesis, Medicinal chemistry, pharmaceutical activity.

Introduction

       
           
       
Figure 1: Structure of anthranilic acid (AA) and analogues of AA.

In this review article the author’s interest is to showcase the classical and advanced synthetic approaches of the molecules derived from anthranilic acid and biological significance of anthranilic derivatives.

       
           
       
Figure 3: A precursor and intermediate molecule for the synthesis of oxazolobenzoxazine (16)

       
           
       
Figure 4: Structure of tricyclic benzoxazinone

Dehydration of anthranilic diacids (14) with acetic anhydride yielded the seven-membered 1,2-dihydro-4,1-benzoxazepine-3,5-dione (15), whereas treatment of the anthranilic diacids (14) with TFAA gave 3a-(trifluoromethyl)-5H-[1,3] oxazolo[3,2-a][3,1]benzoxazine-2,5(1H)-dione 16 (Figure 4)

The synthesis of series of 4-substituted benzenesulfonamides of AA was reported by Prachayasittikul and co-workers. The synthesized compounds were tested for cytotoxic, antifungal and antibacterial activity. The antiproliferative potency was examined against four cell lines (MOLT-3, HepG2, HuCCA-1 and A549) using etoposide and doxorubicin as reference drugs. The synthesized compounds showed cytotoxic activity against MOLT-3 cell line and compound 23 was the most potent one (IC₅₀ = 15.71±0.70 µg/mL). In addition, the antibacterial activity was checked against eighteen strains of gram- negative and gram-positive bacteria (Figure 6). Unfortunately, none of the synthesized compounds possessed antibacterial activity. Nevertheless, all sulfonamides derivatives showed antifungal activity against C. albicans at micro molar concentration (4µg/mL) (26).

       
           
       
R=NO₂, OCH₃, CH₃, Cl

Figure 6: Anthranilic acid containing potent anticancer agents.

Liu and coworkers (2013) prepared a novel series of anthranilamide derivatives and evaluated their anti-proliferative activity against two cell lines, human colon carcinoma cell line (HCT-116) and human breast adenocarcinoma cell line (MDA-MB-231). Compounds 27-28 exhibited a promising inhibition activity against the both cell lines and compound 27 (R=3-CF₃) was the most potent one (IC₅₀=14.6µM and 13.86µM against HTC-116 and MDAMB-231 respectively) (Figure 24). Flow cytometric analysis revealed that compound 27 (R=3-CF₃) suppressed the proliferation of both cell lines through induction of apoptosis in a dose-dependent pattern and arrest G₁ and S phase in the cell cycle [30]. E_l-Shafiey et al designed and prepared two AA derivatives, N-(2-carboxyphenyl) thipheneimine (HL₂) and N-(2-carboxyphenyl) salicylideneimine (H₂L₁) and used them as a ligand for the synthesis of binary Ni (II), Co (II), Cd (II), Fe (III) and UO₂ metal complexes and ternary pyridine metal complexes. The antibacterial activity is exhibited by these synthesized ligands, binary and ternary complexes were evaluated against Bacillus Subtilis, Escherichia Coli and Bacillus Cereus.

Bala & co-workers designed and prepared a novel set of N-aryl AAs containing 1,3,4-oxadiazoles scaffold and evaluated their anti-inflammatory & analgesic activity in addition to molecular docking studies to survey their binding affinity to cyclooxygenase-2 enzyme. The anti-inflammatory activity was carried out using carrageenan-induced rat paw edema assay while the analgesic activity was performed using tail immersion method. The docking study indicated that compound 31 and 32 (Figure 9) formed a good interaction with COX-2 enzyme which explained the higher analgesic and anti-inflammatory activity of them. The authors believed that the good docking scores of compounds 31 and 32 were due to exchangeof the free –COOH which is responsible for gastric side effects with active 1,3,4-oxadiazole moiety [33]].

In 2013, Selley and co-workers studied in vivo analgesic & anti-inflammatory activity of N-(3,4-dimethoxycinnamonyl) anthranilic acid with collagen-induced assay,a mouse model of rhemattoid arthritis (Figure 10). They found that the compound 33 decreased the biological and histological intensity of arthritis, decreased the pain and eradicate the hyperalgesia. It also decreased cell activity in lymph node cell cultures and elevated serum levels of IL-10. The authors also studied the in vitro anti-inflammatory activity of compound 33 and founded that it caused suppression of IFN production and proliferation of both T and B lymphocytes [34].

       
           
       
Figure 10: 2-{[(2E)-3-(3, 4-dimethoxyphenyl) prop-2-enoyl] amino} benzoic acid showing anti-inflammatory activity

Shi and coworkers reported the construction of a novel series of anthranilic diamides derivatives having arylisoxazoline moiety. In vitro anti-cancer potency was evaluated at micromolar, ?M level against cancer cell lines like human lung cancer cell line (NCIH 460), gastric cancer cell lines (SGC-7901 & BGC-823), breast epithelial adenocarcinoma epithelial cell (MCF-7) and hepatocellular liver carcinoma cell line (HepG2) using 5-flouro uracil as a reference drug. Compounds 34 and 35 showed a significant inhibition activity of the tested cell lines at 40 µg/mL and growth inhibition 59.2-76.3 %. Moreover, compound 36 exhibited a selective inhibition activity against human hepatocellular liver carcinoma cell type

       
           
       
Figure 11: Anthranilic acid derivatives with aryl isoxazoline moiety

Onnis and co-workers synthesized a series of esters of  N-(2-(trifluoromethyl)-pyridin4-yl) AA derivatives based on the flufenamic acid scaffold. The prepared compounds were evaluated for anti-proliferative activity with almost 60 cell lines derived from leukemia, colon, lung, melanoma, CNS, renal, ovarian, prostate, and breast human cancers. Compounds 37 and 38 were the promising molecule against the majority o the tested cell lines at nanomolar concentrations (Figure 12). By using compare analysis, the authors believed that the most potent compounds exhibited their antiproliferative activity via COXdependent/independent mechanisms [35].

Ullmann-Goldberg coupling reaction is a metal catalysed C-N bond formation coupling reaction in which copper catalysed coupling reaction is carried out for the preparation of   N-aryl-AAs. In this reaction orthohalobenzoic acid reacts with aryl amine in presence of hot copper. Conversely, the reaction of AA derivatives with aryl halides using copper (metal, oxide, or salt) as a catalyst to produce N-arylAA analogs.  To overcome the drastic reaction conditions (long reaction times and high temperatures), numerous modifications were proceeded on this coupling methodology utilizing a broader range of halobenzoic acid substrates, aryl amines and anilines along with using multiple types of bases and copper & copper complexes. In 2006, wolf et al reported an approach for the synthesis of N-alkyl and N-aryl AAs in high yields and without need of acid protection via regioselective copper-catalyzed amination of bromobenzoic acids with different aromatic and aliphatic amines.

In 2003, Comdom et al also used ultrasonic irradiation for the construction of N-phenyl AAs 32 in high yields and short reaction time via coppercatalyzed Ullmann-Goldberg coupling reaction using water as a solvent

       
           
       

X=Cl, Br

The substituted anthranilic acid can be diazotised like amines, rather zwitter ionic inner salt formed in non-aqueous media. The benzenediazonium carboxylate synthesised in alcohol with isoamyl nitrite, can also be isolated and is stable below -70⁰C. On warming   to 40-60⁰C structure of anthranilic acid fragmented to N2 gas, CO2 and benzyne and can be used as a source of the highly reactive compound. The formation of a closed ring structure, a corresponding 1,2,3-benzotriazine-4-ones has been discussed.  There is a published review on the use of arynes in the preparation covering the years 1990-2002 with refrence to former reviews.

       
           
       
Figure 16: Formation of benzenediazonium carboxylate and benzyne

N, N-dimethylaniline and anthranilic acid diazotised to form diazodye and methyl red like amines. Conversion of anthranilic acid into corresponding 2-nitrobenzoic acids is an example for N-oxidation. Oxidation of anthranilic acid to give 2-nitrobenzoic acid can be carried out using superoxide radicals generated from hydrogen peroxide on soid titanium catalyst. Pperoxyacetic acid or peroxymonosulfuric acid (Caro’s acid) is an oxidising agent used for the oxidation of  anthanilic acid into 2-nitrobezoic acid. Compound 57 can also be synthesised by oxidation of N-hydroxy-anthranilic acid and combination of AA and sodium perborate. Anthranilic acid can be reduced to 2-aminophenylmethanol using sodium amalgam (Na-Hg), lthium aluminium hydride or samarium salts (At room temperature). Reductions of esters to alcohols have been reported with sodiumborohydride or ZnCl₂/NaBH_4. Birch reduction process can also be used to reduce anthranilic acid.

2-[(trifluoroacetyl)amino] benzoic acid (59, Figure 18) prepared by treatment of anthranilic acid with trifluoroactic acid (TFAA) [36] [37], could be activated with dicyclohexylcarbodiimide (DCC) and coupled with alcohols to prepare intermediates for natural product synthesis [38]. Subsequently deprotection of trifluoroacetyl group from 2-[(trifluoroacetyl)amino] benzoic acid processed with sodiumborohydride. N-acetyl anthranilic acid derivatives are obtained after the reaction of sodium salt of anthranilic acid with acetic anhydride followed by acidification of HCl [39]. N-protection in anthranilic acid carried with Boc group, either directly [40], or through reductive condition from 2-azidobenzoic acid [41]. N-Boc-anthranilic acid on reaction with Grignard reagents produce the corresponding 2-aminobenzoketones with optimum yield [42]. N-Tosyl anthranilic acid has been synthesised and used in Friedel crafts acylation of its acid chloride [43]. Deprotection needed strong acidic condition (hot conc. H₂SO₄) [44].

       
           
       
Figure 20: Anticancer agents such as N-hydroxy-2-{[(4-methoxyphenyl) sulfonyl] amino}-3-methylbenzamide derivative, {[2-(amino) benzoyl] amino} acetic acid derivative and 2-[(carboxycarbonyl)amino] benzoic acid derivatives respectively

Synthesis of five membered heterocycles from anthranilic acids

       
           
       
 
       
           
       
Table: Substituent’s (R₁, R₂ & R₃) in Condensations of 2-substituted indoxyls

In spite of the nature of R₁ (Figure 2) only metal alkoxides haave been used as the bases with long time heating when R₁=H.  But when R₁=alkyl the reaction can be carried out at room temperature, where as R₁=aryl the reaction can be carried out at 0⁰C at room temperature.

       
           
       
Figure 22 Condensation yielding 2-substituted indoxyls derivatives (73).

The chemistry indoxyl has been reported [70], but the following some examples are extended reactions of indoxyls. A procedure for hydrolysis/decarboxylation followed by condensation with amines [71], has been used to make the compounds 38 (Figure 23) [72]. Indoxylic derivative (74), the possible intermediate in the Heumann indigo synthesis, that is at the maximum conveniently prepared by hydrolysis of the ethyl ester [73], has been made to react with 2-aminobenzaldehyde to form [3, 2-b] quinoline by decarboxylation condensation [74]. Indoxyl (or N-acetyl indoxyl, or O, N-diacetyl indoxyl) has been condensed with isatin (in its ring opened form) in basic aqueous solution to give the acid [74-77].

The indoxyl derivative (76) has been cyclised to   indoloindol (77) by heating with acetic acid (Figure 24) [78].

On treatment of anthranilic diacids 78 with Vilsmeyer reagent, followed by heating produces indoles 79 in varying yields (Figure 25) [79]. The reaction can also be used to other diacids for the preparation of non-benzofused heterocycles.

In case diazotised anthranilic acid is allowed to react with Na₂S₂ (prepared from Na₂S and S₈), dithiosalicylic acid (81) has been formed (Figure 26)[80].  It can easily be reduced to thiosalicylic acid (85)[80], and afterwards reacted with chloroacetic acid under aqueous basic conditions to produce the diacid under aqueous basic conditions to give the diacid 83 [81]. The prior report demonstrated the direct conversion of 48 to the diacid 83 with reducing agent (sodium dithionite).  Cyclisation by Vilsmeier reagent reactions leading to benzothiophenes and thioindigo. [82], but some preparative procedures carry out reactions without the addition of a reducing agent [83, 84]. Benzo [1,2] dithiol-3-one (84) can be synthesised from 81 [85] or from 85 [81,85,86]. The diacid 80 is made to react with acetic anhydride to give benzothiophene {84), which could be deacetylated to benzothiophene derivative (84). Compound 83 is directly converted to 86 [87].  Like its nitrogen counterpart indoxyl, 86 can be dimerized to the corresponding indigoid compound, in this case thioindigo (88). A mild procedure has been developed with hydrogen peroxide in acetic acid [88]. The dihydrobenzothiophene 86 has been synthesised from 83, and further converted to the thioindigos 88 which is used to synthesise photo activated molecular tweezers [83,84]. Benzothiophenes have been prepared from 83 (Figure 25) 1, 2-Benzothiazol-3(2H)-ones, which exhibit antimicrobial activities, have also been prepared from 81, a review has been reported [89, 90]. An ecofriendly process has been developed to prepare derivatives of 1, 2-Benzothiazol-3(2H)-ones from esters of 81 [91]. There are many syntheses of oxidised derivatives of the sweetener saccharin from anthranilic acid. A recent example utilizes diazotisation of methyl anthranilates [92].

2, 1-Benzisoxazole (90: R₁=R₂=H in Figure 27), also known as anthranil, having special valence tautomerism and aromatic properties [91]. There is a preparation in the literature based on oxidation of methyl anthranilate derivative [92] The treatment of isatoic anhydride with chiral amino alcohols give the anthranilic amides (92) (Figure 28) [93]. These can be ring closed through the compounds (93) to the oxazoles 94 which were utilised in the synthesis of tridentate ligands for asymmetric organometallic catalysis.

The 2,3-diaminobenzoic acid 98 has undergone cyclisation to give the benzimidazole 99 on treatment with formic acid [97, 98], or to give benzimidazolone 100 on treatment with phosgene (Figure 30) [99]

Many benzimidazol-2-ones (102) were prepared on heating the anthraniloyl azides 101)(Figure 31). The products were formed using Curtius arrangement followed by internal capture of the intermediate isocyanate. The precursor 101 are obtained by diazotisaton of the corresponding Anthranilic hydrazides [103], or by ring opening of isatoic anhydride or phthalic anhydride by azide ion [105].

The benzimidazol-2-ones were prepared with high yields (71-99 %) on heating of hydroxamic acids of AA in formamide [106]. This is a modified Lossen rearrangement which appears to give reaction directly from the corresponding hydroxamic acid, circumventing the need to first O-acylate the starting material. The derivatives of hydroxamic acids were synthesised by reaction of appropriately substituted anthranilic ester with hydroxylamine hydrochloride. The sydnones (107) were prepared by dehydration of N-nitroso derivatives (106) (Figure 33).

       
           
       
Figure 34: Synthesis of acridines

An illustration of the first type reaction is cyclisation of the AA derivative (111) (available by Jourdan-Ullman-Goldberg reaction of 2-halobenzoic acid and AA) to the acridone 112, followed by reduction/oxidation to the fully aromatic acridine 113 (Figure 34) [109]. The reaction of 111 with Trifluoroacetic anhydride followed by heating gives ring closure to 112[110].

Synthesis of anti-Herpes simplex acridones are the reactions of anthranilic acid with resorcinols at high temperatures are prepared to yield the acridones 116, which is similar to naturally occurring acridones (R₁ -R₄) are combined with different substituents like H, Cl, Me and OMe, R₅=OH, OMe or Me)(Figure 36)[111].  The former by initial N-thioacylation of anthranilic acid with thiobenzoylthioacetic acid, followed by ring closure in acetic anhydride [112]. The later compound, 77, was directly prepared by heating of anthranilic acid in thioacetic acid [113].

9-disubstituted acridines (118) are formed by the treatment of various substituted esters of N-phenyl anthranilic acid with Grignard reagents (Figure 37), followed by ring closure [117].

       
           
       
Figure 37: Synthesis of 9, 10-dihydroacridine derivatives (Y=H, Cl, F, OCH₃, alkyl and NO₂)

Reaction of ammonium isothiocyanate (NCS)NH₄ with anthranilic acid gives the thiourea derivative [118], but reaction with benzoyl isothiocyanate gives 120 (Figure 38) [119, 120]. The later product can be cyclised under acidic conditions to give benzothiazine 121, which also hydrolysed  to 122, and later transformed by Dimroth rearrangement to give thioxoquinazolinone 123 (under basic or acidic conditions) [121]. The transformations were carried out with several ring substituted derivatives of  120. In the similar way the reactions of anthranilic acid with phenyl isothiocyanates were found to yield benzothiazines or thioxoquinazolinones depending on the substitution pattern in the starting marterial [122].

The following scheme for the synthesis of 1, 2, 3-benzotriazin-4(3H)-one derivatives (formation of benzotriazinones by diazotisation of methyl antthranilate) There was a problem on traditional synthesis of benzotriazinones 127is to prepare the substituted anthranilamide    The problem can be solved by addition of the appropriate amine (Figure 39) [123]

A holistic synthesis of benzotriazine (130) as an additive in peptide couplings have been developed using the reaction of 128 with TMS-protected hydroxylamine to give 129, followed by nitrosation (yielding 36-68?pending on substitution on 129) (Figure 40) [124]. Thermolytic [125,126] and photolytic [127] studies on 1, 2, 3-benzotriazine-4-ones have been published.

       
           
       
Figure 40: Synthesis of 3-hydroxy-1, 2, 3-benzotriazin-4(3H)-one derivative (X=Y= F, Cl, NO₂, alkyl, OCH₃ and H)

When AA is condensed with an aldehyde (R= alkyl or aryl) the following substituted 1, 2-dihydro-4H-3, 1-benzoxazin-4-one (133) have been formed through intramolecular nucleophilic attack and a proton shift (Figure 41). The final product formed is much more stable than the intermediate imine and cannot be isolated normally. The reactivity is simple to prepare such that cyanide opens the ring by addition to the 2-position (under basic conditions the salt of the cyanomethylated anthranilic acid obtained), and the same chemistry has been used to prepare functionalised starting materials for the synthesis of indoxyls (Figure 42)

       
           
       
Figure 41: Synthesis of substituted 1, 2-dihydro-4H-3, 1-benzoxazin-4-one

The derivatives of (4H)-3-benzoxazine-4-ones have been formed  by intramolecular nucleophilic attack in N-acetylated anthranilic acid, but it is not variable process due to low electrophilicity of amide. Rather the reaction is induced by formation of a mixed anhydride of the starting material, usually by heating in acetic anhydride. The amide functions as nucleophile in its iminol form. Which cannot be normally isolated eliminates simpler water molecule to form the product. The chemistry of the resulted compounds sometimes called ‘acylanthranils’ has been reviewed (covering the period 1965-1998) [128]. A suitable way to synthesise (4H)-3-benzoxazine-4-one’s derivatives, i.e., inhibitors of C₁ serine protease, is to treat with anthranilic acid and benzoyl chloride in pyridine [129]. Similar compounds have been isolated from oats (Avena sativum) [130]. The usage of cyanide ring opening (X=H or F). The alkylation procedure has been published in elsewhere [131].

The method in which N-acyl AA is reacted with anilines to give 2, 3-disubstituted-4-quinazolinones catalysed by PCl₃ [132], has been reproduced and the conditions optimised [133]

The Niementowski quinoline synthesis in which 3-hydroxy-2,3-dihydroquinolin-4(1H)-one derivatives have been synthesized from anthranilic acid and acyl derivatives in presence of strong base in turn treating with polyphosphoric acid (PPA) yielding to quinoline derivative (147).

Synthesis of quinolinones 151 from N-hydroxy-2-nitroethanimine and anthranilc acid as prime moiety in which R group can be varied with different substituents. (R₁=SMe and R₂=COAr [134], or R₁=NHPh, R₂=NO₂ [135])

       
           
       
Figure 45: Synthesis of 3 & 4 substituted quinoline moiety.

Synthesis of quinolinones 154 from methyl prop-1-en-1-yl sulfide and AA in presence of cyclising reagent. (R₁=SMe and R₂=COAr [141], or R₁=NHPh, R₂=NO₂ [135])

Synthesis of seven membered rings

The anthranilic acid derivatives 159, with EWG (electron withdrawing groups) attached to the nitrogen of anthranilic acid have been cyclised to the 1,4-benzodiazepine-3,5-diones 160 on reacting with chloroformates and triethylamine (Figure 48) [69]. In the cases where X=NO the nitrogen could be deprotected.

Preparation of benzotriazepinones (X= H, Me or Cl)  in which  benzotriazepinone 164 has been synthesised from anthranilic acid on reaction with a nitrilimine, occurred in situ from 162, followed by ring closure with peptide coupling reagent (Figure 49) [142]. The structure 164 was later confirmed by X-ray crystallography [143].

Synthesis of 4,1-benzoxazepine-5-ones 169 in which compound 168 treated with polyphosphoric acid  (R=Ph or Me) [292]

CONCLUSIONS

The anthranilic acid analogues have shown vital role in the treatment of lethal diseases. The analogues of anthranilic acid have intended to control carcinogenic pathways, metabolic complications related to diabetes, cutting edge antiviral agents and physiologically tolerable anti-inflammatory compounds.

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