Synthesis of New Benzofuran-2-Carboxylic Acid Derivatives

Derivatives of benzofuran-2-carboxylic acid are known for exhibiting various pharmacological activities. Such compounds were found to be selective adenosine A2A receptor antagonists [1], anti-in�ammatory agents [2], and local anaesthetics [3]. Variously substituted 2-benzofurancarboxylic acid derivatives show selective cytotoxicity against human cancer cell line [4]. On the other hand, benzofuran2-carboxylic acids bearing (chlorometyl)indoline or benzoyl nitrogen mustard as DNA-binding group are structural subunits of synthetic analogues of some natural antitumor agents such as CC-1065, duocarmycin, dystamycin, and netropsin [5–8]. Amide derivatives of halo-benzofuran-2carboxylic acid were found to be useful in the treatment of the diseases which are associated with the modulation of H3 receptors [9]. Moreover, 7-piperazinylbenzofuran-2-carboxylic acid amides having halogens or methoxyl groups in benzene ring were patented as partial agonists of 5-HT receptors which should be useful in the treatment of many psychiatric disorders [10]. Many methodologies for the synthesis of benzofuran-2carboxylic acid skeleton were developed and most of them are based on the reaction of salicylaldehydes with haloacetates followed by cyclization of formylphenoxyacetic acid derivatives intermediates [11–17]. e cyclizations were usually carried out in an alcoholic solution in the presence of a basic catalyst such as sodium ethanolate [11], 1,8-diazobicyclo-[5.4.0]-7-undecane [7], potassium carbonate [12, 13], or phosphine [14]. Moreover, ethyl 5-nitrobenzofuran-2-carboxylate was prepared by cyclocondensation of 2-hydroxy-5-nitrobenzaldehyde with bromomalonic acid [18] as well as palladium catalysed carbonylative cyclization of o-alkynylphenols [19–21]. Most of these methods are restricted by the requirement of relatively long reaction time to obtain the expected product in low to moderate yields. We present herein a short synthetic route to obtain halogen-substituted benzofuran-2-carboxylic acid derivatives such as (2-hydroxy-1,1-dimethylethyl)amides as well as esters and amides of benzofuran-2-carboxylic acid containing anticancer mustard type pharmacophore in their structure.


Introduction
Derivatives of benzofuran-2-carboxylic acid are known for exhibiting various pharmacological activities.Such compounds were found to be selective adenosine A 2A receptor antagonists [1], anti-in�ammatory agents [2], and local anaesthetics [3].Variously substituted 2-benzofurancarboxylic acid derivatives show selective cytotoxicity against human cancer cell line [4].On the other hand, benzofuran-2-carboxylic acids bearing (chlorometyl)indoline or benzoyl nitrogen mustard as DNA-binding group are structural subunits of synthetic analogues of some natural antitumor agents such as CC-1065, duocarmycin, dystamycin, and netropsin [5][6][7][8].Amide derivatives of halo-benzofuran-2carboxylic acid were found to be useful in the treatment of the diseases which are associated with the modulation of H3 receptors [9].Moreover, 7-piperazinylbenzofuran-2-carboxylic acid amides having halogens or methoxyl groups in benzene ring were patented as partial agonists of 5-HT receptors which should be useful in the treatment of many psychiatric disorders [10].
Most of these methods are restricted by the requirement of relatively long reaction time to obtain the expected product in low to moderate yields.
We present herein a short synthetic route to obtain halogen-substituted benzofuran-2-carboxylic acid derivatives such as (2-hydroxy-1,1-dimethylethyl)amides as well as esters and amides of benzofuran-2-carboxylic acid containing anticancer mustard type pharmacophore in their structure.

Experimental Section
2.1.General. 1 H and 13 C NMR spectra were recorded on a TM Bruker DPX 400 (400 MHz) instrument, solvent CDCl 3 .Chemical shis  are given from TMS (0 ppm) as an internal standard for 1 H NMR, and CDCl 3 (77.0ppm) for 13 C NMR (100 MHz).Mass spectra were obtained on an Agilent Technologies 6890 N apparatus, equipped with a mass detector 5973 Network and 30 m × 0.25 mm capillary column �lled with a 0.25 m �lm of a 5% MePh silicate.All melting points were determined on a Boetius apparatus and are uncorrected.

General Procedure for Synthesis of 5-[Bis(2-Chloroethyl) Amino]-7-Methoxybenzofuran-2-Carboxylic Acid Derivatives (8a, 8b
).To a solution of compound 8a or 8b (0.6 mmol) in dichloromethane (50 mL) thionyl chloride (0.8 mL) was added.e reaction �ask was sealed and was le� at room temperature for 24 h.Dichloromethane and excess of thionyl chloride were removed by evaporation and the crude product was washed with n-hexane and next crystallized from the same solvent to give product 8a or 8b.Compund 8a: yield-47%, oil product.

Results and Discussion
In order to obtain 5-nitro and halo-substituted benzofurnan-2-carboxylic acid derivatives we have developed a method of synthesis proceeding via corresponding ethyl 2-formylphenoxy acetates as the intermediates.According to the literature reports ethyl 2-formylphenoxyacetate acid was obtained by O-alkylation of salicylaldehyde with ethyl bromoacetate carried out in acetone or dimethylformamide under heating conditions for 20 hours [24,25].e condensation was also carried out in acetone in the presence of tripotassium phosphate but this process was proven to be slow [26].Moreover, the reaction of 2-hydroxy-5-nitrobenzaldehydes with ethyl bromoacetate was also reported to be a slow process lasting for 36 hours [12].
We have previously observed that the reaction of salicylaldehyde with ethyl chloroacetate, carried out in DMF solution and with the use of potassium carbonate as a basic dehydrating agent, led to a mixture of two esters: ethyl 2-formylphenoxyacetate and ethyl benzofuran-2carboxylate in ratio 1 : 1 [12].In the present research we have found that the reactions of 5-bromo-or 3,5dichloro-2-hydroxybenzaldehyde with ethyl chloroacetate, carried out in DMF solution in the presence of 1.5 molar excess of anhydrous potassium carbonate at 92-94 ∘ C for 4 hours, lead only to the corresponding cyclic esters: ethyl 5-bromo-or 5,7-dichlorobenzofuran-2-carboxylate (2a, 2b) (Scheme 1).In both cases the products were obtained in high yields.
On the other hand, the reaction of 2-hydroxy-5-nitro-3-methoxybenzaldehyde (1c) with ethyl chloroacetate performed in the same conditions as for synthesis of 2a (DMF, K 2 CO 3 ) afforded only ethyl 2-formyl-6-methoxy-4-nitrophenoxyacetate ( 4).An attempt to obtain the cyclic product by one-pot synthesis failed, despite longer reaction time (10 hours) and the use of more than double molar excess of K 2 CO 3 .
However, when dried ethyl 2-formyl-6-methoxy-4nitrophenoxyacetate (4) underwent 4-hour cyclization in DMF with 1.3 molar excess of anhydrous potassium carbonate at 92-94 ∘ C the desired ethyl 7-methoxy-5nitrobenzofuran-2-carboxylate (2c) was obtained in good yield (73%) (Scheme 2).ese conditions were established according to the results of several trials of this reaction in which the reaction time and the molar ratio of potassium carbonate to the ester were varied (Table 1).
Cyclization of 2-formylphenoxyacetic acid ethyl ester to the benzofuran derivative is based on base catalyzed intramolecular aldolization of formyl ester via formation of carbiol and next elimination of water (Scheme 3).e presence of halogens as donors of electrons favors this reaction.On the other hand, strongly electrophilic nitro group lower the activity of the carbonyl group and because of that it was not possible to obtain the suitable ester of benzofuran-2carboxylic acid in a one-step process.e desired product 2c could be only obtained starting from previously isolated and  dried ester 4 by heating in DMF in the presence of anhydrous K 2 CO 3 as a base.e obtained 7-methoxy-5-nitrobenzofuran-2-carboxylate (2c) was subsequently reduced to the corresponding amine (5).e reduction was carried out in methanoldimethoxyethane mixture at room temperature under a hydrogen stream, with the use of 10% palladium on activated coal as a catalyst.In order to obtain the desired product in satisfactory yield three attempts of reduction were made.e reduction was performed with different weight ratio of nitrobenzofuran to the catalyst, that is, 1 : 0.1 or 1 : 0.13, for 3.5 or 4 hours.Progress of the reaction was monitored by GC/MS analysis of periodically taken samples.When the reduction was �nished the reaction mixture was le for 24 hours.e catalyst was �ltered off and the solvent was evaporated, affording the crude product which was analyzed by GC/MS and crystallized.e best result of the reduction was achieved when the reaction was carried out at room temperature for 3.5 hours, using a weight ratio of ethyl 7-methoxy-5-nitrobenzofuran-2-carboxylate to the catalyst of 1 : 0.13.In these conditions the crude ethyl 5-amino-7methoxybenzofuran-2-carboxylate (5) was obtained in 91% yield.
In the next step the amino compounds (5 and 6) were transformed into corresponding bis(2-chloroethyl)amino derivatives (8a and 8b) with the use of a two-step strategy: N-hydroxyalkylation with ethylene oxide followed by chlorination with thionyl chloride (Scheme 1).When Nhydroxyalkylation of ethyl 5-amino-7-methoxybenzofuran-2-carboxylate (5) with ethylene oxide was performed with 5-fold molar excess of ethylene oxide in methanol and the reaction mixture was le at 6-7 ∘ C for two weeks, the crude product consisted of two compounds, that is, the desired ethyl 5-[bis(2-hydroxyethyl)amino]-7-methoxybenzofuran-2-carboxylate (60%) and ethyl 5-[mono-(2-hydroxyethyl) amino]-7-methoxybenzofuran-2-carboxylate (40%).erefore, in the next attempt a larger, 10-fold molar excess of ethylene oxide was employed while the other conditions of the reaction remain unchanged.e product was analyzed by CG/MS and it turned out that only the target ethyl 5-[bis(2-hydroxyethyl)amino]-7-methoxybenzofuran-2-carboxylate (7a) was obtained.Next, the reaction of amine 6 with ethylene oxide was carried out in the same conditions to give derivative 7b in good yield.
Bis(2-hydroxyethyl)amino derivatives 7a and 7b were next subjected to the reaction with thionyl chloride.To obtain  the desired products 8a and 8b in good yields several attempts to this reaction were done using benzene or methylene chloride as solvents and changing reaction temperature and the time.e desired products 8a and 8b were �nally obtained in the highest yields of 47% and 50%, respectively, when the reagents were le in methylene chloride at room temperature for 24 hours.

Con�ic� of �n�eres�s
e authors declare that they do not have a direct �nancial relation with the commercial identity mentioned in this paper that might lead to a con�ict of interests.