Methyl 4-(benzofuran-2-yl)-2,4-dioxobutanoate

The synthesis of novel derivatives of pyrazole-3-carboxylate (3-5) from methyl 4-(benzofuran-2-yl)-2,4-dioxobutanoate (1)is reported. 

The synthesis of the starting compound, methyl 4-(benzo-furan-2-yl)-2,4-dioxobutanoate 1 achieved in quantitative yields by the reference method.³⁹ 39 N.J. Siddiqui, M. Idrees, N.T. Khati and M.G. Dhonde, Bull. Chem. Soc. Ethio., 2013, 27, 85-94.   

The IR spectrum of 1 showed the enolic -OH stretch at 3475 cm^-1 and C=O stretching in ester group at 1759 cm^-1. The ¹H NMR spectrum showed a singlet³⁹ at δ 14.24 ppm due to -OH proton, singlet at ö 3.95 ppm due to OCH₃, multiplet at ö 7.27-7.72 ppm due to aromatic protons and singlet at ö 7.10 ppm confirms vinylic =CH proton.

A chemical shift value in ¹³C NMR is observed at ö 53.3 ppm due to methoxy carbon; the carbon atoms connected to methoxy group are observed at the range of δ 156-167 ppm, signal at δ 167.9 ppm is due to C-1 carbon in C=O of the ester group whereas C-4 carbon in C=O group under the influence of strong electronegative environment appears downfield at δ 181.0 ppm; the aromatic carbons were observed in expected region. 

The mass spectrum of this product reveals a molecular ion at m/z 247 [M+H] + is in consistent with the molecular formula C₁₃H₁₀O₅.

Methyl 4-(benzofuran-2-yl)-2,4-dioxobutanoate 1 on reaction with hydroxyl amine hydrochloride and sodium acetate in absolute ethanol gave methyl 4-(benzofuran-2-yl)-2-(hydroxy-imino)-4-oxobutanoate 2a. The ¹H NMR spectrum showed a singlet at δ 12.39 ppm corresponding to enolic -OH proton, multiplet a δ 7.20-7.73 due to aromatic protons, this confirmed that cyclization has not occurred to form an isoxazole ring. Hence, 2a was heated in 50 mL absolute ethanol in presence of conc. HCl for 2 hours to get methyl 5(benzofuran-2-yl)-isoxa-zole-3-carboxylate 2, where a multiplet at δ 7.23-7.75 ppm due to aromatic protons, and a quartet at ö 4.41-4.46, a triplet at δ 1.39-1.42 ppm due to the presence of the COOCH₂CH₃ group confirmed that trans esterification has also occurred simultaneously.

Synthesis of Methyl 4-(Benzofuran2-yl)-2,4-dioxobutanoate (1 )

Diethyloxalate (1.46 mL, 10 mmol) was gradually added with stirring to a solution of 2-acetyl benzofuran (1.6 g, 10 mmol) and sodium methoxide (0.23 g Na in 5 mL methanol, 10 mmol) in N,N-Dimethylformamide (100 mL). The reaction mixture was stirred for 12 h at room temperature, the product obtained was acidified by 1:1 ice-cold HCl, filtered, washed with water and recrystallized from acetone to get yellow crystalline solid 1 (85 %); 

m.p.: 131-133 °C; R = 0.66 (ethylacetate/n-hexane 1:2); 

IR (KBr, n_max): 3475 (-OH), 3059, 3020 (ArH), 2968, 2879 (CH₃) 1805, 1759 (C=O, ester), 1624,1573,1521 (C=c) cm^-1; 

¹HNMR (CDCl₃): δ 3.95 (s, 3H, CH₃), 7.10 (s, 1H, =CH), 7.27-7.72 (m, 5H, ArH), 14.24 (s, 1H, -OH) ppm; 

¹³C NMR (CDCl₃): 53.3,99.3,112.4,114.1, 123.3, 124.2, 127.3, 128.8 , 150.8, 156.2, 162.3, 167.9, 181.0 ppm;

ESI(+)-MS: m/z 247 (M+H)+. Anal. calcd. for C₁₃H₁₀O₅: C, 63.41; H, 4.06 %. Found: C, 62.52; H, 4.13 %.

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