The condensation of 7-ethyl-4-methylbenzofuran-2-carbaldehyde (I) with 4-acryloylbenzoic acid methyl ester (II) by means of 3-benzyl-5-(hydroxymethyl)-4-methylthiazolium chloride (BHMT) in hot DMF gives the 1,4-butanedione (III), which is cyclized with ammonium acetate in refluxing methanol to yield the substituted pyrrole (IV). Finally, the ester group of (IV) is hydrolyzed with NaOH in refluxing ethanol/water.
The condensation of 2-hydroxy-3,6-dimethylbenzaldehyde (I) with 2-bromoacetaldehyde dimethyl acetal (II) by means of K2CO3 in DM gives the phenoxyacetaldehyde acetal (III), which is cyclized by means of acetic acid to yield 4,7-dimethylbenzofuran (IV). The condensation of (IV) with 4-acryloylbenzoic acid methyl ester (V) by means of 3-benzyl-5-(hydroxymethyl)-4-methylthiazolium chloride (BHMT) and TEA in DMF affords the butanedione derivative (VI), which is cyclized to the pyrrole derivative (VII) by means of NH4OAc in refluxing methanol. Finally, the methyl ester group of (VII) is hydrolyzed with NaOH in ethanol to afford the target carboxylic acid.
This compound has been obtained by two related ways: 1) The Grignard reaction of 5,8-dimethylnaphthalene-2-carbaldehyde (I) with vinylmagnesium bromide in ether/THF gives the expected carbinol (II), which is oxidized with MnO2 in dichloromethane yielding the propenone (III). The condensation of (III) with 4-formylbenzoic acid methyl ester (IV) by means of 3-benzyl-5-(hydroxymethyl)-4-methylthiazolium chloride (BHMT) in refluxing ethanol affords the 1,4-butanedione (V), which is cyclized with ammonium acetate in refluxing methanol to give the substituted pyrrole (VI). Finally, the ester group of (V) is hydrolyzed with NaOH in refluxing ethanol/water. 2) 1,4-Butanedione (V) can also be obtained by condensation of 5,8-dimethylnaphthalene-2-carbaldehyde (I) with 4-acryloylbenzoic acid methyl ester (VII) by means of BHMT in hot DMF.
The Grignard reaction of 7-fluoro-4-(trifluoromethyl)benzofuran-2-carbaldehyde (I) with vinylmagnesium bromide in ether/THF gives the expected carbinol (II), which is oxidized with MnO2 in dichloromethane yielding the propenone (III). The condensation of (III) with 4-formylbenzoic acid methyl ester (IV) by means of 3-benzyl-5-(hydroxymethyl)-4-methylthiazolium chloride (BHMT) in refluxing ethanol affords the 1,4-butanedione (V), which is cyclized with ammonium acetate in refluxing methanol to give the substituted pyrrole (VI). Finally, the ester group of (V) is hydrolyzed with NaOH in refluxing ethanol/water.
From the starting carbaldehyde (I) two different pathways can be followed: 1) A solution of carbaldehyde (I) in ether is treated with the vinyl Grignard reagent (A) in THF and oxidized with activated manganese dioxide to afford enone derivative (II). The derivative is then coupled with methyl 4-formylbenzoate (III) in refluxing ethanol in presence of 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium chloride (B) to yield diketone (IV). 2) Methyl 4-formylbenzoate (III) is treated with vinylmagnesium bromide (A) in THF, and oxidized with pyridinium dichromate to give methyl 4-acryloylbenzoate (V), which is coupled with carbaldehyde (I) by means of 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium chloride (B) and triethylamine in DMF to yield directly diketone (IV). The final step is the cyclization of diketone (IV) to afford the pyrrole-benzoic acid derivative by treatment of the diketone with ammonium acetate in refluxing methanol and hydrolysis with NaOH in refluxing ethanol followed by neutralization with dilute HCl.