Synthesis and properties of 5-styryl-2-furancarboxylic acids

5-(4-X-Styryl)-2-furancarboxylic acids (X = H, Br, NO2, COOCH3, (CH3)2N) were prepared by oxidation of the appropriate 5-(4-X-styryl)-2-furaldehydes, or alternatively by alkaline hydrolysis of the corresponding methyl esters. The latter were obtained by reacting the 4-substituted benzaldehydes with 5-methoxycarbonylfuryltriphenylphosphonium chloride. The UV and IR spectra of products are commented. The apparent pK values of the synthesized acids were employed for calculation of the transmission coefficient π' through the benzene-furan system bridged by a -CH=CH- group.