Simple route to synthesize (E)-3-propyl-4-oxo-2-butenoic acid esters through the Z isomer

Esters of 3-alkyl-4-oxo-2-butenoic acid, which are very important synthons, are not equally accessible in both E and Z configurations. The (Z)-isomers can be easily obtained from 3-alkyl-4-hydroxybutenolides, in turn prepared by aminoalkylation of aliphatic aldehydes with glyoxylic acid. The (E)-isomers, on the contrary, result from laborious procedures: the condensation of aldehydes with glyoxylic acid, followed by separation from γ-hydroxybutenolide by-product and esterification, or of aldehyde enamines with glyoxylic esters, followed by Z ester by-product conversion into γ-aminobutenolide and purification. Here, we describe a straightforward route to the title compounds, applied to methyl (E)-3-propyl-4-oxo-2-butenoate, avoiding any problematic by-product or isomer chromatographic separation: pentanal and glyoxylic acid are condensed to 3-propyl-4-hydroxybutenolide, which is converted to methyl (Z)-3-propyl-4-oxo-2-butenoate and then isomerized to E ester under acidic conditions.