Propanal → 3-Formyl-crotonic acid butyl ester

836 G. of the obtained glyoxalic acid butyl ester are introduced into 376 g. of propionaldehyde. The mixture is treated dropwise at 60° C. with 40.8 g of di-n-butylamine. In so doing, the reaction temperature should not rise higher than 105° C. The reaction mixture is then stirred for 2 hours at 116°-111° C., cooled and taken up in ether. The diethyl ether extract is washed successively with 500 ml. of 1 N sulphuric acid, 700 ml. of water, 1000 ml. of 5% by weight aqueous sodium bicarbonate solution and subsequently with 1000 ml. of water, dried over sodium sulphate and evaporated under reduced pressure. The remaining 3-formylcrotonic acid butyl ester boils, after rectification, at 93°-105° C./14 mm. Hg; nD25 = 1.