Dehydrobromination of the ethyl ester of 2-bromo-2-propylpentanoic acid (I) by means of a tertiary amine yields a mixture of the isomers of 2-propyl-2-pentenoic acid ethyl ester (IIa) and (IIb), from which the major component, the trans-isomer (IIa), is separated and saponified to obtain trans-2-propyl-2-pentenoic acid (trans-2-en-valproic acid) (H. Schafer, personal communication).