Abstract: A novel process for the preparation of pent-4-enoic acids of the formula ##STR1## wherein X is selected from the group consisting of hydrogen, cyano and alkoxycarbonyl of 2 to 5 carbon atoms and Y is selected from the group consisting of hydrogen, cyano, alkoxycarbonyl 2 to 5 carbon atoms, acyl of an organic carboxylic acid of 2 to 7 carbon atoms, alkylsulfonyl of 1 to 4 carbon atoms and arylsulfonyl of 6 to 7 carbon atoms with the proviso that X and Y are not both hydrogen comprising reacting a compound of the formula ##STR2## wherein R is selected from the group consisting of acyloxy of an organic carboxylic acid of 1 to 7 carbon atoms, alkylsulfonyl of 1 to 4 carbon atoms and arylsulfonyl of 6 to 7 carbon atoms with an anion derivative of a compound of the formula ##STR3## wherein X.sub.1 and Y.sub.