Prepn from butyric acid: Williams, Krynitsky, Org. Synth. coll. vol. III, 165 (1955); Kuwajima, Mukaiyama, J. Org. Chem. 29, 1385 (1964). Manuf by catalytic hydrogenation of crotonic acid: Smith, Hunter, US 2492403 (1949 to Celanese); by catalytic carbonylation of the corresponding acid ester: Reppe, Friederich, US 2730546 (1956 to BASF).