NEW MATERIAL:Compounds of formula I (R is 1W4C straight or branched-chain alkyl).

EXAMPLE: 2-Acetylamino-4,5,5-trifluoro-5-ethoxypentanoic acid.

USE: Synthetic intermediate of fluorine-containing aminoacids or intermediate of medicines and agricultural chemicals.

PREPARATION: A β-trifluorovinylalanine derivative of formula II (R' is methyl, ethyl) and an alcohol of ROH are heated in the presence of a base such as potassium or sodium hydroxide and the reaction mixture is treated with an inorganic acid such as dilute hydrochloric acid to produce the compound of formula I. The reaction is effected by dissolving the compound of formula II in an excess amount of the alcohol (5W20 times the weight of the compound) and adding the base 1.5W2 times the molar amount of the compound of formula II. The objective compound is prepared readily by the one-step process.