The invention relates to a process for preparing a compound of the formula (I), in which R is H or C1-C6 alkyl, by reaction of at least one compound of the formula (II) in which R has the same definition as in the formula (I) and in which R1 is H, C1-C12 alkyl or C3-C12 cycloalkyl, with a compound of the formula (III) in which R2 is H or C(O)R3, in which R3 is H or C1-C12 alkyl, in the presence of at least one enzyme suitable for transesterification.