Advantageously, formula (I) esters in which B is hydrogen; W is oxygen and A is COOR3 wherein R3 is a saturated C1 -C12 alkyl, C3 -C6 cycloalkyl or benzyl substituent, and R1, R2, X, Y and Z are as defined above; can be prepared by reaction of the corresponding acid, i.e., where A is COOH, with an appropriate alcohol in the presence of a catalytic amount of a strong mineral acid such as hydrochlor