definitions with the proviso that R is not methyl, ethyl, n-propyl or n-butyl when R.sub.1 is methyl and R is not methylwhen R.sub.1 is ethyl comprises reacting a compound of the formula ##STR5## with a compound of the formulawherein A is acyl of an organic carboxylic acid of 2 to 18 carbon atoms and X is a halogen and saponifying the resulting ester to obtain a compound of formula I.sub.A '.The