Compounds of the formula (I) A is OH or a carbonyl group, n is 0 or 1, R is an alkyl radical having from 1 to 4 carbon atoms, in particular methyl, ethyl, n- propyl, iso-propyl, n-butyl or t-butyl or a residue of a hydroxy carboxylic acid, in particular the residue -CH(OH)COOH, -CH(OH)CH2COOH, -CH2CH(OH)COOH or -CH(OH)CH(OH)COOH and R" is independently selected from H or OH.