Compounds of the formula are disclosed: ##STR1## wherein R.sup.1, R.sup.2 and R.sup.3 are identical or different and represent an optionally substituted alkyl, alkenyl, cycloalkyl or an aryl group and R.sup.4 represents a monosubstituted or polysubstituted aryl group, containing at least one substituent which favours electrophilic aromatic substitution.