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