wherein Y stands for --O--, --S-- or ##STR3## and R is a phenyl or naphthyl radical optionally substituted by alkyl or isoalkyl groups or by halogen, or a cycloalkyl group having from 5 to 12 ring carbon atoms, or an alkyl chain having from 1 to 60 carbon atoms, optionally interrupted by ether, thioether, carboxylic acid ester groups and/or --C???C double bonds, and optionally substituted by an (