lent when R.sub.0 and R.sub.0 are covalently bonded to each other and to the same dioxy carbon atom to form a fused polycyclic ring or fused polycyclic ring substituted with an alkyl, alkoxy or alkenyl when R.sub.0 is an oxa, alkyleneoxy or alkenyleneoxy and R.sub.0 is aryloxy, aralkyleneoxy, aralkenyleneoxy or aralkylene