pendently selected from an unsubstituted or substituted alkyl, aryl, amino or alkoxy group or a 5-10 membered heterocyclic ring which contains one or more heteroatomsselected from nitrogen, oxygen and sulfur, which ring is unsubstituted or substituted.When R' and/or R are an amino or alkoxy group they may, for example, be substituted with a halogen, aryl-oxy or alkyl- or aryl-sulfonyl group.