(b) in general structure (iv), when Z is oxygen, X1 is NR13, wherein R13 is hydrogen, alkyl, C1-C4 alkanoyl or aryl and X2 is methylene, then R6 is other than hydrogen or a substituent on the ring member at position 2 selected from: hydroxyl, C1-C4-alkyloxy, C1-C4-alkyloxycarbonyl, aryloxy and NR9R10;