According to a further aspect the invention provides a compound of formula (II): wherein A is ???NH, ???CON3, ???CO2H, ???N???C???O, ???N???C???S, ???NHCOL, halogen or ???NHCOCBr3 and L is a leaving group, n, X and Y are as defined in formula (I), and R1??? and R7??? are R1 and R7 as defined in formula (I) or groups convertible thereto.