Chem., 1999, 42, 2180-2190) with a compound of formula L???R6 wherein L is a leaving group, e.g. halo, sulfonate or triflate, will yield both isomers of the compounds of formula (I) where X1 or X2 is NR6 in which R6 is other than hydrogen, the isomers can be separated by chromatographic methods. [0042] During the synthesis of the compounds of formula (I) labile functional groups in the intermediat