Chem., 1999, 42, 2180-2190) with a compound of formula L???R10 wherein L is a leaving group, e.g. halo, sulfonate or triflate, will yield both isomers of the compounds where X1 or X2 is NR10 in which R10 is other than hydrogen, the isomers can be separated by chromatographic methods (Scheme 3). [0069] Compounds of formula (I) where R3 is ???CH2NHCOR7 may be prepared according to Scheme 4.