ecting group, or R4 and R5 together are an oxo group and R6 is hydrogen or fluorine and R7 is fluorine, or R4 is hydrogen, R5 is hydroxy, R6 is hydrogen and R7 is amino, amino substituted by a protecting group which is readily cleavable under physiological conditions, azido or fluorine or R6 and R7 each are fluorine or together are oximino or an oxo group, R8 is hydroxymethyl, alkylhydroxymethyl,