The invention provides compounds of the formula:R1 is OR11, or NR5R5'；R2 is F；R5 is H, C1-C6alkyl, OH, C(=O)R6, O(C=O)R6 or O(C=O)OR6；R5' is H or C1-C6alkyl；R6 is C1-C22alkyl or C3-C7cycloalkyl；R13 is H, phenyl, pyridyl, benzyl, indolyl or naphthyl wherein the phenyl, pyridyl, benzyl, indolyl and naphthyl is optionally substituted with 1, 2 or 3 R22；and the other variables are as defined in the claims,which are of use in the treatment of cancer, and related aspects.
