R3 is hydrogen, C1 -C6 alkyl or C2 -C6 alkanoyl; and the pharmaceutically acceptable salts thereof; and wherein, when, at the same time, R2 is hydrogen, C1 -C6 alkyl, halogen or cyano and R3 is hydrogen, R1 and n being as defined above, then at least one of R4, R5 and R6 is other than hydrogen.