Compounds of the formula I in which it represents: R 1, R 2 and R 3, independently of one another H or an acyl radical having 1 to 10 carbon atoms; and R 4 is H or -C (O) (CH 2) n COOH, wherein n is 1 to 7; with the exception that R1, R2, R3 and R4 do not all mean X; as well as their physiologically acceptable salts. The application contains 15 more patent claims.