. . . . "The invention relates very especially preferably to compounds of formula I wherein R1 is hydrogen, lower alkyl or lower alkoxy-lower alkyl, R2 is lower alkenoyl or halo-lower alkanoyl, or, when R1 is lower alkoxy-lower alkyl, R2 may also be hydrogen, R3 and R4 are each hydrogen, R5 is chlorine, R6 is hydrogen, X is a direct bond and Y is 1,2-ethylene, and pharmaceutically acceptable salts thereof." . .