"Compounds of the formula I, wherein n is zero or the number 1, R1 is C1 -C4 alkyl, methoxymethyl or phenyl, R2, R3, R4, R5, R6, R7 and R8 are a each hydrogen, X is an oxygen atom and Y is the radical ##STR9## in which Z is ethylene and R11 is a hydrogen atom or methyl, represent particularly preferred compounds." . . . . . . .