Abstract: Derivatives having the formula: ##STR1## where R.sub.1 denotes C.sub.1 -C.sub.4 alkyl; X denotes oxygen or methylene; R.sub.2 denotes C.sub.1 -C.sub.4 alkyl or CF.sub.3 ; R.sub.3 denotes a CH.sub.2 R.sub.4 group in which R.sub.4 is (i) a pentagonal or hexagonal aromatic heterocyclic group comprising one or two nitrogen atoms and optionally fused with a benzene ring or (ii) a group having the formula ##STR2## where n=0 or 1, and R.sub.5 and R.sub.6 independently denote C.sub.1 -C.sub.4 alkyl or benzyl, and R.sub.5 and R.sub.6 also, together with the nitrogen atom to which they are bonded, form a piperidino or pyrrolidino or morpholino or N-methyl piperazino radical, and their N-oxide forms and acid addition salts of these derivatives and their N-oxide forms, the derivatives being in various stereoisomeric forms or in the form of a mixture of these forms, including the racemic form.