Abstract: The present invention relates to compounds of formula ##STR1## in which formula I X stands for O, S, ##STR2## R.sub.1 and R.sub.2 which can be the same or different stand for hydrogen, straight or branched, saturated or unsaturated, unsubstituted or substituted C.sub.1- C.sub.8 -alkyl, aryl or for ar-C.sub.1 -C.sub.4 -alkyl, aryl and ar being unsubstituted or substituted phenyl; R.sub.3, R.sub.4, R.sub.5, and R.sub.6 are the same or different and stand for hydrogen, halogen, pseudo halogen, cyano, nitro, amino, carboxy, hydroxy, alkyl, alkoxy; or R.sub.5 and R.sub.6 form an aromatic ring which is fused to the pyridyl ring, and which aromatic ring may substituted; provided that R.sub.1 and R.sub.2 cannot be hydrogen at the same time, and provided that when R.sub.5 and R.sub.6 both are chlorine and R.sub.1 is hydrogen, then R.sub.2 cannot be n-propyl; and salts and bioreversible derivatives thereof.

Abstract: The present invention relates to compounds of formula ##STR1## in which formula I X stands for O, S, ##STR2## R.sub.1 and R.sub.2 which can be the same or different stand for hydrogen, straight or branched, saturated or unsaturated, unsubstituted or substituted C.sub.1 -C.sub.8 -alkyl, aryl or for ar-C.sub.1 -C.sub.4 -alkyl, aryl and ar being unsubstituted or substituted phenyl; R.sub.3, R.sub.4, R.sub.5, and R.sub.6 are the same or different and stand for hydrogen, halogen, pseudo halogen, cyano, nitro, amino, carboxy, hydroxy, alkyl, alkoxy; or R.sub.5 and R.sub.6 form an aromatic ring which is fused to the pyridyl ring, and which aromatic ring may substituted; provided that R.sub.1 and R.sub.2 cannot be hydrogen at the same time, and provided that when R.sub.5 and R.sub.6 both are chlorine and R.sub.1 is hydrogen, then R.sub.2 cannot be n-propyl; and salts and bioreversible derivatives thereof.