The derivatives have the following formula: ##STR1## wherein R1, R2 and R3 represent each a hydrogen or C1 C6 alkyl group; R4 represents an ethyl, {HEAD ~ 2 fluoroethyl}, vinyl, isopropyl, {TAIL ~ isopropenyl} or cyclopropyl group; and X represents CH, C F, C Cl or N characterized in that R2 and R3 are not hydrogen at the same time 
The derivatives have the following formula : # # STR1 # # wherein R1 , R2 and R3 represent each a hydrogen or C1 C6 alkyl group ; R4 represents an ethyl , { HEAD ~ 2 fluoroethyl } , vinyl , isopropyl , { TAIL ~ isopropenyl } or cyclopropyl group ; and X represents CH , C F , C Cl or N characterized in that R2 and R3 are not hydrogen at the same time
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