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, 2 fluoroethyl, vinyl, isopropyl, isopropenyl or cyclopropyl group; and {HEAD ~ X} represents {TAIL ~ CH}, C F, C Cl or N characterized in that R2 and R3 are not hydrogen at the same time 
represents
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 , 2 fluoroethyl , vinyl , isopropyl , isopropenyl or cyclopropyl group ; and { HEAD ~ X } represents { TAIL ~ CH } , C F , C Cl or N characterized in that R2 and R3 are not hydrogen at the same time
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