Benzylidène‐5‐hydroxy‐4‐dihydro‐2,5‐thiophènecarboxanilides‐3‐( Z ): Synthèse et isomérisation en benzyl‐5‐hydroxy‐4‐thiophènecarboxanilides‐3

( Z )‐5‐Benzylidene‐2,5‐dihydro‐4‐hydroxy‐3‐thiophenecarboxanilides: Synthesis and Isomerization into 5‐Benzyl‐4‐hydroxy‐3‐thiophenecarboxanilides Upon treatment with an arylamine in boiling xylene, the esters I yield predominantly the corresponding anilides II, along with a small but variable amount of the isomeric thiophene derivatives III ( Table 1 ). On the other hand, the derivatives III can be readily prepared by base‐ or acid‐catalyzed isomerization of II . Esters I can also be isomerized to the corresponding thiophene derivatives IV (Scheme 6) , but only in the presence of a strong acid (Table 4) . The two series of isomers reported in Tables 2 and 3 present spectral differences which allow unambiguous structural assignments. The ( Z )‐configuration for compounds of Table 2 is confirmed by a NOE study carried out on the O ‐methyl derivatives 6a and 7a .