2-Cyano-3,5-dihydroxyhex-2-enecarboxamide derivatives

A compound of the formula I

• in any stereoisomeric form, or a salt thereof,
• wherein
• R1 is a hydrogen atom or (C1-C4)-alkyl,
• R2 is pyridyl,
• R3 is
• a) —CF3,
• b) —O—CF3,
• c) —S—CF3,
• d) —OH,
• e) —NO2,
• t) halogen,
• g) benzyl,
• h) phenyl,
• i) —O-phenyl,
• j) a hydrogen atom,
• k) —CN, or
• l) —C-phenyl, mono- or polysubstituted by
• 1) (C1-C4)-alkyl,
• 2) halogen,
• 3) —O—CF3 or
• 4) —O—CH3,
• R4 is
• a) (C1-C4)-alkyl,
• b) halogen, or
• c) a hydrogen atom, and
• n is the integer zero, 1, 2, 3 or 4.