Spirocyclic tetronic acid derivatives

A compound of formula (I)

• in which
• R1, R2, R3 independently of one another represent alkyl, halogen, haloalkyl, alkoxy, cyano or CO2-alkyl, where if R1 and R2 represent methyl, R3 represents ethyl, propyl or methoxy;
• or
• R1 and R2 together with the carbon atom to which they are attached represent an unsubstituted or substituted cycle which may optionally be interrupted by 1 or 2 heteroatoms
• R3 represents C1-C4-alkyl;
• or
• R1, R2 and R3 together with the carbon atom to which they are attached represent optionally substituted adamantyl.