A method for the preparation of a compound of formula (II) wherein R
1
and R
2
are independently H, alkyl, alkoxy, oxoalkyl, alkenyl, aryl or arylalkyl whether unsubstituted or substituted, optionally interrupted by one or more hetero atoms, straight chain or branched chain, hydrophilic, hydrophobic or fluorophilic; R
3
, R
4
, R
5
and R
6
are independently or all hydrogen or halogen; provided that at least two of the R
3
, R
4
, R
5
and R
6
are halogens.