| http://www.w3.org/ns/prov#value | - Process according to claim 11 characterised in that a compound of formula 1, wherein R1 and R2 represent positively charged counterion(s), is preferably obtained from previously prepared compounds of formula 1 wherein R1 and R2 represent an alkyl, or R1 is a hydrogen atom or a positively charged counterion and R2 is an alkyl, in the reaction with trimethylsilyl halide, particularly with bromotrime
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