In the process according to the present invention, compounds of formula 1,wherein R1 and R2 represent a hydrogen atom associated with amine, are preferably obtained from previously prepared compounds of formula 1, wherein R1 and R2 represent an alkyl or R1 is a hydrogen atom associated with amine and R2 is an alkyl in the reaction with trimethylsilyl halide, particularly with bromotrimethylsilane.