N-alkylation of N-aryl-alpha-amino acid methyl esters by trialkyloxonium tetrafluoroborates

In this work we present the results obtained for the N-alkylation of a series of N-arylsulfonyl-alpha-amino acid methyl esters bearing different substituents at the 4-position of the sulfonamide aromatic ring. In particular, we compare the reactivity of these species with diazomethane and trimethyloxonium tetrafluoroborate in N-methylation processes. Diazomethylation is unsuccessful for N-arylsulfonamide derivatives containing electron-releasing groups on the aromatic ring. In these cases trimethyloxonium tetrafluoroborate is the reagent of choice for the direct and quantitative N-methylation. Further we extend our evaluation to the use of triethyloxonium tetrafluoroborate. This reagent shows to be very efficient in order to prepare N-ethyl derivatives of N-arylsulfonyl-alpha-amino acid methyl esters. An experimental protocol similar to that used for N-methylation with trimethyloxonium tetrafluoroborate is applied for the N-ethylation.