Tetrafluoroboric acid, a useful deprotecting reagent in peptide synthesis

We have found that tetrafluoroboric acid (HBF4) in trifluoroacetic acid (TFA) in the presence of thioanisole cleaves various protecting groups currently used in peptide synthesis. HBF4 in TFA cleaves an amino acid amide from 4-methylbenzhydrylamine resin more effectively than trifluoromethanesulfonic acid in TFA. Lamprey gonadotropin-releasing hormone (a 10-residue peptide amide) was synthesized using 1 M HBF4-thioanisole in TFA by both solution-phase and solid-phase methods.