Trimethylsilyl bromide (TMSBr) in trifluoroacetic acid (TFA) was found to have the ability to cleave benzyl-type protecting groups, i.e., benzyloxycarbonyl (Z), benzyl (O-Bzl) and p-methoxybenzyl (S-MBzl). The reaction was best accelerated by addition of thioanisole, compared with other soft nucleophiles so far examined. The rate of the cleavage reaction with TMSBr/TFA was judged to be somewhat slower than that with trimethylsilyl trifluoromethanesulfonate/TFA. However, TMSBr/TFA reduced Met (O) efficiently and gave almost no side reaction of Asp (succinimide formation). This deprotecting procedure was applied to the synthesis of human gastrin-releasing peptide.
The properties of two aspartic acid β-esters (Asp(OR), R=cycloheptyl (Chp) and cyclooctyl(Coc)) were examined. These two protecting groups were found to be stable to trifluoroacetic acid(0°C, 2 h), but were cleaved by HF or 1 M trifluoromethanesulfonic acid-thioanisole in trifluoro-acetic acid (0°C, 60 min). Using a model peptide, Z(OMe)-Ala-Asp(OR)-Gly-OBzl, the behavior of the Asp(OR) residue with base or acid was examined. These esters were less susceptible to succinimide formation than the Bzl group in Et3N treatment. In acid deprotection, succinimide formation from these peptides was less than 7% in both cases.