SN2 Deprotection of Synthetic Peptides with a Low Concentration of HF in Dimethyl Sulfide: Evidence and Application in Peptide Synthesis

Article abstract of DOI:10.1021/ja00359a014

An S_N2 deprotection reaction for synthetic peptides was observed when the weak base dimethyl sulfide was used as a diluent for HF.Kinetic studies of the deprotection of O-benzylserine revealed that there was a sharp changeover in mechanism from A_AL1 to A_AL2 when the concentration of HF in dimethyl sulfide was below 55percent.The changeover in mechanism was also found in the deprotection of O-benzyltyrosine.At higher HF concentrations (>55percent), the A_AL1 cleavage mechanism, which generates carbonium ions, led to significant 3-benzyltyrosine side product.However, at low HF concentrations, the side product was minimal as a result of an A_AL2 cleavage mechanism in which carbonium ions are not formed.A sharp increase of side product was seen when the HF concentration reached the critical changeover concentration.The HF-dimethyl sulfide reagent was also found to reduce methionine sulfoxide to methionine and, in the presence of a thiol, to deprotect Nⁱ-formyltryptophan to tryptophan.Both of these reactions were also dependent on the concentration of HF and were optimal at low concentrations.Furthermore, deprotection of aspartic and glutamic acid side chain benzyl esters at the low HF concentration also minimized the A_AC1 mechanism and the accompanying acylation side reactions.A practical mixture for the S_N2 deprotection reaction was found to be HF-dimethyl sulfide-p-cresol (25:65:10 v/v).For the deprotection of Trp(For)-containing peptides, the reagent was adjusted to HF-dimethyl sulfide-p-cresol-p-thiocresol (25:65:7.5:2.5 v/v) so that the Nⁱ-formyl could be removed concomitantly with other protecting groups.The low-acidity function, S_N2 reaction was also effective for solid-phase peptide synthesis.The same protecting groups were removed as in solution, and in addition the bond holding the peptide to the resin support was cleaved.For more resistant anchoring bonds and protecting groups a combined low-high HF procedure was developed, in which most of the precursors of harmful carbonium ions are removed by a S_N2 mechanism before the final strong-acid, S_N1, step begins.The new deprotection procedure was tested on three synthetic model peptides, methionine-enkephalin, bovine growth hormone fragment (128-131), and C-terminal pentagastrin amide, and was found to provide efficient deprotection and significant reduction in the level of alkylation side reactions, the rearrangement to aspartimide, and the acylation of aromatic scavengers by glutamic acid.