[Development of new deprotecting methodologies for peptides and application to studies on signaling mechanism]

This review summarizes the development of deprotecting methodologies for peptides and their practical application to the synthesis of disulfide bond- or phosphoamino acid-containing peptides. Acidic deprotecting systems utilizing Brønsted acid (HF, trifluoromethanesulfonic acid (TFMSA) and HBr etc.) have been used for the removal of protecting groups in peptide chemistry; however, these reagents are not always applicable to all of the peptides including cystine- or phosphoamino acid-containing peptides. Our attempt to utilize Lewis acid for the deprotective reaction resulted in the development of efficient and practical reagent systems (1 M trimethylsilyl trifluoromethanesulfonate (TMSOTf)-sulfide in trifluoroacetic acid (TFA) and 1 M trimethylsilyl bromide (TMSBr)-sulfide in TFA) suitable for peptide synthesis. A new disulfide bond-forming reaction using Tl(OCOCF3)3 was developed for the synthesis of cystine peptides. The use of TMSOTf or TMSBr-mediated deprotecting system in conjunction with the disulfide bond-forming reaction utilizing Tl (III) provides a procedure for the practical synthesis of cystine peptides. A two-step deprotection method consisting of high acidic (1 M TMSOTf-thioanisole in TFA, m-cresol, ethanedithiol) and low acidic (high acidic system + dimethyl sulfide--TMSOTf) treatments was successfully applied to the deprotection of protected phosphopeptide with dimethyl-protected phosphoamino acids. Furthermore, we synthesized phosphatase-resistant phosphoamino acid isosters bearing the substitution of a phosphate oxygen with a difluoromethylene. The syntheses of peptides possessing these nonhydrolyzable phosphoamino acids were achieved utilizing two-step deprotecting methodologies. Additionally, we demonstrated the usefulness of phosphatase-resistant phosphopeptides as biochemical tools for understanding signal transduction.