Mechanisms for the removal of benzyl protecting groups in synthetic peptides by trifluoromethanesulfonic acid-trifluoroacetic acid-dimethyl sulfide

The need to improve peptide synthesis has led to the development of less acidic and milder sN2 deprotection conditions to remove benzyl protecting groups and to avoid reactive carbocations associated with the conventional sN1 conditions. To further understand the relationships of both mechanisms and the acidity of the reaction, the SN1 and sN2 deprotection mechanisms for synthetic peptides with 0-benzyl protecting groups were studied in a ternary mixture of trifluoromethanesulfonic acid (TFMSA)-trifluoroacetic acid (TFA)-dimethyl sulfide (DMS). Kinetic studies of the deprotection rate-acid profiles of 0-benzylserine in sets of experiments containing predetermined amounts of DMS revealed sharp changeover points in the mechanism from sN2 (A,2) to sN1 (AALI) when the concentration of TFMSA in TFA was increased. Similar changeover points in mechanism were also found in the deprotection product-acid profiles of 0-benzyltyrosine. The activities of DMS required for the sN2 reaction were determined by IH NMR, and the acidities of the reaction media were calculated from the Yates-McClelland equation. The changeover points were found to be in the range where DMS activities were approaching zero. In general, the reaction mechanism depended on the activity of DMS and the acidity of the reaction mixture. The sN1 deprotection mechanism predominated at high acidities and low DMS activities. sN2 reaction mechanisms were observed at moderate acidities and high DMS activities. On the basis of the changeover points, a mechanism-reagent composition diagram could be constructed in the form of an equilateral triangle in which the sN1 and SN2 regions could be defined as a function of reagent composition. Furthermore, from the mechanistic considerations, a practical mixture for the sN2 deprotection reaction was found to be TFMSA-TFA-DMS-m-cresol (10:50:30:10 (v/v)). For the deprotection of Trp( For)-containing peptides, the reagent was adjusted to TFMSA-TFA-DMS-m-cresol-ethanedithiol (10:50:30:8:2 (v/v)) so that the N'-formyl could be removed concomitantly with other protecting groups. Both deprotection mixtures also converted Met(0) to Met efficiently. The fundamental design of most protecting group strategies in peptide synthesis is based on the differential lability of protecting groups in acids. ',* A popular and widely used version is the (1) Related papers of this work (a) Tam, J. P.; Heath, W. F. ; Merrifield, R. B. Tetrahedron Lett. 1982,23,2939-2942. (b) Heath, W. F. ; Tam, J. P.; and Merrifield, R. B. J. Chem. Soc. , Chem. Commun. 1982, 896-897. (c) Tam, J. P.; Heath, W. F. ; Merrifield, R. B. Tetrahedron Letf . 1982, 23, 4435-4438. (d) Tam, J. P.; Heath, W. F. ; Merrifield, R. B. J . Am. Chem. Soc. 1983, I U S , 6442-6455. (e) Tam, J. P.; Heath, W. F. ; Merrifield, R. B. Int. J. Peptide Protein Res. 1983, 21, 57-65. (0 Tam, J. P.; Heath, W. F. In Proceedings of the 7th American Peptide Symposium; Hruby, V. J . , Rich, D. H.; Ed.; Pierce Chemical Co.: Chicago, 1983; pp 83-86. 0002-7863/86/1508-5242$01.50/0 combination of Nu-tert-butyloxycarbonyl and side-chain benzyl protection. Usually, a strong anhydrous acid in an SN1 mechanism is employed in the final step to remove all the benzyl protecting group^.^^^ Because of the strong acidity attendant with the SN1 (2) (a) Merrifield, R. B. J. Am. Chem. SOC. 1963, 85, 2149-2154. (b) Merrifield, R. B. ; Vizioli, L. D. ; Boman, H. G. Biochemistry 1982, 21, 5020-5031. (c) Anderson, G. W. ; Blodinger, J.; Welcher, A. D. J . Am. Chem. SOC. 1952, 74, 5309-5312. (d) Schwyzer, R. ; Sieber, P. Nature (London) (3) Sakakibara, S. In Chemisfry and Biochemistry ofAmino Acids, Peptides, and Proteins; Weinstein, B. , Ed.; Dekker: New York, 1971; Vol. 1, pp 1963, 199, 172-174.