Perfluoro-tert-butanol for selective on-resin detritylation: a mild alternative to traditionally used methods

• Published in: Amino acids Vol. 53; no. 9; pp. 1455 - 1466
• Main Authors: Wester, Anita, Hansen, Anna Mette, Hansen, Paul R., Franzyk, Henrik
• Format: Journal Article
• Language: English
• Published: Vienna Springer Vienna 01.09.2021; Springer Nature B.V
• Subjects: Acidity; Analytical Chemistry; Biochemical Engineering; Biochemistry; Biomedical and Life Sciences; Butanol; Chain branching; Chains; Dichloromethane; Life Sciences; Low concentrations; Lysine; Molecular Structure; Neurobiology; Original Article; Peptides; Peptides - chemical synthesis; Protecting groups; Proteomics; Reaction time; Residues; Resins; Resins, Synthetic - chemistry; Serine; Solid phase methods; Solid phase synthesis; Solid-Phase Synthesis Techniques - methods; Tert-butanol; tert-Butyl Alcohol - chemistry; Trichloroacetic acid; Trifluoroacetic acid; Trifluoroacetic Acid - chemistry; Trifluoroethanol; Trityl Compounds - chemistry; butanol; On-resin deprotection; Selective detritylation; 4-methoxytriphenylmethyl; trityl; 4-methyltriphenylmethyl; Perfluoro; N-(4-methoxytriphenylmethyl); N-(4-methyltriphenylmethyl); O-trityl; Perfluoro-tert-butanol
• ISSN: 0939-4451; 1438-2199
• DOI: 10.1007/s00726-021-03059-8

Solid-phase synthesis of cyclic, branched or side-chain-modified peptides typically involves introduction of a residue carrying a temporary side-chain protecting group that undergoes selective on-resin removal. In particular, N α -Fmoc-N ε -(4-methyltriphenylmethyl) (Mtt)-protected lysine and its shorter analogues are commercially available and extensively used in this context. Nevertheless, rapid reliable methods for on-resin removal of Mtt groups in the presence of tert -butyloxycarbonyl (Boc) groups are needed. Current commonly used conditions involve low concentrations (1–3%) of trifluoroacetic acid (TFA) in dichloromethane, albeit adjustment to each specific application is required to avoid premature removal of Boc groups or cleavage from the linker. Hence, a head-to-head comparison of several deprotection conditions was performed. The selected acids represent a wide range of acidity from TFA to trifluoroethanol. Also, on-resin removal of the N -(4-methoxytriphenylmethyl) (Mmt) and O -trityl groups (on serine) was investigated under similar conditions. The mildest conditions identified for Mtt deprotection involve successive treatments with 30% hexafluoroisopropanol (HFIP) or 30% perfluoro- tert -butanol [(CF 3 ) 3 COH] in dichloromethane (3 × 5 or 3 × 15 min, respectively), while 30% HFIP, 30% (CF 3 ) 3 COH, or 10% AcOH–20% trifluoroethanol (TFE) in CH 2 Cl 2 (3 × 5 min) as well as 5% trichloroacetic acid in CH 2 Cl 2 (3 × 2 min) enabled Mmt removal. Treatment with 1% TFA with/without 2% triisopropylsilane added (3 × 5 min), but also prolonged treatment with 30% (CF 3 ) 3 COH (5 × 15 min), led to selective deprotection of an O-Trt group on a serine residue. In all cases, the sequences also contained N-Boc or O- t Bu protecting groups, which were not affected by 30% HFIP or 30% (CF 3 ) 3 COH even after a prolonged reaction time of 4 h. Finally, the optimized conditions involving HFIP or (CF 3 ) 3 COH proved applicable also for selective deprotection of a longer resin-bound peptide [i.e., Ac-Gly-Leu-Leu-Lys(Mtt)-Arg(Pbf)-Ile-Lys(Boc)-Ser(tBu)-Leu-Leu-RAM-PS] as well as allowed for an almost complete deprotection of a Dab(Mtt) residue.