Novel class of silicon-based protective groups for the side chain of tyrosine

A novel class of silyl-based protective groups compatible with the Bpoc/t-Bu strategy has been developed for the side chain of tyrosine. Carbobenzyloxy (CBZ) and biphenylisopropyloxy (Bpoc)-O-β-trimethylsilylethyl-tyrosine (10 and 12) and CBZ-O-β-dimethylphenylsilylethyl-tyrosine 14 were prepared in reasonable yields and in very high purity. The trimethylsilylethyl (TMSE) group proved to be 3–4 times more stable than the tert-butyl ether group towards 0.5% TFA. The latter is removed up to 4% during the acidolysis of the Bpoc group. As expected, the dimethylphenylsilylethyl (DMPSE) group was even more resistant towards 0.5% TFA (five time greater than the TMSE analog). Both silyl protective groups were found to be resistant towards a variety of reagents used in peptide synthesis, such as trialkylamines, hydroxybenzotriazole, trialkylphosphine and nucleophiles. They are readily removed in neat TFA in 5–20 min in the absence of cation scavengers, without any detectable alkylation of the phenolic ring. The application of the new silyl-based protective group was demonstrated by the synthesis of the C-terminal 29 amino acid peptide of the basic pancreatic trypsin inhibitor by the prior thiol capture methodology. The protected octapeptide Boc-C(Acm)QT(tBu)FW(TMSE)GG-PO-dibenzofuranthiol was synthesized by solid-phase peptide synthesis using Bpoc-(O-TMSE)-Tyr-OH in greater than 90% yield and coupled to an unprotected 21-mer. The partially blocked, purified peptide was deprotected quantitatively in neat TFA in 1 h.