Crown ethers as catalysts of fluoride-anion-mediated reactions in peptide synthesis. Part 1. Protection of tryptophan by benzyloxycarbonyl and 2,4-dichlorobenzyloxycarbonyl groups

Protection of the indole ring of tryptophan by benzyloxycarbonyl and 2,4-dichlorobenzyloxycarbonyl groups is described. Unsolvated fluoride anion, generated by the solubilization of potassium fluoride with 18-crown-6, acts as a powerful base which abstracts the proton from the indole nitrogen atom, and the indolide ion can then be acylated with suitable carbonates. No racemization occurs during acylation. The protecting groups can be removed by catalytic hydrogenation or with liquid hydrogen fluoride or hydrazine.