Studies on peptides. LXXIX. By-products derived from N.ALPHA.-protected tryptophan by acids.

Four by-products formed during the treatment of Z (OMe)-Trp-OH with trifluoroacetic acid were isolated and their structures were assigned by nuclear magnetic resonance and mass spectra. It was found that the major by-product was H-Trp (2'-p-methoxybenzyl)-OH. Confirmatively, H-Trp (1'-tert-butyl)-OH was identified as the major by-product from Boc-Trp-OH. Such side reaction was found to be efficiently suppressed by the use of the scavenger system, such as, thioanisole containing 2% ethanedithiolskatole or dimethylsulfide containing 2% ethanedithiol-skatole, during the deprotection with trifluoroacetic acid or preferably with 4N ethanesulfonic acid.