An improved method has been described for the solid-phase synthesis of tryptophan-containing peptides. Hydrogen chloride in formic acid has been introduced as a reagent for the cleavage of the t-butyloxycarbonyl group owing to the stableness of tryptophan and Ni-formyltryptophan in this system. The effectiveness of the new reagent has been demonstrated in the synthesis of the tryptophan-containing heptapeptide, lysylalanylglycyl-leucylglycyltryptophylleucine.

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Ni-Formyltryptophan has proved to be more resistant than tryptophan against oxidation mediated by hydrogen chloride in acetic acid. Nα-t-Butyloxycarbonyl-Ni-formyltryptophan has been synthesized via nonaqueous acylation reaction and used for the solid-phase synthesis of tryptophan-containing peptides, in combination with hydrogen chloride in formic acid as the reagent for cleavage of the t-butyloxycarbonyl group. The validity of the method has been demonstrated, on the basis of several criteria, in the synthesis of the tryptophan-containing octapeptide, Lys–Gly–Val–Leu–Ala–Gly–Trp–Leu.

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