A Novel Synthesis of 2‐thioether Derivatives of Tryptophan

The reaction of L‐3a‐hydroxy‐1,2,3, 3a, 8,8a‐hexahydropyrrolo[2,3‐b]indole‐2‐carboxylic acid (Hpi) with methanethiol, ethanethiol, mercaptoethanol or 3‐mercaptopropionic acid in warm aqueous acetic acid gives the corresponding 2‐thioether derivatives of tryptophan in 50–80% yield (based on Hpi). Better yields may be obtained in 25% trifluoroacetic acid at room temperature. Cysteine reacts with Hpi to give the double amino acid 2‐(L‐3‐alanylthio)‐L‐tryptophan (tryptathionine), which is a constituent of the highly poisonous cyclopeptides of Amanita phalloides, such as phalloidin. Reaction of a moderate excess of Hpi with cysteine‐SH groups of a tripeptide (glutathione) and a protein (reduced ribonuclease) has also been effected, giving the respective S‐tryptophanylated peptide or protein. In both cases, reaction occurred specifically with the ‐SH groups of cysteine and virtually quantitative covalent binding of tryptophan was verified. The extent of the reaction is easily quantitated by spectrophotometry or by amino acid analysis of the content of oxindolylalanine in the hydrolysate with hot 3‐N p‐toluenesulfonic acid of the S‐tryptophanylated peptide or protein. The reaction should be useful in the field of peptide synthesis, providing a simple method for establishing a cross‐link between tryptophan and cysteine, as a basic step in the chemical synthesis of toxic peptides of Amanita phalloides.