Preparation of gamma-glutamylcysteine, a precursor of glutathione

A method to prepare γ-L-Glutamyl-L-Cysteine or bis-γ-L-Glutamyl-L-Cystine, comprising:

• preparing an aqueous solution of cysteine or cystine;
• adding Di-tert-butyl (S)-(2,6-dioxotetrahydro-2H-pyran-3-yl)iminodicarbonate (formula 5) to the aqueous solution of cysteine or cystine to obtain a reaction mixture;
• maintaining a pH of the reaction mixture of from 7 to 8 to obtain a γ ring-opening condensation of the cyteine or cystine amino group with the Di-tert-butyl (S)-(2,6-dioxotetrahydro-2H-pyran-3-yl)iminodicarbonate;
• isolating a bis(tert-butoxycarbonyl) protected γ-L-Glutamyl-L-Cysteine or bis-(tert-butoxycarbonyl) protected γ-L-Glutamyl-L-Cystine;
• and
• hydrolyzing the bis(tert-butoxycarbonyl) groups to obtain γ-L-Glutamyl-L-Cysteine (formula 8) or bis-γ-L-Glutamyl-L-Cystine (formula 11)