A method for preparing N,N'-
diacetyl-L-
cystine disodium salt, the method comprising: (i) mixing hydroxyalkane (0.5-100 L, preferably 0.5-10 L, more preferably 1-3 L, most preferably 1 L / mole
cystine) with
sodium base (4.0
molar equivalents / mole
cystine) to form a cold solution at a temperature of 5-10°C; (ii) adding cystine (1
molar equivalent) to the cold solution and stirring for a
sufficient time to form an alkaline cystine solution; (iii) optionally cooling the cystine solution to 5°C; (iv) adding
acetyl chloride (2
molar equivalents / mole cystine) in batches while maintaining the temperature at 3-50°C, preferably at 5-35°C, more preferably below 10°C, most preferably at 5°C, thereby obtaining a white suspension; (v) stirring the white suspension and warming the suspension to
room temperature of 15°C-50°C, preferably at 20-35°C, more preferably 20°C, thereby obtaining N,N'-
diacetyl-L-cystine disodium salt product dissolved in solution, and a
sodium chloride byproduct precipitated
solid in the suspension. This specification discloses an exemplary method on a small
laboratory scale (500 mg cystine isolate, 688 mg product, 90% yield) (Specification page 13, Example 1).