Functionalization of cysteine derivatives by unsymmetrical disulfide bond formation

We have developed a convenient method for the synthesis of unsymmetrical disulfides of L-cysteine under mild conditions in good to excellent yields. The described method is based on the straightforward preparation of 5,5-dimethyl-2-thioxo-1,3,2-dioxaphosphorinane-2-sulfenyl bromide from readily available bis(5,5dimethyl-2-thioxo-1,3,2-dioxaphosphorinan-2-yl) disulfide. The unsymmetrical disulfides can be obtained for L-cysteine derivatives and thiols bearing neutral, basic, or acidic functionalities.