Description
The phosphite triester formed in the coupling step can be converted to the corresponding phosphorothioate triester by treatment with 0.2 M solution of phenylacetyl disulfide (PADS) in acetonitrile and 3-picoline (V / V = 1 : 1). Typically, a 1.5-column volume of PADS solution is used, and sulphurization is complete within 3 minutes, at which time excess reagent is recovered from the reaction vessel by washing with acetonitrile.
An alternative sulfur-transfer reagent is 3-amino-1,2,4-dithiazole-5-thione (ADTT, also known as Xanthane Hydride) which is suitable for solid-phase synthesis.
