Trifluoroacetic acid → (S)-6-(2-Mercaptopropyl)-1,3-dimethyl-5-phenyl-1H-pyrrolo[3,4-d]pyrimidine-2,4(3H,6H)-dione

To crude (S)-1,3-dimethyl-5-phenyl-6-(2-(tritylthio)propyl)-1H-pyrrolo[3,4-d]pyrimidine-2,4(3H,6H)-dione (step 3)(1.35 mg, 1.06 mmol) in DCM (30 ml) was added TFA (4.09 ml, 53.1 mmol) and triethylsilane (1.7 ml, 10.6 mmol) giving a yellow solution. The mixture was stirred at RT overnight. The reaction mixture was evaporated under vacuum. The residue was partitioned between DCM/water and the phases separated. The organic phase was passed through a hydrophobic frit and evaporated under vacuum. The solid was dissolved in a minimal volume of DCM and purified via ISCO column chromatography, 40 g silica, liquid load, 0-60% iso-hexane:EtOAc, product eluting at 50% affording 7 pure fractions. Corresponding fractions were reduced in vacuo to yield the title compound as a white solid.