31P NMR studies on the putative chemoselective activation of nucleoside-3'-thiophosphate-O-esters.

P-diastereomerically pure O-esters of N(Bz)-5'-DMT-dA-3'-monothiophosphate, having charged S=P-O(-) moiety, have been synthesized. Chemoselectivity of their activations by formation of different mixed anhydrides, followed by couplings with N(Bz)-3'-levulinyl-dA, were studied by 31P NMR spectroscopy.