Synthesis of nucleoside borane conjugates by “chemical ligation” of nucleoside borane acceptors and borane donors: 17–24. i) 2, 5, or 6, CuSO4·5H2O/potassium ascorbate, tert-butyl alcohol/water 1:1; ii) 7 or 8, CuSO4·5H2O/potassium ascorbate, tert-butyl alcohol/water 1:1