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Adapting a literature known protocol (Osby and Ganem, Tetrahedron Lett., 1985, 26(52), 6413-6416), a mixture of nickel(II) dichloride hexahydrate (NiCl2 6H2O) (1.19 g, 5.0 mmol) in methanol (MeOH) (80 mL) is sonicated to effect complete dissolution. Solid sodium borohydride (NaBH4) (567 mg, 15.0 mmol) is added in small portions upon a black precipitate (Ni2B) is generated immediately and hydrogen gas is generated (exotherm!). After about 30 min, a solution of 5-[2-(2-methyl-5-nitro-phenyl)-3-nitro-propyl]-1-trityl-tetrazole (20b) (5.35 g, 10.0 mmol) in MeOH (20 mL) is added followed by portion-wise addition of more NaBH4 (1.13 g, 30.0 mmol) at room temperature. The reaction is monitored by TLC and/or LCMS to completion. The reaction mixture is filtered over Celite® 545 and the filter residue is washed with additional MeOH. The combined filtrates are evaporated to dryness under reduced pressure using a rotary evaporator. The crude material (20c) is used directly in the next step without further isolation and purification.

1. customis sonicated
2. dissolutiondissolution
3. filtrationThe reaction mixture is filtered over Celite® 545
4. washthe filter residue is washed with additional MeOH
5. customThe combined filtrates are evaporated to dryness under reduced pressure
6. customa rotary evaporator
7. customThe crude material (20c) is used directly in the next step without further isolation and purification