Preparation of Nucleosides Derived from 2-Nitroimidazole and d-Arabinose, d-Ribose, and d-Galactose by the Vorbrüggen Method and Their Conversion to Potential Precursors for Tracers To Image Hypoxia

2-Nitroimidazole was silylated using hexaethyldisilazane and then reacted with 1-O-acetyl derivatives of D-arabinose, D-ribose, and D-galactose in acetonitrile at mild temperatures (-20 °C to rt), catalyzed by triethylsilyl triflate (Vorbrüggen conditions). The α-anomer was formed in the former case and the β-anomers in the latter two cases (highly) selectively. When D-arabinose and D-ribose were silylated with tert-butyldiphenylsilyl chloride in pyridine at the hydroxyl groups at C-5 and acetylated at the other ones in a one-pot reaction, mixtures of anomeric 1-O-acetyl derivatives were obtained. These were coupled by the Vorbrüggen method and then deblocked at C-5 and tosylated to give precursors for tracers to image hypoxia in four steps without using Hg(CN)(2) necessary for other methods. The Vorbrüggen conditions enable a shorter route to azomycin nucleoside analogues than the previous coupling procedures.