A general synthetic method of 5-carboranyluracil nucleosides with potential antiviral activity and use in neutron capture therapy

Previous biochemical and pharmacological studies indicated that 5-o-carboranyl-2'-deoxyuridine is a lead candidate for boron neutron capture therapy. This prompted the development of a rapid and stereoselective N-1-glycosylation reaction of silylated 5-o-carboranyluracil with a variety of protected sugars. The key intermediate, 5-o-carboranyluracil (6), was prepared from 5-iodouracil in six steps. A novel coupling procedure of the 2,4-dimethoxy-5-ethynylpyrimidine (4) with decaborane without activator was used. Silylated 6 was coupled with a variety of carbohydrates under mild conditions to produce several carborane containing nucleosides. Tn each case, the stereochemistry and stereoselectivity of the glycosylation reaction was not affected by the presence of the carborane at the 5-position of the uracil and produced exclusively close [closo-1,2-C2B10H12 cage] nucleosides. This was confirmed by X-ray structure determination of racemic 5-carboranyl-2',3'-dideoxy-3'-thiauridine. This compound demonstrated an anti-conformation with the oxathiolane ring in a pseudo C-2'-endo conformation. The toxicity profile of the new compounds and their precursors was determined in three cell culture systems, two of human origin (PBM and CEM cells) and one of monkey origin (Vero cells). The compounds were also evaluated for their potential antiviral activity against human immunodeficiency virus and herpes simplex virus in vitro. 5-o-Carboranyl-xylofuranosyluracil (12) demonstrated low toxicity in culture and in mice.