Synthese stereospecifique de nucleosides pyrimidiques par la methode de Hilbert-Johnson

Article abstract of DOI:10.1139/v82-410

Condensation of 2,4-dimethoxy pyrimidine with bromoglucosides derived from methyl 4-deoxypentopyranosides 2-7 and from 3,5,6-tri-O-benzoyl glucofuranose, 20, leads in a stereospecific manner to the nucleosides 8-13 and 22 with the trans 1',2'-configuration.The stereochemistry of this reaction is not influenced by the structure of the starting sugar but rather is directed by the presence of the C-2 hydroxyl group.The synthesis of epoxide 18, obtained by the action of NaH on the bromohydrine 17, derived from 7, supports the intervention of a reaction assisted by an epoxide-like intermediate derived from the trans-bromohydrine.On the other hand, the cis-bromohydrine undergoes direct substitution of anomeric halogen by base.The possibility of the formation of a carbocation-like intermediate is discounted since S_N1 type glycosylation reactions of 19 by MeOH or t-BuOH lead preferentially to products with the cis-configurations.