Abstract
Reaction of 2, 3, 5-tri-O-benzyl-D-ribofuranose (4) with carboalkoxychloromethylenetriphenyl-phosphoranes (5a, b) gave an E, Z mixture of the corresponding alkyl 4, 5, 7-tri-O-benzyl-2-chloro-2, 3-dideoxy-D-ribo-hept-2-enonates (6a, b). Cyclization of 6a, b with triethylamine afforded exclusively the β-C-glycosides 7a, b which were diastereomeric at the carbon bearing chlorine. Hydrolysis of 7a, b with 5% aq. sodium hydroxide in dioxane gave the carboxylic acid 8. Treatment of 8 with thionyl chloride (or trifluoroacetic anhydride) in the presence of triethylamine, followed by reaction with the 2-arylideneaminopyridine 2a or ethyl N-(2-pyridyl) formimidates (2b, c) gave the corresponding fully protected pyrido [1, 2-a] pyrimidine C-nucleosides (15a-c), which were deprotected with boron trichloride to give the C-nucleosides (17b, c).
