Bromide ion → beta-D-Ribofuranosyl bromide, 2,3,5-triacetate

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A solution of 10 g of 5,6-O-isopropylidene- 1,2'3,6'-tetrakis-N-(trifluoroacetyl)neamine (3) in 330 ml of benzene and 220 ml of nitromethane is heated to boiling and 110 ml of distillate collected. 2,3,5-Tri-O-acetyl-β-D-ribofuranosyl bromide is prepared from 16.5 g of 1,2,3,5-tetra-O-acetyl-β-D-ribofuranose, as described above. The bromide is dissolved in 30 ml of nitromethane. One third of this solution and 7 g of mercuric cyanide is added to the original reaction mixture. The mixture is heated to reflux. Two similar additions of bromide and mercuric cyanide are made after 1 and 2 hours of reflux. The mixture is refluxed a final hour. The cooled reaction mixture is diluted with 750 ml of ethyl acetate and extracted with two 500 ml portions of 5% KHCO3 solution. The organic layer is washed with brine and filtered through Na2SO4. The solvent is evaporated under vacuum. The residue is chromatographed over 1 kg of silica gel, and eluted with chloroform-methanol (20:1). Fractions of 50 ml are collected and monitored by tlc. Fractions 1-30 contain fast moving impurities and are discarded. Fractions 30-40 (27A) weigh 13.4 g. while fractions 41-48 (27B) weigh 3.23 g. A 12.4 g portion of 27A is rechromatographed over 1 kg of silica gel using chloroform-methanol (30:1) for elution. The center cut based on tlc of the fractions is combined and evaporated to give 6.66 g of blocked analog (12β). CMR data, after removal of the blocking group as described infra, shows this compound to be chiefly the 3' β isomer.

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