The synthesis of fragments, comprising two and four repeating units, of the title polysaccharide Is described.
A recent report' describing the synthesis of the Me compounds prompts us to disclose our own work, which differs in some aspects from the synthesis published by Hoogerhout et a/'.
In our synthetic approach, the disaccharides, formed by highly stereoselective glycosidatlon of the ribofuranosyf halides 6 and the protected rlbltd 4 or 5, were joined by a phosphotriester linkage via a phosphoramldite approach2 using diisopropylmethylphosphonamk$c chlorkfe as phosphltylatlng reagent. The protective groups at each terminus (O-3 of ribf and O-5 of the ribiid chain) of the disaccharide could be removed selectively, which allowed using a block-synthesls strategy in the assemblage of larger fragments. The known diethyl dithloacetal of D-rlbose3 was treated wlth monomethoxytrltyl chloride and silver nitrate in THF to gfve the corresponding 5-Cmonomethoxytrityiated ribose4 in 73% yield. Subsequent benzyiatlon, using sodium hydride and benzyi bromkfe In DMF, gave the fully protected rfbose 1 (Scheme 1). Treatment of 1 wlth a catalytic amount of ptoluenesulfonic add In methanol afforded 2, which was plvaloyfated (1.5 equiv plvaloyi chloride, 3 equlv pyrldine, 0.15 equiv DMAP, CH2C12, r.t.) to provide compound 3 In 98% yield. Treatment of 1 or 3 with 2.5 equlv HgC12 and CdC03 in acetone-H20 (5:l V/V), followed by reduction of the resulting aldehyde with either NaBH3CN In dlmethoxyethane or NaBH4 In methanol, gave the protected D-rlbltols 4 ([a]23D -18.4', c 10.9 In CHC$) and 55 ([a]23D +4.4', c 11.8 In CHC13). No racemlzation of 5, caused by mlgratlon of pivalate group from O-5 to O-1, occurred during the reduction step, which was ascertained by comparing the "F NMR spectrum of the Masher ester of 5 (113.45 ppm) with the corresponding ester of the racemic ribltol 5 (113.39 and 113.47 ppm) derhred from meso-2,3,4-t&O-benzyl ribltd by successive pivaloylation and esteriflcation with (-)-a-methoxy-a-trifluoromethylphenylacetlc add/DC@.