Synthetic Studies of the Cororubicin Oligosaccharide: Glycosylation of Branched Amino and Nitro Sugars

Methods for 2-Deoxyglycoside Synthesis

“…In studies directed at the synthesis of the cororubicin trisaccharide, Giuliano and co-workers found that treating an l -Oliose bromide donor 19 with various decilonitrose analogues ( 20a – c ) as acceptors resulted in variable selectivities ( Scheme 6 ). 53 While the N -acetyl-protected acceptor 20a reacted with the donor to afford the target 21a in good yield (69%), the selectivity of the reaction was modest (2:3, α:β). Changing the nitrogen protecting group on the acceptor from an acetate to a trifluoroacetate ( 20b ) permitted coupling to the same donor in similar yield, but with vastly improved selectivity (70%, 30:1 α:β), while the use of a nitro group in this position afforded the product disaccharide 21c as a single α-isomer in modest yield (20%).…”

“…86 They further demonstrated that the fluoride could be activated using a combination of AgClO 4 and SnCl 2 for highly efficient glycosylations. To demonstrate the utility of this approach, the authors used the deoxy-sugar fluorides for the synthesis of the disaccharide from avermectin B 1a (53) and attachment of this compound to the aglycone 54 (Scheme 17). Interestingly, the selectivity in the reaction appears to be highly dependent on the nature of the silver salt.…”

Glycosylation of Branched Amino and Nitro Sugars. 2. Synthesis of the Cororubicin Trisaccharide

“…The first glycosylations involving nitro sugars were reported by Baer, in which nitro hexopyranosides were glycosylated by the Koenigs−Knorr method. In a recent paper from our laboratory, we described the first synthesis of α-linked disaccharides containing the nitro sugar decilonitrose, in which derivatives ( 1 ) of the branched amino sugar methyl 3-amino-2,3,6-trideoxy-3- C -methyl-α- l - ribo -hexopyranoside and its 3-nitro analogue (decilonitrose) were coupled with glycosyl donors ( 2 ) of 3,4-di- O -acetyl-2,6-dideoxy- l -fucopyranose to give α-linked disaccharides . Stereoselective glycosylations were developed using the fucosyl bromide, activated with silver triflate, and the fucosyl oxysilane, activated with trimethylsilyl triflate (Scheme ).…”

“…Our previous study of α-disaccharide synthesis using branched amino sugar derivatives revealed a dependence of stereoselectivity on the nitrogen protecting group; thus, it was desirable to have another derivative of general structure 1 for the glycosylation. Acetamido, trifluoroacetamido, and carbobenzyloxy derivatives of 1 had been prepared from the amino alcohol in our previous study …”