Synthesis of Monosaccharide Azasugar

The pyranone ring in trisaccharide 88 is perfectly situated for further elaboration into a rhamnose ring with the desired acylation pattern for deistrioside-5 and -6. This was accomphshed by Luche reduction and acylation or chloro-acylation to give 89 and 90. A subsequent dihydroxylation and ortho-ester-mediated C-2 acylation gave trisaccharides 91 and 92, which could be converted into cleistrioside-5 and -6 (72 and 73) by a selective removal of the chloro-acetates over the other acetates (removed with thiourea) and acetonide-protecting groups (hydrolyzed with ACOH/H2O). 

It is worth noting that the route to any of the cleistetrosides is quite comparable to the two previously reported routes to cleistetroside-2 in terms of total number of steps. What distinguished it from these more traditional routes is its flexibility to diverge to any of the possible natural product isomers. Thus, we have found that this divergent approach is particularly amenable to medicinal chemistry studies. In this regard, our synthetic access to these eight natural products (two 

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