Synthesis of Heptosylated Oligosaccharides

To give an overview of the state of the art of this field, we selected five complementary approaches presented in their chronological order. A more exhaustive presentation has been published by Kosma and Oscarson [4, 5]. 

Synthesis of the Core Tetrasaccharideof Neisseria meningitidis Lipopolysaccharide 

Synthesis of a Branched Heptose- and Kdo-Containing Common Tetrasaccharide Core Structure of Haemophilus influenzae Lipopolysaccharides 

Barbier reaction [31, 33a], The key anhydro bridge was realized through an internal glycosylation of a 6-O-trimethylsilylated ethyl thioheptoside using NlS/TfOH as a promoter. The 3- and 4-substituents were readily introduced into the 1,6-anhydro intermediate by glycosylation reactions using thioglycosides 108 and 111 as donors and NIS/TfOH as a promoter. 

Interestingly, this transformation was not possible using acceptors with equatorial 3,4-substituents. Acetolysis of the anhydro bridge followed by conversion into the ethyl thioglycoside afforded a ttisaccharide donor, which, in NlS/TfOH-promoted couplings to the spacer and to a Kdo acceptor followed by deprotection, efficiently gave the target compounds. 

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