Acetamido groups, glycoside

As expected, unprotected MOP donors containing a 2-acetamido group give 1,2-trans-glycosides (Scheme 3), presumably by the intermediacy of an oxazolinium ion 15. 

Scheme 3 l,2-/rcwj-Glycoside synthesis using an unprotected MOP glycosyl donor containing a 2-acetamido group. 

In contrast, the Barbier reactions of 2-acetamido-2-deoxy sugars with ketones showed a preference for the formation of 1,2-m-C-glycoside products (Scheme 7.39). This was attributed to the complexation of the C2-acetamido group to the glycosyl samarium, thereby reducing the configurational lability of the organometallic species.78,80,81... 

The initial purpose of the acetylation of the C5 acetamido group is to prevent M-methy-lation in the course of MeOTf-promoted glycosidation using methylthioglycoside of Neu-5Ac-a-(2-3)-Gal as a glycosyl donor. Without the acetyl protection, the glycosidation with a glucosyl acceptor produced Af-acetyl-Al-methyl trisaccharide in 70% yield. AW-Diacetyl... 

The transformation is only moderately stereoselective in this situation and the stereochemical outcome of the reaction is explained by a complexation of the samarium atom by the acetamido group in the kinetic a-Sm(IIl) species 186, slowing down the isomerization to the /3-species. The anomeric selectivity is completely lost when operating with the acetylated sulfone 187. A solution to restore a selective reaction and to access to C-glycosides mimicking the biologically ubiquitous /3-GlcNAc motif, relies on this intermolecular samarium-Barbier reaction followed by an oxidation-isomerization sequence [91] (O Scheme 39). 

The a-C-glycoside derivative 27 was first synthesized in six steps from N-acetylglucosamine as described by Cipolla et al (Scheme 3) (54). The 6-hydroxyl group of compound 27 was then protected as the 6-TBDPS silyl ether followed by 3,4-isopropylidenation to afford compound 28. Based on our previous experience, we decided to double protect the acetamido group at C-2 position in order to avoid cyclic hemiaminal formation later in the scheme when the C-6 position will be oxidized to the corresponding aldehyde. Thus, 28 was treated... 

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