The Stereodirecting Effect of the Glycosyl C5-Carboxylate Ester: Stereoselective Synthesis of β-Mannuronic Acid Alginates

Glycosylations of mannuronate ester donors proceed highly selectively to produce the 1,2-cis-linked products. We here forward a mechanistic rationale for this counterintuitive selectivity, based on the remote stereodirecting effect of the C5-carboxylate ester, which has been demonstrated using pyranosyl uronate ester devoid of ring substituents other than the C5- carboxylate ester. It is postulated that the C5-carboxylate ester prefers to occupy an axial position in the oxacarbenium intermediate, thereby favoring the formation of the 3H4 half-chair over the 4H3 conformer. Nucleophilic attack on the 3H4 half-chair intermediate occurs in a β-fashion, providing the 1,2-cis-mannuronates with excellent stereoselectivity. The potential of the mannuronate ester donors in the formation of the β-mannosidic linkage has been capitalized upon in the construction of a mannuronic acid alginate pentamer using a convergent orthogonal glycosylation strategy.