Recent advances in β-l-rhamnosylation

Abstract

This review describes various methodologies for the stereoselective 1,2-cis glycosylation of l-rhamnose leading to β-l-rhamnosides and their applications in oligosaccharide synthesis.

Progress and challenges in the synthesis of sequence controlled polysaccharides

“…In analogy to mannose, β-rhamnosylation is a highly challenging reaction. However, while the β-mannosylation reaction can benefit from 4,6- O -benzylidene PG stereocontrol, β-rhamnosylation cannot [ 323 ]. To date, only few reports of successful β-rhamnans synthesis are available, mostly limited to short oligosaccharides [ 324 ].…”

Synthesis of a Pentasaccharide Repeating Unit of Lipopolysaccharide Derived from Virulent E. coli O1 and Identification of a Glycotope Candidate of Avian Pathogenic E. coli O1

“…Structurally, the O1A pentasaccharide contains two 1,2‐ cis ‐β‐glycosidic linkages, which are difficult to form with high stereoselectivity due to an unfavorable anomeric effect and steric hindrance from the C2 axial group [8] . The installation of the β‐rhamnosidic linkage is particularly challenging due to its 6‐deoxy structure, which prevents the use of stereoselective glycosylation strategies using donors with 2,6‐, 3,6‐, or 4,6‐cyclic O‐protection [9] . To overcome these problems, significant efforts have been devoted to developing efficient β‐rhamnosylation methods, and both indirect and direct methods have been developed [9] .…”

“…The installation of the β‐rhamnosidic linkage is particularly challenging due to its 6‐deoxy structure, which prevents the use of stereoselective glycosylation strategies using donors with 2,6‐, 3,6‐, or 4,6‐cyclic O‐protection [9] . To overcome these problems, significant efforts have been devoted to developing efficient β‐rhamnosylation methods, and both indirect and direct methods have been developed [9] . As an example of an indirect method, Ito and co‐workers reported an intramolecular aglycon delivery (IAD) method via a naphthylmethyl tether mechanism for the stereoselective synthesis of β‐rhamnosides [10] .…”

Synthesis of a Pentasaccharide Repeating Unit of Lipopolysaccharide Derived from Virulent E. coli O1 and Identification of a Glycotope Candidate of Avian Pathogenic E. coli O1

“…[8] Thei nstallation of the b-rhamnosidic linkage is particularly challenging due to its 6-deoxy structure, which prevents the use of stereoselective glycosylation strategies using donors with 2,6-, 3,6-, or 4,6-cyclic Oprotection. [9] To overcome these problems,s ignificant efforts have been devoted to developing efficient b-rhamnosylation methods,a nd both indirect and direct methods have been developed. [9] As an example of an indirect method, Ito and coworkers reported an intramolecular aglycon delivery (IAD) method via an aphthylmethyl tether mechanism for the stereoselective synthesis of b-rhamnosides.…”

“…[9] To overcome these problems,s ignificant efforts have been devoted to developing efficient b-rhamnosylation methods,a nd both indirect and direct methods have been developed. [9] As an example of an indirect method, Ito and coworkers reported an intramolecular aglycon delivery (IAD) method via an aphthylmethyl tether mechanism for the stereoselective synthesis of b-rhamnosides. [10] This method has excellent b-stereoselectivity,b ut the mixed acetal intermediates need further conversion to the desired glycoside products.S uccessful direct approaches include hydrogen bond-mediated aglycon delivery (HAD), [11] use of ab isthiourea catalyst, [12] use of a N-benzoylglycine/thiourea cooperative catalyst, [13] and use of several modified donors, such as a2 -alkynyl-4-nitrobenzoate donor, [14] as uper armed donor, [15] or ad onor with 2,3-or 3,4-cyclicO -protection.…”