Abstract
Regioselective glycosylations allow planning simpler strategies for the synthesis of oligosaccharides, and thus reducing the need of using protecting groups. With the idea of gaining further understanding of such regioselectivity, we analyzed the relative reactivity of the OH-3 and OH-4 groups of 2,6-diprotected methyl α- and β-galactopyranoside derivatives in glycosylation reactions. The glycosyl acceptors were efficiently prepared by simple methodologies, and glycosyl donors with different reactivities were assessed. High regioselectivities were achieved in favor of the 1→3 products due to the equatorial orientation of the OH-3 group. A molecular modeling approach endorsed this general trend of favoring O-3 substitution, although it showed some failures to explain subtler factors governing the difference in regioselectivity between some of the acceptors. However, the Galp-(β1→3)-Galp linkage could be regioselectively installed by using some of the acceptors assayed herein.
Highlights
Given the importance of carbohydrates in living systems, oligosaccharides and other glycoconjugates are needed to carry out the corresponding glycobiological studies
Following a methodology previously applied to ᴅ-glucosamine acceptors [8] with some modifications, in the present study, we evaluated the model of ᴅ-galactose and analyzed the relative reactivity of the OH-3 and OH-4 groups of methyl α- and β-galactose derivatives 1α/β and 2α/β in glycosylation reactions with glycosyl donors 3–5 (Figure 1)
We synthesized derivatives 1α/β and 2α/β in order to compare the differences in the regioselectivity of the glycosylation reaction due to the different electron-withdrawing/-donating properties and anomeric configurations
Summary
Given the importance of carbohydrates in living systems, oligosaccharides and other glycoconjugates are needed to carry out the corresponding glycobiological studies. Following a methodology previously applied to ᴅ-glucosamine acceptors [8] with some modifications, in the present study, we evaluated the model of ᴅ-galactose and analyzed the relative reactivity of the OH-3 and OH-4 groups of methyl α- and β-galactose derivatives 1α/β and 2α/β in glycosylation reactions with glycosyl donors 3–5 (Figure 1). The use of acetyl groups was avoided, both in the donors and acceptors, to preclude migration during the glycosylation reactions [14,15].
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