Organic Chemistry 2 · Carbohydrate structure and equilibria
Methyl β-D-ribofuranoside by kinetic Fischer glycosylation
Conditions: CH3OH, HCl (cat.), short reaction time
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Protonate the anomeric OH
HCl, the strongest acid present, protonates the anomeric hydroxyl of β-D-ribofuranose so that it can leave as water. The anomeric OH is the one that leaves because the ring oxygen next to it stabilizes the cation that forms.
The curved arrows
- A lone pair on the anomeric oxygen takes the proton from HCl.
- The H–Cl bond pair leaves with chlorine as chloride.
Water leaves; an oxocarbenium ion forms
The furanose ring oxygen pushes its lone pair toward C1 and expels water. The resulting oxocarbenium ion is planar at C1 and resonance-stabilized by the ring oxygen.
The curved arrows
- A ring-oxygen lone pair forms the C=O⁺ π bond.
- The C1–O bond breaks and the electrons leave with neutral water.
Methanol adds to C1
Methanol attacks the planar furanosyl oxocarbenium ion from the face away from the C2 hydroxyl, giving the β glycoside. With a short reaction time this kinetic furanoside is isolated before it can rearrange to the more stable pyranosides.
The curved arrows
- A lone pair on the methanol oxygen bonds to the oxocarbenium carbon.
- The C=O⁺ π pair returns to the ring oxygen, making it neutral again.
Water removes the oxonium proton
The water released earlier deprotonates the methyloxonium ion; it is a stronger base than chloride. The proton ends on water as hydronium, so HCl is regenerated.
The curved arrows
- A water lone pair takes the proton on the glycoside oxygen.
- The O–H pair stays on the glycoside oxygen.
Methyl β-D-ribofuranoside
C1 is now a furanoside acetal carbon with the OCH3 cis to the CH2OH, the β arrangement of nucleosides. Longer heating in methanolic HCl would reopen the ring and give the thermodynamic methyl ribopyranosides instead.
What you'll practice
Draw the HCl-catalysed Fischer glycosylation of β-D-ribofuranose with methanol, through the furanosyl oxocarbenium ion, to methyl β-D-ribofuranoside.
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