Electrochemical Characterisation of 6-Iodomaltose, 6′-Iodomaltose and 6-Iodomaltotriose on a Silver Cathode and Their One-Pot Electrochemical Dimerisation to New Mixed O/C Maltotetraose and Maltohexaose Mimics

The electrochemical reduction on silver of peracetylated 6‐iodo‐6‐deoxy‐β‐maltose (2), 6‐iodo‐6‐deoxy‐β‐maltotriose (3) and 6′‐iodo‐6′‐deoxy‐β‐maltose (4) has been investigated by cyclic voltammetry and performed on a preparative scale, according to the stoichiometry, CH2I(2–4)+e−→CH2.+I−. In agre...

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Bibliographic Details
Published in:Chemistry : a European journal 2009-08, Vol.15 (32), p.8005-8014
Main Authors: Alberti, Angelo, Macciantelli, Dante, Naggi, Annamaria, Urso, Elena, Torri, Giangiacomo, Vismara, Elena
Format: Article
Language:English
Subjects:
Carbohydrate Sequence
carbohydrates
Electrochemistry
Electrodes
Electron Spin Resonance Spectroscopy
maltomimics
Maltose - analogs & derivatives
Maltose - chemistry
Models, Molecular
Molecular Structure
radicals
silver
Silver - chemistry
Stereoisomerism
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Summary:The electrochemical reduction on silver of peracetylated 6‐iodo‐6‐deoxy‐β‐maltose (2), 6‐iodo‐6‐deoxy‐β‐maltotriose (3) and 6′‐iodo‐6′‐deoxy‐β‐maltose (4) has been investigated by cyclic voltammetry and performed on a preparative scale, according to the stoichiometry, CH2I(2–4)+e−→CH2.+I−. In agreement with the preparative electrolysis results, cyclic voltammetry showed different profiles for the reducing terminal‐iodinated 2 and 3 and for the non‐reducing terminal‐iodinated 4. Compounds 2 and 3 partly dimerised to maltotetraoses mimics 7 (6,6‐dimer) and 8 (5′,5′‐dimer) in 38 % overall yield and to maltohexaose mimics 12 (6,6‐dimer) and 13 (5′,5′‐dimer) in 30 % overall yield, respectively. Compounds 7 and 12 came from the dimerisation of CH2., primary radicals at C‐6, which could also H‐5′, becoming CH3 and generating the C‐5′ quaternary radicals that dimerised in 8 and 13, respectively. These products were accompanied by the maltose derivatives 9, 10 and 11 a/b in 42 % overall yield and by the maltotriose derivatives 14, 15 and 16 in 48 % overall yield, respectively. Compounds 9, 14 and 10, 15 came from CH2. disproportionation to CH3 and CH2C, respectively (exocyclic double bond C‐6/C‐5). Compounds 11 a/b and 16 came from C‐5′ radical reduction, followed by acetate anion elimination (double bonds C‐6′/C‐5′ and C‐5′/C‐4′). In turn, 4 afforded only the 6′,6′‐dimer maltotetraose mimic 17 in 60 % yield, accompanied by the reduced maltose 18 in 20 % yield, in which the starting CH2I became CH3. Compounds 7, 8, 12, 13 and 17 belong to a class of mixed O/C malto‐mimic oligosaccharides wherein an unnatural CC bond between two saccharide units increases metabolic stability compared to their O‐analogues and modulates the sugar chain conformational flexibility, a fundamental parameter in determining protein–carbohydrate binding. Direct and spin‐trapping EPR studies substantiated the radical‐based nature of the dimerisation processes and allowed the identification of some of the paramagnetic species involved. Mixed mimics: The electroreduction of halo‐monosaccharides on a silver cathode is a well‐established technique providing a mild, clean and one‐pot method for the preparation of double sugar units through the formation of stable interglycosidic CC bonds. By extending this technique to oligosaccharides, mixed O/C‐maltotetraose and ‐hexaose mimics were prepared (see graphic).
ISSN:0947-6539
1521-3765
DOI:10.1002/chem.200900825