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-beta-maltose (2), 6-iodo-6-deoxy-beta-maltotriose (3) and 6'-iodo-6'-deoxy-beta-maltose (4) has been investigated by cyclic voltammetry and performed on a preparative scale, according to the stoichiometry, -CH(2)I(2-4) + e(-) --> -CH(2)(*) + 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 -CH(2)(*), primary radicals at C-6, which could also abstract H-5', becoming CH(3) 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 -CH(2)(*) disproportionation to CH(3) and CH(2)=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 CH(2)I became CH(3). 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.
Alberti
Angelo
A
ISOF-CNR Area di Ricerca via Pietro Gobetti 101, 40129 Bologna (Italy).
Macciantelli
Dante
D
ISOF-CNR Area di Ricerca via Pietro Gobetti 101, 40129 Bologna (Italy).
Naggi
Annamaria
A
Istituto di Ricerche Chimiche e Biochimiche "G. Ronzoni" via Giuseppe Colombo 81, 20133 Milano (Italy).
Urso
Elena
E
Istituto di Ricerche Chimiche e Biochimiche "G. Ronzoni" via Giuseppe Colombo 81, 20133 Milano (Italy).
Torri
Giangiacomo
G
Istituto di Ricerche Chimiche e Biochimiche "G. Ronzoni" via Giuseppe Colombo 81, 20133 Milano (Italy).
Vismara
Elena
E
Dipartimento di Chimica, Materiali et Ingegneria Chimica "G. Natta", Politecnico di Milano (Italy), Fax: (+39) 0223993180/3080.
D002240:Carbohydrate Sequence; D004563:Electrochemistry; D004566:Electrodes; D004578:Electron Spin Resonance Spectroscopy; D008320:Maltose / Q000031:analogs & derivatives* / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D012834:Silver / Q000737:chemistry; D013237:Stereoisomerism
D012834:Silver; D008320:Maltose