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24
P.Dudziński,K.Koroniak-Szejn
2.Mel,NaHCO
1.H
H
2
O,CH
5
lO
DMF
6
,CrO
3
CN
3
3
19R=Ac
20R=Lev
17R=Ac
18R=Lev
Fig.15.Oxidationof17and18followedbyesterformationinvolvingmethyliodide
AfterselectivedeprotectionoftheOH-4groupof20usinghydrazine
acetate,aglycosylationreactionbetweentheresultingcompound21and
glycosyldonor22wascarriedouttoaffordHAdisaccharide23invery
goodyield(86%).ItisworthrecallingthatattempttodeprotecttheOH-4
groupof19withsodiummethoxidewasnotsuccessfulbecauseitledto
thedesiredsaccharide21aswellastheeliminationproductinroughly
equalamounts.Thedisaccharide23canserveaseitherareducingendor
aprecursorforpreparationofhighersaccharides.Thisrequiresdeprotec-
tionandinstallationofananomericleavinggroup.Moreover,theOH-3
groupcanbereadilyandselectivelydeprotectedtoprovideaglycosyl
acceptor(Fig.16).
21
22
CH
TMSOTf
2
Cl
2,
0°C
23
Fig.16.Theglycosylationreactionbetweenglycosyldonor21andacceptor22leadingto
disaccharide23
Additionally,anotherHAdisaccharidesubunitcontainingaglucosa-
minereducingendwassynthesized.Theglucosamineacceptor26was
elaboratedfromallylglucoside24.Thisanomericprotectinggroupwas
chosenduetoitscompatibilitywithperiodateoxidationreactioncarried
outinafurtherstageofthedisaccharidesynthesis.First,theOH-3
groupwastemporarilyprotectedwithlevulinicacid.Then,selective
openingof4,6-benzylideneringgavecompound25.Finally,acetylation
followedbytreatmentwithhydrazineacetateprovidedtheOH-3group
forglycosylationreaction(Fig.17).
