- Orthogonally protected d-galactosamine thioglycoside building blocks via highly regioselective, double serial and double parallel inversions of β-d-thiomannoside
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An efficient route for the synthesis of orthogonally protected d-galactosamine thioglycosides via one-pot double serial and double parallel displacements of the d-mannosyl-2,4-bis-trifluoromethanesulfonates by azide and nitrite ions is described. These bu
- Emmadi, Madhu,Kulkarni, Suvarn S.
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- Total Synthesis of the Repeating Unit of Bacteroides fragilis Zwitterionic Polysaccharide A1
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Zwitterionic polysaccharides isolated from commensal bacteria are endowed with unique immunological properties and are emerging as immunotherapeutic agents as well as vaccine carriers. Reported herein is a total synthesis of the repeating unit of Bacteroides fragilis zwitterionic polysaccharide A1 (PS A1). The structurally complex tetrasaccharide unit contains a rare sugar 2-acetamido-4-amino-2,4,6-trideoxy-d-galactose (AAT) and two consecutive 1,2-cis glycosidic linkages. The repeating unit was efficiently assembled by rapid synthesis of d-galactosamine and AAT building blocks from cheap and abundant d-mannose via a one-pot SN2 displacement of 2,4-bistriflates and installation of all of the glycosidic bonds in a highly stereoselective manner. The total synthesis involves a longest linear sequence of 17 steps with 3.47% overall yield.
- Pathan, Ennus K.,Ghosh, Bhaswati,Podilapu, Ananda Rao,Kulkarni, Suvarn S.
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p. 6090 - 6099
(2021/05/06)
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- Stereoselectivity of glycosylations of conformationally restricted mannuronate esters
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Glycosidation of conformationally unrestricted mannuronate ester donors proceeds in a highly β-selective fashion, whereas condensations of mannuronate ester donors, which are conformationally constrained by a 3,4-butanedimethylacetal or a 2,3-isopropylidene function, provide α-selective products. We hypothesize that the difference in stereochemical outcome of these condensations results from the different conformations of the product forming oxacarbenium intermediate. The formation of the β-linked products from the flexible mannuronates is thought to originate from the most favorable 3H4 oxacarbenium ion, which is not accessible from the conformationally restrained donors. Although an α-triflate intermediate is formed upon activation of the 3,4-butanedimethylacetal protected mannuronate ester thio donor, this is not the product forming intermediate. The anomeric triflate serves as a reservoir for the 4H3 oxacarbenium ion, which is glycosidated to provide the α-linked mannuronates.
- Codée, Jeroen D.C.,de Jong, Ana R.,Dinkelaar, Jasper,Overkleeft, Herman S.,van der Marel, Gijsbert A.
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experimental part
p. 3780 - 3788
(2009/09/08)
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