246875-57-0Relevant academic research and scientific papers
Stereoselectivity of glycosylations of conformationally restricted mannuronate esters
Codée, Jeroen D.C.,de Jong, Ana R.,Dinkelaar, Jasper,Overkleeft, Herman S.,van der Marel, Gijsbert A.
experimental part, p. 3780 - 3788 (2009/09/08)
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.
