The Stereochemical Outcome of Galactosylations Is Influenced by Both the Position and Electron-Withdrawing Power of
Line Juul-Madsen1, Nikolai Cordua1, Rasmus D Greve1
1Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 C Aarhus, Denmark.
Abstract:
The stereodirecting effect of various distal benzoyl esters on the anomeric selectivity in galactopyranosylations was investigated. It was found that esters on O-6 of galactosyl donors of the phenyl thioglycoside type had a negligible influence on the anomeric selectivity. Instead, α-selective galactosylations were observed with 4-O-benzoyl, 3,4-di-O-benzoyl, and 4,6-di-O-benzoyl protected galactosyl donors, with the highly electron-withdrawing p-nitrobenzoyl (pNO2Bz) protecting group providing the most α-selective galactosylations. Furthermore, the α-selectivity was enhanced by replacing the thiophenyl aglycon functionality with the highly reactive cyclohexyl aglycon functionality. These findings enabled the successful synthesis of the biologically relevant α-d-Gal(1→4)Gal linkage. The obtained results do not suggest distal participation as a course for the observed anomeric selectivities.
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