We reported an α-selective glycosylation using the o-xylylene-bridged compound 1(Motoyama, et. al. Synthesis 2018, 50, 282). The bridge in 1 arches over the β-face of the pyranose ring, and, consequently, it hinders the β-face approach of an alcohol to produce the α-selectivity under kinetic conditions using Cp2ZrCl2and AgClO4in the presence of 4A molecular sieves (MS) in diethyl ether (Suzuki glycosylation) (Matsumoto, et. al. Tetrahedron Lett. 1988, 29, 3567). However, the bridge locked the pyranose ring into the 3S1form, which directed the 2-O-benzyl group axially toward the α-face thus becoming an adverse steric hindrance. Therefore, the α-selectivity and the yield of glucosides decreased using elevated reaction temperatures, and the use of more sterically hindered alcohols. It was considered that this issue could be resolved by increasing the steric hindrance at the β-face and reducing the overhang of the 2-O-benzyl group on the α-face. To satisfy both of these requirements, a modification of the pyranose conformation would be appropriate by introducing a longer bridge than the o-xylylene group. With this consideration in mind, the 1,1'-(ethane-1,2-diyl)dibenzene-2,2'-bis(methylene) and longer groups were designed so that it would be removable by catalytic hydrogenolysis. The glycosylation reactions using glucosyl fluoride possessing the groups 2 afforded α-selectivity even at room temperature.

Glycosylation reaction using glucosyl fluorides equipped with bridges of varying length

2019 Fall National Meeting

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