A completely β-selective glycosylation that does not rely on neighboring group participation is described. The novelty of this work is the design of the glycosyl donor locked into the axial-rich form by the o-xylylene bridge between the 3-O and 6-O of d-glucopyranose. The synthesized 2,4-di-O-benzyl-3,6-O-(o-xylyene)glucopyranosyl fluoride could efficiently react with various alcohols in a SnCl(2)-AgB(C(6)F(5))(4) catalytic system. The mechanism composed of the glycosylation and isomerization cycles was revealed through comparative experiments using acidic and basic molecular sieves. The achieved perfect stereocontrol is attributed to the synergy of the axial-rich conformation and convergent isomerization caused by HB(C(6)F(5))(4) generated in situ.
A new O-benzylation reagent, benzyl N-phenyl-2,2,2-trifluoroacetimidate, has been developed. It even reacts with sterically hindered alcohols and base-sensitive hydroxy esters to afford the corresponding benzyl ethers catalyzed by TMSOTf in 1,4-dioxane. This reagent was more stable than benzyl 2,2,2-trichloroacetimidate, a known benzylation reagent.
Protection O 0345 Benzyl N-Phenyl-2,2,2-trifluoroacetimidate: A New and Stable Reagent for O-Benzylation. -The title compound (II) even reacts with sterically hindered alcohols and base-sensitive hydroxy esters to afford the corresponding benzyl ethers (III). -(OKADA, Y.; OHTSU, M.; BANDO, M.; YAMADA*, H.; Chem. Lett. 36 (2007) 8, 992-993; Sch. Sci. Technol., Kwansei Gakuin Univ., Sanda, Hyogo 669, Japan; Eng.) -H. Toeppel 01-045
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