Regioselective De‐O‐benzylation of Phenylsulfonylethylidene (PSE) Acetals‐Containing Benzylated Monosaccharides Using Triisobutylaluminum (TIBAL).

Roizel, Berengere Chevalier‐du; Cabianca, Elena; Rollin, Patrick; Sînaÿ, Pierre

doi:10.1002/chin.200308192

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“…The expected 4- O -ether derivative 4c was isolated with 51% yield, and the two byproducts obtained ( 9 , 11%; 10 , 9%) were identified as partial debenzylated derivatives of the major product. Chelation-controlled selective de- O -benzylation of polybenzylated carbohydrates with Lewis acids including triisobutylaluminum (TIBAL) and diisobutylaluminum hydride (DIBAL-H) is well-documented. According to these literature results, the formation of 9 can proceed via the pentacoordinated Al complex I-9 (Scheme ).…”

Section: Results and Discussionmentioning

confidence: 99%

Systematic Study of Regioselective Reductive Ring-Opening Reactions of 4,6-O-Halobenzylidene Acetals of Glucopyranosides

et al. 2021

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Reductive openings of cyclic acetals are widely used in modern synthetic organic chemistry for the regioselective introduction of protecting groups. A systematic study was performed on the applicability and efficacy of various hydride donor and protic or Lewis acid reagent combinations in the reductive ring opening of glucosidic 4,6-halobenzylidene acetals bearing an ortho-, meta-, and para-chloro- or -bromo substituent on the benzene ring. Most of the reagent combinations tested cleaved the 4,6-O-halobenzylidene acetal rings at O4 or O6 efficiently and with the expected regioselectivity. The LiAlH4–AlCl3 and the BH3·THF–TMSOTf combinations produced the 4-O-halobenzyl ether/6-OH products with complete regioselectivity and high yields. The use of Me3N·BH3–AlCl3 reagent system in toluene was also effective in cleaving the acetal ring at O6 but was accompanied by Al-chelation-assisted debenzylation side reactions. The NaCNBH3–HCl and the Et3SiH-BF3·Et2O combinations were highly effective in yielding the 6-halobenzyl ether/4-OH derivatives. Et3SiH, in combination with TfOH, produced the 6-O-ether/4-OH products in rapid reactions but also triggered silylation and reductive halobenzylation as secondary transformations. Reductive opening of the 1,3-dioxane ring of pyranosidic 4,6-O-halobenzylidene acetals by the proper reagent combination was found to be an efficient method for the regioselective introduction of versatile halobenzyl protecting groups onto the pyranose ring.

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Section: Results and Discussionmentioning

confidence: 99%