Palladium-Catalyzed Glycal Imidate Rearrangement:  Formation of α- and β-N-Glycosyl Trichloroacetamides

Abstract

A novel palladium(II)-catalyzed stereoselective synthesis of alpha- and beta-N-glycosyl trichloroacetamides has been developed. The alpha- and beta-selectivity at the anomeric carbon depends on the nature of the palladium-ligand catalyst. While the cationic palladium(II) promotes the alpha-selectivity, the neutral palladium(II) favors the beta-selectivity.

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“…Recent reports focused on the use of trichloroacetimidate8 as a directing group9 and a zinc reagent10 was chosen to soften the O‐nucleophiles (Scheme ). However, the rearrangement by‐products3b, 11 reduced the yields and additives often complicate the reaction process. Another promising directing group was O ‐picoloyl which is reported to strongly coordinate metal centers12 as well as undergo selective glycosylation with pyranosyl donors through hydrogen‐bond assistance (Scheme ) 13.…”

An Overview on the ChemicalN‐Functionalization of Sugars and Formation ofN‐Glycosides

“…Next, Nguyen group in 2007 first explored palladium catalysed formation of stereoselective αas well as -N-glycosyl trichloroacetamides from glycal C3-imidates rearrangement without the involvement of acceptors and showed the effect of palladiumligand complex to control the anomeric selectivity (Scheme 51). [69] The treatment of glucal imidate 170 with [Pd(PhCN) 2 Cl 2 ], tris(2,4,6-trimethoxyphenyl) phosphine (TTMPP) bulky ligand, as well as salicylaldehyde, undergo rearrangement in CH 2 Cl 2 at room temperature afforded corresponding exclusively -N-glycosyl trichloroacetamides 171a-171d in good to excellent yields. But with cationic palladium [Pd(CH 3 CN) 4 ](BF 4 ) 2 catalyst in the absence of ligand, furnished exclusively α-N-glycosyl trichloroacetamide 172 and proved that cationic palladium(II) afforded the α-selectivity, the neutral palladium(II) promotes the -selectivity.…”

Glycosylation via Transition‐Metal Catalysis: Challenges and Opportunities

“…In 2007, Nguyen and co‐workers first reported a novel palladium(II)‐catalyzed stereoselective synthesis of α‐ and β‐ N ‐glycosyl trichloroacetamides using glycal C3‐imidates in the absence of acceptors . As shown in Table , in the presence of 2.5 mol‐% of [Pd(PhCN) 2 Cl 2 ], 2.5 mol‐% tris(2,4,6‐trimethoxyphenyl)phosphine (TTMPP), and 10 mol‐% salicylaldehyde, d ‐glucal C3‐imidates 179 can undergo rearrangement in CH 2 Cl 2 at room temperature to afford corresponding β‐ N ‐glycosyl trichloroacetamides 180 – 183 bearing 2,3‐unsaturation in good to excellent yields with poor to good β‐selectivity.…”