title: Highly stereoselective β-mannopyranosylation via the 1-α-glycosyloxy-isochromenylium-4-gold(I) intermediates.
abstract: While the gold(I)-catalyzed glycosylation reaction with 4,6-O-benzylidene tethered mannosyl ortho-alkynylbenzoates as donors falls squarely into the category of the Crich-type β-selective mannosylation when Ph3 PAuOTf is used as the catalyst, in that the mannosyl α-triflates are invoked, replacement of the (-) OTf in the gold(I) complex with less nucleophilic counter anions (i.e., (-) NTf2 , (-) SbF6 , (-) BF4 , and (-) BAr4 (F) ) leads to complete loss of β-selectivity with the mannosyl ortho-alkynylbenzoate β-donors. Nevertheless, with the α-donors, the mannosylation reactions under the catalysis of Ph3 PAuBAr4 (F) (BAr4 (F) =tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) are especially highly β-selective and accommodate a broad scope of substrates; these include glycosylation with mannosyl donors installed with a bulky TBS group at O3, donors bearing 4,6-di-O-benzoyl groups, and acceptors known as sterically unmatched or hindered. For the ortho-alkynylbenzoate β-donors, an anomerization and glycosylation sequence can also ensure the highly β-selective mannosylation. The 1-α-mannosyloxy-isochromenylium-4-gold(I) complex (Cα), readily generated upon activation of the α-mannosyl ortho-alkynylbenzoate (1 α) with Ph3 PAuBAr4 (F) at -35 °C, was well characterized by NMR spectroscopy; the occurrence of this species accounts for the high β-selectivity in the present mannosylation.
authors.affiliation: State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032 (China).
authors.forename: Yugen
authors.initials: Y
authors.lastname: Zhu
authors.affiliation: State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032 (China). byu@mail.sioc.ac.cn.
authors.forename: Biao
authors.initials: B
authors.lastname: Yu
mesh_terms: D002384:Catalysis; D006031:Glycosylation; D006046:Gold / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008358:Mannose / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D006046:Gold; D008358:Mannose
keywords: beta-mannopyranoside; glycosyl ortho-alkynylbenzoate; glycosylation; gold; non-coordinating counter anion