Mannosylations of various acceptors with donors possessing an electron-withdrawing o-trifluoromethylbenzenesulfonyl, benzylsulfonyl, p-nitrobenzoyl, benzoyl, or acetyl group at O-3, O-4, or O-6 positions were found to be beta-selective except when donors had 3-O-acyl and 6-O-acetyl groups, which afforded alpha-mannosides as major products. The alpha-directing effect of 3-O-acyl and 6-O-acetyl groups was attributed to their remote participation, and the isolation of a stable bicyclic trichlorooxazine ring resulting from the intramolecular trapping of the anomeric oxocarbenium ion by 3-O-trichloroacetimidoyl group provided evidence for this remote participation. The triflate anion, counteranion of the mannosyl oxocarbenium ion, was essential for the beta-selectivity, and covalent alpha-mannosyl triflates with an electron-withdrawing group at O-3, O-4, or O-6 were detected by low-temperature NMR. The strongly electron-withdrawing sulfonyl groups, which exhibited a higher beta-directing effect in the mannosylation, made the alpha-mannosyl triflates more stable than the weakly electron-withdrawing acyl groups. We therefore proposed the mechanism for the beta-mannosylation and the origin of the beta-directing effect: the electron-withdrawing groups would stabilize the alpha-mannosyl triflate intermediate, and the subsequent reaction of the alpha-triflate (or its contact ion pair) with the acceptor would afford the beta-mannoside. The beta-selective mannosylation of a sterically demanding acceptor was achieved by employing a donor possessing two strongly electron-withdrawing benzylsulfonyl groups at O-4 and O-6 positions.

Section A: General Methods S10 Section B: Synthesis and characterization data Synthesis and characterization data for S1 S11 Synthesis and characterization data for S2 S12 Synthesis and characterization data for S3 S13 Synthesis and characterization data for S4 S14 Synthesis and characterization data for S5 S14 Synthesis and characterization data for 6 S15 Synthesis and characterization data for 7 S15 Synthesis and characterization data for 8 S16 Synthesis and characterization data for 1 S17 S2 Synthesis and characterization data for 2 S17 Synthesis and characterization data for 3 S18 Synthesis and characterization data for 4 S19 Synthesis and characterization data for 5 S19 Synthesis and characterization data for S6 S20 Synthesis and characterization data for S7 S21 Synthesis and characterization data for S8 S22 Synthesis and characterization data for S9 S22 Synthesis and characterization data for S10 S23 Synthesis and characterization data for S11 S23 Synthesis and characterization data for 9 S24 Synthesis and characterization data for 10 S25 Synthesis and characterization data for 11 S25 Synthesis and characterization data for 12 S26 Synthesis and characterization data for S12 S27 Synthesis and characterization data for S13 S27 Synthesis and characterization data for S14 S28 Synthesis and characterization data for S15 S29 Synthesis and characterization data for S16 S30 Synthesis and characterization data for S17 S31 Synthesis and characterization data for 13 S31 Synthesis and characterization data for 14 S32 Synthesis and characterization data for 15 S33 Synthesis and characterization data for 16 S33 References S88 Section C: 1 H NMR and 13 C NMR spectra 1 H NMR and 2D NMR spectra for 74 S202Section A: General Methods All reactions were conducted under a positive pressure of dry argon with dry, freshly distilled solvents unless otherwise noted. All reagents were purchased from commercial suppliers and used without further purification unless otherwise noted. Dichloromethane was distilled from calcium hydride. Flash column chromatography was performed employing 230-400 mesh silica gel. Thin-layer chromatography was Allyl 2,3,6-Tri-O-benzyl-4-O-(o-trifluoromethylbenzenesulfonyl)-D-mannopyranoside (S1). To a solution of allyl 2,3,6-tri-O-benzyl-D-mannopyranoside 1 (763 mg, 1.55 mmol) in CH 2 Cl 2 (10 mL) were added o-CF 3 PhSO 2 Cl (0.48 mL, 3.11 mmol), pyridine (0.50 mL, 6.22 mmol), and DMAP (38 mg, 0.311 mmol) at 0 o C. The resulting solution was stirred at 0 o C for 10 min and at room temperature for further 2 h. The reaction mixture was quenched with saturated aqueous NH 4 Cl, and then extracted with CH 2 Cl 2 . The combined organic layer was washed with 1N HCl and brine, dried over MgSO 4 , and concentrated in vacuo. The residue was purified by flash column chromatography