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Synlett 2005(15): 2325-2328  
DOI: 10.1055/s-2005-872678
LETTER
© Georg Thieme Verlag Stuttgart · New York

Highly β-Selective Mannosylation towards Manβ1-4GlcNAc Synthesis: TMSB(C6F5)4 as a Lewis Acid/Cation Trap Catalyst

Shin-ichi Tanaka, Mamoru Takashina, Hiroomi Tokimoto, Yukari Fujimoto, Katsunori Tanaka, Koichi Fukase*
Department of Chemistry, Graduate School of Science, Osaka University, Machikaneyama 1-1, Toyonaka, Osaka 560-0043, Japan
Fax: +81(6)68505419; e-Mail: koichi@chem.sci.osaka-u.ac.jp;
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Publication History

Received 30 June 2005
Publication Date:
07 September 2005 (online)

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Abstract

Highly β-selective mannosylation of 4,6-O-benzylidene-mannopyranosyl-N-phenyltrifluoroacetimidate (α/β, 5:95, 93% yield) has been realized by utilizing a dual Lewis acid/cation trap catalyst, TMSB(C6F5)4. The obtained Manβ1-4GlcNTroc unit can be readily converted to a variety of N-linked oligosaccharides.

Key words

stereoselective mannosylation - β-glycoside - oligosaccharides - Lewis acid - TMSB(C6F5)4 - Manβ1-4GlcNAc synthesis

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Preparation of TMSB(C6F5)4: AgNO3 (73.9 mg, 0.44 mmol) dissolved in distilled H2O (2.2 mL) was mixed with a solution of LiB(C6F5)4 (150 mg, 0.220 mmol) in Et2O (3.3 mL) in a separatory funnel. The biphasic solution was vigorously shaken for 5 min and the organic phase was collected. The solvent was removed in vacuo on a rotary evaporator and the residue was then placed on a vacuum pump for 10 min. To the AgB(C6F5)4 obtained 4 Å MS , Et2O (2.2 mL), toluene (2.2 mL), and TMSCl (31.7 µL) were added; the resulting mixture was used directly for mannosylation. Similarly, HB(C6F5)4 was prepared by the treatment of AgB(C6F5)4 with t-BuBr in Et2O-toluene.

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α/β-Mannoside 4: To a suspension of 4 Å MS, glycosyl acceptor 3 (20.0 mg, 34.7 µmol), and donor 5 (32.1 mg, 52.2 µmol) in CH2Cl2 (1.0 mL) was added TMSB(C6F5)4 (50 mM solution) in toluene-Et2O (1:1, 0.13 mL) at -78 °C under Ar. After the mixture was stirred at -78 °C for 20 min, the reaction was quenched by the addition of sat. aq NaHCO3 solution. The resulting mixture was extracted with EtOAc, washed with brine, dried over MgSO4, filtered, and concentrated in vacuo to give the crude product. The residue was purified by preparative TLC (silica gel; hexane-EtOAc, 7:3) to afford a mixture of α/β-mannoside 4 (32.4 mg, 93%). α-Anomer: 1H NMR (500 MHz, CDCl3): δ = 7.51-7.49 (m, 2 H, PhCH), 7.39-7.20 (m, 21 H, PhCH, Bn), 7.15-7.13 (m, 2 H, Bn), 5.94-5.87 (m, 1 H, OCH2CH=CH2), 5.61 (s, 1 H, PhCH), 5.29 (dd, J = 17.2, 1.4 Hz, 1 H, OCH2CH=CH2), 5.24 (d, J = 1.1 Hz, 1 H, H1′), 5.23 (dd, J = 11.7, 1.1 Hz, 1 H, OCH2CH=CH2), 5.08 (d, J = 9.9 Hz, 1 H, NH), 4.87 (d, J = 3.7 Hz, 1 H, H1), 4.80 (d, J = 12.2 Hz, 1 H, PhCH2), 4.70 (d, J = 11.9 Hz, 1 H, PhCH2), 4.68 (d, J = 11.3 Hz, 1 H, PhCH2), 4.63-4.54 (m, 5 H, PhCH2, Cl3CCH2OCO), 4.42 (d, J = 11.9 Hz, 1 H, PhCH2), 4.24 (d, J = 11.9 Hz, 1 H, PhCH2), 4.23 (t, J = 8.7 Hz, 1 H, H6a′), 4.19 (dd, J = 12.8, 5.3 Hz, 1 H, OCH2CH=CH2), 4.12 (dd, J = 9.6, 4.1 Hz, 1 H, H3′), 4.05 (td, J = 10.2, 3.6 Hz, 1 H, H2), 4.00 (dd, J = 12.9, 6.2 Hz, 1 H, OCH2CH=CH2), 3.92 (1 H, H5′), 3.88 (1 H, H4), 3.85 (dd, J = 9.2, 4.2 Hz, 1 H, H6b′), 3.83-3.76 (m, 4 H, H2′, H4′, H6ab), 3.73-3.69 (m, 2 H, H3, H5). HRMS (MALDI-TOF): m/z calcd for C53H56Cl3NO12 [M + Na]+, 1028.2752; found, 1028.2747. β-Anomer: 1H NMR (500 MHz, CDCl3): δ = 7.49-7.41 (m, 3 H, PhCH, Bn), 7.39-7.20 (m, 21 H, PhCH, Bn), 7.15-7.13 (m, 1 H, Bn), 5.95-5.87 (m, 1 H, OCH2CH=CH2), 5.49 (s, 1 H, PhCH), 5.30 (dq, J = 17.2, 1.5 Hz, 1 H, OCH2CH=CH2), 5.23 (dd, J = 11.7, 1.1 Hz, 1 H, OCH2CH=CH2), 5.01 (d, J = 10.1 Hz, 1 H, NH), 4.91 (d, J = 3.7 Hz, 1 H, H1), 4.87 (d, J = 12.1 Hz, 1 H, PhCH2), 4.81 (d, J = 11.9 Hz, 1 H, PhCH2), 4.78-4.71 (m, 3 H, PhCH2), 4.66-4.54 (m, 4 H, PhCH2, Cl3CCH2OCO), 4.43 (s, 1 H, H1′), 4.34 (d, J = 12.1 Hz, 1 H, PhCH2), 4.17 (ddt, J = 12.9, 5.3, 1.4 Hz, 1 H, OCH2CH=CH2), 4.08-3.93 (m, 4 H, H4′, H6a′, OCH2CH=CH2, H2), 3.77-3.65 (m, 2 H, H5, H2′), 3.60 (t, J = 9.6 Hz, 1 H, H3), 3.56-3.50 (m, 2 H, H6ab), 3.46 (t, J = 10.2 Hz, 1 H, H6b′), 3.37 (dd, J = 9.8, 2.9 Hz, 1 H, H3′), 3.06 (td, J = 9.5, 4.8 Hz, 1 H, H5′). HRMS (MALDI-TOF): m/z calcd for C53H56Cl3NO12 [M + Na]+, 1028.2752; found, 1028.2790.